Lathe Features

Our PWL-12-16 was specially designed for welding a wide range of circumferential parts or components for use in both plasma and TIG welding applications.

This lathe can be used in a standard production environment as well as laboratory settings, which require a versatile welding tool.

Standard Features:

• Alignment adjustments on head & tail stock


• Use in horizontal or vertical welding positions


• Pneumatic tail stock & torch retract


• SCS-102 Solid State Speed Control with Closed Loop Speed Regulation
  (Custom speeds available on request)


• Torch positioners for versatile torch movement


• Three-jaw 6” scroll chuck allows concentricity adjustment–Chuck includes two-piece hardened reversible jaws
  (special jaws on request)


• Handles lock torch carriage & tail stock in place

Head Stock Standard & Custom Features
Our chuck is a 6” three-jaw with reversible hardened jaws. Unique micro-adjustment screws enable concentricity to be centered dead true for accurate rotation of parts for welding.

The plate behind the threaded lathe spindle has built-in adjustment screws for head stock alignment. The spindle hole is 1.5” and the spindle threads are 2.250”-8.

Special Custom Tooling can be Adapted to the Lathe Spindle Such as:

• Face plates


• 5C collet chucks


• Air collet closer for 5C collets


• Expanding 5C collets


• 5C step chucks

Custom Welding Lathe

Process Welding Systems’ experienced staff can conceptualize, design, and manufacture a custom lathe, positioner or complete system to meet your specified welding needs. Our in-house lab will provide you with weld samples, feasibility studies as well as welding assistance.

PWS Can Quote Special Lathes
With Features Such as:

• Dual head stocks


• Extended lathe bed lengths


• Special tooling for the tail stock


• Gas back-up


• Air collet closers


• Special rotation speeds

Specifications:

Weight:
230 lbs.
Weld Current Capacity:
300 Amps
Distance Between Centers:
16.0”Torch Stroke Weld/Retract:
4.0”
Maximum Diameter Part:
12.0”
Tail Stock Stroke:
3.0”
Lathe Bed:
1.5” diameter hardened rods
Pneumatics:
Equipped with speed controls
Tooling:
3 jaw 6.0” Set-Tru chuck & live center
(special live center points optional)

Plasma Welding Equipment

The Plasma Arc Welding Advantages

The plasma welding process offers two prime benefits: Improved weld quality and increased weld output. Plasma welding offers advanced levels of control, arc stability and weld consistency for high quality welds either in miniature or precision applications.

The plasma process is equally suited to manual and automatic applications. It has been used in a variety of operations ranging from high volume welding of micro components, to precision welding of surgical instruments, to automatic repair of jet engine blades to the manual welding for repair of components in the tool, die and mold industry.

Microplasma Arc Welding Advantages – How Plasma Welding Works
The system requires a power supply and welding torch. In the plasma welding torch a Tungsten electrode is located within a copper nozzle having a small opening at the tip. A pilot arc is initiated between the torch electrode and nozzle tip. This arc is then transferred to the metal to be welded.

By forcing the plasma gas and arc through a constricted orifice, the torch delivers a high concentration of heat to a small area. With high performance welding equipment, the plasma process produces exceptionally high quality welds on a variety of materials.















Plasma Arc Welding Advantages – Features and Benefits

• Protected electrode, offers long times before electrode maintenance (usually one 8 hour shift)


• Low amperage welding capability (as low as 0.1 amp)


• Arc consistency and gentle arc starting produce consistent welds, time after time


• Stable arc in arc starting and low amperage welding


• Minimal high frequency noise issues, HF only in pilot arc start, not for each weld


• Arc energy density reaches 3 times that of GTWA. Higher weld speeds possible


• Weld times as short as .1 secs


• Energy density reduces heat affected zone, improves weld quality


• Length of arc benefits due to arc shape and even heat distribution


• Diameter of arc chosen via nozzle orifice

Preventing Arc Blow

Arc blow can cause a number of welding problems, including excessive spatter, incomplete fusion, porosity and lower quality. What is it and how can it be prevented? In this article, we will examine arc blow and discuss ways to troubleshoot and eliminate this phenomenon to create a better weld.


Arc blow occurs in DC arc welding when the arc stream does not follow the shortest path between the electrode and the workpiece and is deflected forward or backward from the direction of travel or, less frequently, to one side.

First, let's examine some of the terms associated with arc blow. Back blow occurs when welding toward the workpiece connection, or the end of a joint, or into a corner. Forward blow is encountered when welding away from the workpiece connection, or at the starting end of the joint. Forward blow can be especially troublesome with SMAW iron-powder electrodes, or other electrodes that produce large slag coverings, where the effect is to drag the heavy slag or the crater forward and under the arc.

There are two types of arc blow - magnetic and thermal. Of the two, magnetic arc blow is the type causing most welding problems, so we will study that one first.

Magnetic Arc Blow

Magnetic arc blow is caused by an unbalanced condition in the magnetic field surrounding the arc. This unbalanced condition results from the fact that at most times, the arc will be farther from one end of the joint than another and will be at varying distances from the workpiece connection. Imbalance also exists because of the change in direction of the current as it flows from the electrode, through the arc, and into and through the workpiece.

Visualizing a Magnetic Field


To understand arc blow, it is helpful to visualize a magnetic field. Figure 3-37 shows a DC current passing through a conductor (which could be an electrode or the plasma stream between an electrode and a weld joint). Surrounding the conductor a magnetic field, or flux, is set up with lines of force that can be represented by concentric circles in planes at right angle to the direction of the current. These circular lines of force diminish in intensity the farther they are from the electrical conductor.

The concentric flux fields will remain circular when they can stay in one medium expansive enough to contain them until they diminish to essentially nothing . But if the medium changes (such as from steel plate to air), the circular lines of force are distorted and tend to concentrate in the steel where they encounter less resistance. At a boundary between the edges of a steel plate and air, there is a squeezing of the magnetic flux lines, causing deformation in the circular lines of force. This squeezing can result in a heavy concentration of flux behind or ahead of a welding arc. The arc then tends to move in the direction that would relieve the squeezing and restore the magnetic field balance. It veers away from the side of magnetic flux concentration. This veering is observed as arc blow.


Figure 3-38 illustrates the squeezing and distortion of flux fields at the start and finish of a seam weld. At the start, the magnetic flux lines are concentrated behind the electrode. The arc tries to compensate for this imbalance by moving forward which creates forward arc blow. As the electrode approaches the end of the seam, the squeezing is ahead of the arc, with a resultant movement of the arc backwards, and the development of back blow. At the middle of a seam in two members of the same width, the magnetic field would be symmetrical, and there would not be any back or forward arc blow. But, if one member should be wide and the other narrow, side blow could occur at the midpoint of the weld.

Understanding the Effect of Welding Current Returning Through the Workpiece


Another "squeezing" phenomenon results from the current returning back towards the workpiece connection within the workpiece. As shown in Figure 3-39, a magnetic flux is also set up by the electrical current passing through the workpiece to the workpiece lead. The heavy line represents the path of the welding current while the light lines represent the magnetic field set up by the current. As the current changes direction, or turns the corner from the arc to the work, a concentration of flux occurs at x, which causes the arc to blow, as indicated, away from the workpiece connection

The movement of the arc because of this effect will combine with the movement resulting from the concentration previously described to give the observed arc blow. The effect of the returning current may diminish or increase the arc blow caused by the magnetic flux of the arc. In fact, control of the direction of the returning current is one way to control arc blow, especially useful with automatic welding processes.


In Figure 3-40(a), the workpiece cable is connected to the starting end of the seam, and the flux resulting from the returning welding current in the work is behind the arc. The resulting arc movement would be forward. Near the end of the seam, however, the forward arc movement would diminish the total arc blow by canceling some of the back blow resulting from concentration of the flux from the arc at the end of the workpiece, see


In Figure 3-40(b), the work cable is connected to the finish end of the seam, which results in back blow. Here, it would increase the back blow of the arc flux at the finish of the weld. The combination of "squeezed" magnetic fluxes is illustrated in Figure 3-41(b). A workpiece connection at the finish of the weld, however, may be what the welder needs to reduce excessive forward blow at the start of the weld.

Because the effect of welding current returning through the workpiece is less forceful than concentrations of arc-derived magnetic flux at the ends of workpieces, positioning of the workpiece connection is only moderately effective in controlling arc blow. Other measures must also be used to reduce the difficulties caused by arc blow when welding.

Other Problem Areas

• Corner and Butt Joints with deep Vee grooves

Where else is arc blow a problem? It is also encountered in the corners of fillet welds and in weld joints which use deep weld preparations. The cause is exactly the same as when welding a straight seam - concentrations of lines of magnetic flux and the movement of the arc to relieve such concentrations. Figures 3-42 and 3-43 illustrate situations in which arc blow with DC current is likely to be a problem.

• High Currents

There is less arc blow with low current than with high. Why? Because the intensity of the magnetic field a given distance from the conductor of electric current is proportional to the square of the welding current. Usually, serious arc blow problems do not occur when stick electrode welding with DC up to approximately 250 amps (but this is not an exact parameter since joint fitup and geometry could have major influence.)

• DC Currents

The use of AC current markedly reduces arc blow. The rapid reversal of the current induces eddy currents in the base metal, and the fields set up by the eddy currents greatly reduce the strength of the magnetic fields that cause arc blow.

• Magnetically Susceptible Materials

Some materials, such as 9%nickel steels, have very high magnetic permeability and are very easily magnetized by external magnetic fields, such as those from power lines, etc. These materials can be very difficult to weld due to the arc blow produced by the magnetic fields in the material. Such fields are easily detected and measured by inexpensive hand - held Gauss meters. Fields higher than 20 Gauss are usually enough to cause welding problems.

Thermal Arc Blow

We've already examined the most common form of arc blow, magnetic arc blow, but what other forms might a welder encounter? The second type is thermal arc blow. The physics of the electric arc require a hot spot on both the electrode and plate to maintain a continuous flow of current in the arc stream. As the electrode is advanced along the work, the arc will tend to lag behind. This natural lag of the arc is caused by the reluctance of the arc to move to the colder plate. The space between the end of the electrode and the hot surface of the molten crater is ionized and, therefore, is a more conductive path than from the electrode to the colder plate. When the welding is done manually, the small amount of "thermal back blow" due to the arc lag is not detrimental, but it may become a problem with the higher speeds of automatic welding or when the thermal back blow is added to magnetic back blow.

Arc Blow with Multiple Arcs

Some recent welding process advances involve the use of multiple welding arcs for high speed and improved productivity. But, this type of welding can also cause arc blow problems. Specifically, when two arcs are close to each other, their magnetic fields react to cause arc blow on both arcs.

When two arcs are close and have opposite polarities, as in Figure 3-44(a), the magnetic fields between the arcs causes them to blow away from each other. If the arcs are the same polarity, as in Figure 3-44(b), the magnetic fields between the arcs oppose each other. This results in a weaker field between the arcs, causing the arcs to blow toward each other.

Usually, when two arcs are used, it is suggested that one be DC and the other AC, as shown in Figure 3-44(c). In this case, the flux field of the AC arc completely reverses for each cycle, and the effect on the DC field is small. As a result, very little arc blow occurs.

Another commonly used arrangement is two AC arcs. Arc blow interference here is avoided to a large extent by phase-shifting the current of one arc 80 to 90 degrees from the other arc. A so-called "Scott" connection accomplishes this automatically. With the phase shift, the current and magnetic fields of one arc reach a maximum when the current and magnetic fields of the other arc are at or near minimum. As a result, there is very little arc blow.

How To Reduce Arc Blow

• 
  If DC current is being used with the shielded metal-arc process - especially at rates above 250 amps - a change to AC current may eliminate problems.


• Hold as short an arc as possible to help the arc force counteract the arc blow.


• Reduce the welding current - which may require a reduction in arc speed.


• Angle the electrode with the work opposite the direction of arc blow, as illustrated in Figure 3-45.


• Make a heavy tack weld on both ends of the seam; apply frequent tack welds along the seam, especially if the fitup is not tight.


• Weld toward a heavy tack or toward a weld already made.


• Use a back-step welding technique, as shown in Figure 3-46.


• Weld away from the workpiece connection to reduce back blow; weld toward the workpiece connection to reduce forward blow.


• With processes where a heavy slag is involved, a small amount of back blow may be desirable; to get this, weld toward the workpiece connection.


• Wrap the work cable around the workpiece so that the current returning to the power supply passes through it in such a direction that the magnetic field set up will tend to neutralize the magnetic field causing the arc blow.

The direction of the arc blow can be observed with an open-arc process, but with the submerged arc process it is more difficult to diagnose and must be determined by the type of weld defect.

Back blow is indicated by the following:

• Spatter


• Undercut, either continuous or intermittent


• Narrow, high bead, usually with undercut


• An increase in penetration


• Surface porosity at the finish end of welds on sheet metal


• Forward blow is indicated by:


• A wide bead, irregular in width


• Wavy bead


• Undercut, usually intermittent


• A decrease in penetration

The Effects of Fixturing on Arc Blow
Another precaution the weld operator needs to be aware of with arc blow is its relationship to fixturing. Steel fixtures for holding the workpieces may have an effect on the magnetic field around the arc and on arc blow and may become magnetized themselves over time. Usually, the fixturing does not cause any problems with stick-electrode welding when the current does not exceed 250 amps. Fixtures for use with higher currents and with mechanized welding should be designed with precautions taken so that an arc blow-promoting situation is not built into the fixture.

Each fixturing device may require special study to ascertain the best way to prevent the fixture from interfering with the magnetic fields. The following are some points to note:

• Fixtures for welding the longitudinal seam of cylinders (Figure 3-47) should be designed for a minimum of 1-in. clearance between the supporting beam and the work. The clamping fingers or bars that hold the work should be nonmagnetic. Do not attach the workpiece cable to the copper backup bar; make the work connection directly to the workpiece if possible.


• abricate the fixture from low-carbon steel. This is to prevent the buildup of permanent magnetism in the fixture.


• Welding toward the closed end of "horn type" fixtures reduces back blow.


• Design the fixture long enough so that end tabs can be used if necessary.


• Do not use a copper strip inserted in a steel bar for a backing, as in Figure 3-48. The steel part of the backup bar will increase arc blow


• Provide for continuous or close clamping of parts to be seam-welded. Wide, intermittent clamping may cause seams to gap between clamping points, resulting in arc blow over the gaps.


• Do not build into the fixture large masses of steel on one side of the seam only. Counter-balance with a similar mass on the other side.

By understanding the mechanics of arc blow and how to correctly diagnose it in the weld, operators should be able to eliminate it from their applications and be able to create welds without the problems normally associated with arc blow.

Information from The Lincoln Electric Company
source : lincolnelectric.com

Ten Steps to Reducing Your Welding Costs

Many companies strive to get the best possible price on welding equipment and consumables. Although this is an admirable goal, these companies may be overlooking the big picture which says that rather than aim for a savings based on a one-time purchase price, look for ways to get productivity savings. By reducing overall welding costs, the productivity savings that are realized multiply year after year. Productivity savings will allow a company to keep saving even when the price of equipment, consumables or welding accessories goes up.
Looking at the typical work cell model, you will notice that only 20 percent of the cost of welding is related to materials, while the bulk of the costs - more than 80 percent - are attributed to labor and overhead. Hence, if a company saves 10 percent on the material costs of welding, the company is only saving two percent of the total welding costs. But, if a company can save 10 percent on the costs associated with labor and overhead, the company will achieve an eight percent savings on the total welding costs in the work cell model. The work cell information is valid for manual or semi-automatic welding process mild steel application.
Outlined below are 10 steps that companies can take to reduce welding costs and realize productivity savings in the cost of doing business. These are some of the most common items that Lincoln Electric examines when auditing a company.

• Analyze the delivery of consumables and accessories to the welding points


In many shops, the operator has to go to a tool room or supply area for a new contact tip, coil of wire or other welding accessory. This takes valuable time away from the welding cell and slows down overall productivity. To improve the operating efficiency and minimize wasted time, companies should stock at least a limited supply of all necessary items near the welding station - this includes shielding gas, flux and wire. Another helpful productivity enhancing tip is to switch to larger spools of wire such as from 25 lb. spools to 44 or 60 lb. spools to even larger packages of 1,000 lb. reels or 1,000 lb. drums. A simple switch like this means less changeover time, which adds up over the weeks, months and years.

Shops should also be on the lookout for shielding gas waste. A simple device called a surge turbine can be placed at the end of the gun to provide a digital readout of the gas surge and flow rate. If the surge rate is high, investing in a surge guard can reduce the pressure, eliminating gas surges and waste.

Leaks in the gas delivery system can also create a potential loss of money. By looking at the amount of consumables purchased each year and then examining the total gas purchased, a company can determine if there is a significant loss. Welding manufacturers and distributors should be able to provide average utilization figures so that loss can be detected. If there is a loss suspected, one of the easiest ways to check for leaks is to shut off the gas delivery system over the weekend. Check the level on Friday evening and then again on Monday morning to determine if gas was used while the system was in shut down mode.


• Analyze whether material handling is effective


Delivery of parts to the welding station in an organized and logical fashion is also a way to reduce welding costs. For example, one company was manufacturing concrete mixing drums. In the fabrication process, the company produced 10 parts for one section, then went on to make 10 parts of another drum section, etc. As pieces came off the line, they were put onto the floor of the shop. When it was time to weld, the operator had to hunt for the pieces needed and sort through them. When the outside welding expert pointed out the amount of time being wasted in this process, the company started to batch each one on a cart. In this way, the pieces needed to weld one drum were stored together and could easily be moved to the welding area.

This type of scenario is also true for companies that may outsource parts to a vendor. Though it may cost more to have parts delivered in batches, it may save more in time than having to organize and search through parts to be able to get to the welding stage.

How many times each piece is handled in the shop may be an eye-opener to reducing wasted time. To measure such an intangible as this, operators are asked to put a soapstone mark on the piece each time it is touched - some companies are surprised to find out how many times a part is picked up, transported and laid down in the manufacturing process. In the case of one company, moving the welding shop closer to the heat treatment station eliminated four extra times that the part was handled. Basically, handling a part as few times as possible and creating a more efficient production line or work cell will reduce overall costs.


• Look for ways to correct overwelding


One of the "cardinal sins" that almost every shop does is overweld. This means that if the drawing calls for a 1/4" fillet weld, most shops will put down a 5/16" weld. The reasons? Either they don't have a fillet gauge and are not exactly sure of the size of the weld they are producing or they put in some extra to "cover" themselves and make sure there is enough weld metal in place.

But, overwelding leads to tremendous consumable waste. Let's look again at our example. For a 1/4" fillet weld, the typical operator will use .129 lbs. per foot of weld metal. The 5/16" weld requires .201 lbs. per foot of weld metal - a 56 percent increase in weld volume compared to what is really needed. Plus, you must take into account the additional labor necessary to put down a larger weld. Not only is the company paying for extra, wasted consumable material, a weld with more weld metal is more likely to have warpage and distortion because of the added heat input. It is recommended that every operator be given a fillet gauge to accurately produce the weld specified - and nothing more. In addition, changes in wire diameter may be used to eliminate overwelding.


• Enhance current welding processes and procedures


Look for ways to create more efficiencies in the welding process. This includes examining such things as wire diameter, wire feed speed, voltage, travel speed, gas type, transfer mode, etc. For instance, if the shop is currently welding with a short arc process and a 75/25 blend of shielding gas, it may be more effective to switch to a different gas and a spray mode of transfer. Or, a change in process may be warranted based on the condition of the part. If there is oxide on the part, it may be easier to change to a process that will overcome contamination problems rather than try to clean each part before welding. Your welding supplier should be up to date on the latest technology and be able to advise you on new processes, machinery and consumables that can optimize welding at the shop.


• Optimize joint preparation


In some cases, it may be better to double bevel a joint to prepare it for welding rather than single bevel it. It is recommended to double bevel any material that is more than 3/4" in thickness. Just this simple change in procedure can save quite a bit in weld metal. On a 3/4" thick piece, a double bevel will use 1.45 lbs. per foot of weld metal while a single bevel will use 1.95 lbs. per foot.


• Eliminate any extra welds from the design


Look for ways to modify product designs to eliminate unnecessary welds. For example, one company that manufactured boxes originally had a design that called for welded lift handles on each side of the box. By simply changing the design of the box to cut out lifting slots, it eliminated the need for welding the handles - saving time and money. In another instance, rather than making a part with an open corner, the design was changed to accommodate a closed corner, which meant 1/3 less metal required to fill the corner.


• Look for items that can be welded rather than cast


We've already discussed ways to eliminate welds to create efficiencies, but what about adding welds? In some cases, it may be more cost effective to weld metal pieces to a part rather than cast the entire component in a costly alloy or exotic metal. For example, a company that originally used a part cast in a high-nickel alloy found that 50 percent of the part could be composed of standard, structural steel which allowed a savings in material and thus a savings in total cost. Also, the company was further able to redesign the part so that it was more efficient.


• Look for ways to eliminate costly record keeping


Many companies get completely "bogged down" in the paperwork required to run a business. But with today's latest technological advances, there are items that can be a great help. For instance, Lincoln Electric offers something called ArcWorks software which can document procedures, create drawings everyone in the shop can access, keep track of welding operator's qualifications, and many other things. Software such as this can be tailored to the individual company's needs and provide great efficiencies and also eliminate mistakes.


• Adding robotics or hard automation to the operation


Today's technological advances offer many options. Robotics can be justified when the volume of parts a company produces is so great that it can offset the monies spent on a robot. Robotics can also be considered if there are a number of different parts that are similar enough in nature to be able to be handled by the same robot.

If robots are not justified, a company might determine that fixturing or hard automation could be used to increase efficiency or quality. One company incorporated fixturing and clamps to hold down a tank while the seam was being welded. In another case, an automotive manufacturer decided that automation was necessary because of the amount of parts and intricate angles and welding positions.


• Examine safety concerns


Although it may not lead to immediate welding cost reductions, operating under proper safety techniques will save money in the long run by reducing employee accidents. Safety items to consider may include chaining gas cylinders so they can't fall, installing flash arrestors to eliminate blow back when oxyfuel cutting or labeling piping to avoid mishaps.

Conclusion

These are just some of the items that are considered when The Lincoln Electric Company performs its Guaranteed Cost Reduction program. Under this program, a team of Lincoln welding experts visits a facility and performs an audit. A menu of cost reduction ideas is then presented to the company from which they choose and prioritize. Lincoln will calculate the savings and actually guarantee a certain amount of savings if the ideas presented are implemented. If those savings are not realized, Lincoln will write a check for the difference.

As the old saying goes, "don't be penny wise and pound foolish" -- look for ways to decrease welding costs, increase efficiencies and improve productivity, these are the savings items that will reap benefits time and time again.

By James Rosenthal, District Sales Manager, The Lincoln Electric Company
lincolnelectric.com

MIG Welding Stainless Steel

Although welding stainless steel may not be as difficult as welding aluminum, the metal does have its specific properties that vary from your more common steels.
When MIG welding on stainless, you usually have three choices of transfer depending on your equipment: spray-arc, short-circuiting, or pulsed-arc transfer.

Spray-Arc Transfer
Electrode diameters as great as 1/16-in., but usually 0.045", 0.035", and 0.030", are used with relatively high currents to create the spray-arc transfer. A current of approximately 300-350 amperes is required for a 1/16-in. electrode, depending on the shielding gas and type of stainless wire being used. The degree of spatter is dependent upon the composition and flow rate of the shielding gas, wire-feed speed, and the characteristics of the welding power supply. DCEP (Direct Current Electrode Positive) is used for most stainless-steel welding. A 1or 2% argon-oxygen mixture is recommended for most stainless steel spray arc welding.

On square butt welds, a backup strip should be used to prevent weld-metal drop through. When fitup is poor or copper backing cannot be used, drop-through may be minimized by short-circuit welding the first pass.

Forehand techniques are beneficial when welding with a semiautomatic gun. Although the operator's hand is exposed to more heat, better visibility is obtained. For welding plate ¼-in. and thicker, the gun should be moved back and forth in the direction of the joint and at the same time moved slightly from side to side. On thinner metal, however, only back and forth motion along the joint is used.

The more economical short-circuiting transfer process for thinner material should be used in the overhead and horizontal position for, at least, the root and first passes. Although some operators use a short digging spray arc to control the puddle, the weld is apt to be unduly porous.
Power supply units with slope, voltage, and inductance controls are recommended for the welding of stainless steel with short-circuiting transfer. Inductance, in particular, plays an important part in obtaining proper puddle fluidity.

The shielding gas recommended for short-circuiting welding of stainless-steel contains 90% helium, 7.5% argon, and 2.5% carbon dioxide. The gas gives the most desirable bead contour while keeping the CO2 level low enough so that it does not influence the corrosion resistance of the metal. High inductance in the output is beneficial when using this gas mixture.

Single-pass welds may also be made by using argon-CO2 gas. The CO2 in the shielding gas will affect the corrosion resistance of multipass welds made with short-circuiting transfer.

Wire extension or stickout should be kept as short as possible. Backhand welding is usually easier on fillet welds and will result in a neater weld. Forehand welding should be used for butt welds. Outside corner welds may be made with a straight motion. A slight backward and forward motion along the axis of the joint should be used. Short-circuiting transfer welds on stainless steel made with a shielding gas of 90% He, 7-1/2% A, 2-1/2% CO2 show good corrosion resistance and coalescence. Butt, lap, and single fillet welds in material ranging from 0.60-in. to .125-in. in 321, 310, 316, 347, 304, 410, and similar stainless steels can be successfully made.
Pulsed-Arc Transfer

The pulsed arc process is normally a process wherein one small drop of molten metal is transferred across the arc for each high current pulse of weld current. The high current pulse must be of sufficient magnitude and duration to cause at least one small drop of molten metal to form and be propelled by the pinch effect from the end of the wire to the weld puddle. During the low current portion of the weld cycle the arc is maintained and the wire is heated, but the heat developed is not adequate to transfer metal. For this reason, the time duration at the low current value must be limited otherwise metal would be transferred in the globular mode.

Wire diameters of 0.030", 0.035", and 0.045" are most commonly used with this process. Gases for pulsed arc welding are argon plus 1% oxygen, the same as used for spray arc welding. These and other wire sizes can be welded in the spray transfer mode at lower average current with pulsed current than with continuous weld current. The advantage of this is that thin material can be welded in the spray transfer mode which produces a smooth weld with less spatter than the short circuiting mode. Another advantage is that for a given average current, spray transfer can be obtained with a larger wire. Larger diameter wires are less costly than smaller sizes, and the lower ratio of surface to volume reduces the possibility of weld contamination from surface oxides.

Pulsed MIG welding characteristics are excellent with lower currents. There are many advantages with the process including low spatter, penetration without melt-through and excellent operator appeal.
Source: Adapted from The Procedure Handbook of Arc Welding. The Lincoln Electric Company, 1994.

Laser welding of plastics

Laser welding was first demonstrated on thermoplastics in the 1970's, but has only recently found a place in industrial scale situations. The technique, suitable for joining both sheet film and moulded thermoplastics, uses a laser beam to melt the plastic in the joint region. The laser generates an intense beam of radiation (usually in the infra red area of the electromagnetic spectrum) which is focussed onto the material to be joined. This excites a resonant frequency in the molecule, resulting in heating of the surrounding material. Two forms of laser welding exist; CO2 laser welding and transmission laser welding. CO2 laser radiation is readily absorbed by plastics, allowing quick joints to be made, but limiting the depth of penetration of the beam, restricting the technique to film applications. The radiation produced by Nd:YAG and diode lasers is less readily absorbed by plastics, but these lasers are suitable for performing transmission laser welding. In this operation, it is necessary for one of the plastics to be transmissive to laser light and the other to absorb the laser energy, to ensure that the beam focuses on the joint region. Alternatively, an opaque surface coating may be applied at the joint, to weld two transmissive plastics. Transmission laser welding is capable of welding thicker parts than CO2 welding, and since the heat affected zone is confined to the joint region no marking of the outer surfaces occurs.

The technique
CO2 laser welding

The CO2 laser is a well established materials processing tool, available in power outputs of up to 60kW, and most commonly used for metal cutting. The CO2 laser radiation (10.6µm) is rapidly absorbed in the surface layers of plastics. Absorption at these photon energies (0.12eV) is based on the vibration of molecular bonds. The plastics will heat up if the laser excites a resonant frequency in the molecule. In practice the absorption coefficients for the CO2 laser with most plastics is very high. Very rapid processing of thin plastic film is therefore possible, even with fairly modest laser powers (<1000w).> 2 laser beam cannot be transmitted down a silica fibre optic, but can be manipulated around a complex process path using mirrors and either gantry or robotic movement.





A CO2 laser weld in 100µm polyethylene film at 100m/min with 100W laser power




Transmission laser welding - Nd:YAG laser

The Nd:YAG laser is also well established for material processing, and recent developments have led to increases in the power available to above 6kW and reduced the physical size of the laser. In general, the light from Nd:YAG lasers is absorbed far less readily in unpigmented plastics than CO 2 laser light. The degree of energy absorption at the Nd:YAG laser wavelength (1.064µm, 1.2eV photon energy) depends largely on the presence of additives in the plastics. If no fillers or pigments are present in the plastics, the laser will penetrate a few millimetres into the material. The absorption coefficient can be increased by means of additives such as pigments or fillers, which absorb and resonate directly at this photon energy or scatter the radiation for more effective bulk absorption. The Nd:YAG laser may therefore be used for heating plastics to depths of a few millimetres or for heating a more highly absorbent medium (either metal or a plastic containing suitable additives) through or within the transmissive plastic part. The Nd:YAG laser beam can be transmitted down a silica fibre optic enabling easy flexible operation with gantry or robot manipulation.

Transmission laser welding - Diode laser

High power diode lasers (>100W) have been available since early 1997. They are now available up to 6kW and are competitively priced compared to CO2 and Nd:YAG lasers. The production of the diode laser light is a far more energy efficient process (30%) than CO2 (10%) or Nd:YAG (3%) lasers. The interaction with plastics is very similar to that of the Nd:YAG lasers, and applications overlap. The beam from a diode laser is typically rectangular in shape, which, while being preferential for some applications, limits the minimum spot size and maximum power density available. The diode laser source is small and light enough to be mounted on a gantry or robot for complex processing.








Diagram of transmission laser welding







Comparison of commercially available laser sources for plastics processing

Laser Type	CO2	Nd:YAG	Diode
Wavelength (µm)	10.6	1.06	0.8-1.0
Max. power (W)	60,000	6,000	6,000
Efficiency	10%	3%	30%
Beam Transmission	Reflection off mirrors	Fibre optic and mirrors	Fibre optic and mirrors
Minimum spot size * (mm)	0.2-0.7 diam.	0.1-0.5 diam.	0.5x0.5
Capital Cost * (£k)	100W: £20k 1000W: £50k	100W: £40k 1000W: £80k	100W: £15-20k 1000W: £80-100k
Running Cost *(£/hr)	100W: £0.2-0.5 1000W: £2-4	100W: £0.1 1000W: £3-5	100W: £0.1-0.2 1000W: £1-2
Interaction with Plastics	Complete absorption at surface in <0.5mm	Transmission and bulk heating for 0.1-10mm	Transmission and bulk heating for 0.1-10mm

* Approximate figures for general case. Other equipment variants exist with different properties.

Scope

Laser welding is a high volume production process with the advantage of creating no vibrations and generating minimum weld flash. The technique relies on the initial outlay for a laser system, however, the benefits of a laser system include; a controllable beam power, reducing the risk of distortion or damage to components; precise focussing of the laser beam allowing accurate joints to be formed; and a non contact process which is both clean and hygienic. Laser welding may be performed in a single-shot or continuous manner, but the materials to be joined require clamping. Weld speeds depend on polymer absorption. It is possible to create joints in plastics over 1mm thick (with transmission laser welding) at up to at least 20m/min whilst rates of up to 750 m/min are achievable in the CO2 laser welding of films.

Adaptations of laser welding
Clearweld®

The Clearweld® process was invented, and has been patented, by TWI. It is being commercialised by Gentex Corporation. The process uses commercially available lasers in conjunction with infrared absorbing welding consumables.

The carbon black absorber traditionally used is replaced by a colourless, infrared absorbing medium thus expanding the applicability of the technique to clear plastics. The infrared absorbing medium is either printed/painted onto one surface of the joint, encompassed into the bulk plastic, or produced in the form of a film that can be inserted into the joint. It absorbs infra-red laser light allowing an almost invisible weld to be produced between materials that are required to be clear or have a predetermined colour. The process is especially suitable where the appearance of a product is important. In the case of fabrics joining, positioning of the infrared absorbing medium at the joint restricts melting to the interface rather than through the full thickness of the joint as occurs in other welding methods for fabrics. Consequently, flexible seams are produced making the process suitable for the joining of fabrics for clothing applications.

Additional information can be found on the Clearweld® website - www.clearweld.com.

Laser Welding

Plastics are laser-welded by passing laser light through a (laser transparent) top part onto a (laser absorbent) bottom part. The absorbed laser energy softens and melts both parts. With externally applied clamping pressure, the parts are bonded upon cooling. Typically, diode lasers having a wavelength in the (infra-red) range of 800nm-1000nm are used in this process.

Advantages of laser welding:

• Joint design need only be surface to surface. There is no need for energy directors or collapse of the weld joint.


• Weld lines can be as narrow as 0.1mm (0.004 in.)


• Good welds can be achieved, even to a hermetic seal. Tensile strength is that of the unreinforced base resin


• There is no relative motion between the parts as happens with vibration welding. There are no vibrations that could damage electronic components


• Three-dimensional geometries can be welded


• There is no part marking or bleed through


• The joint has less flash than with other methods

The four main laser welding methods:

• Spot laser welding


• Line laser welding


• Mask laser welding


• Simultaneous through welding

In spot welding, a circular spot of laser energy traverses a pre-programmed contour path. The simultaneous line method creates a laser line for welding, while the mask method blocks the laser line in a predefined pattern. Simultaneous through welding delivers laser energy to the entire surface via a fiber optic head and typically runs a three to five second cycle. (ticona.com)

Carbon dioxide laser

by Paul Hilton

The carbon dioxide (CO2 ) gas laser, is one of the most versatile for materials processing applications, and emits infra red radiation with a wavelength between 9 and 11µm, although emission at 10.6µm is the most widely used. Of the several types of CO2 laser that are available, the waveguide, the low power sealed tube and the transversely excited atmospheric (TEA) lasers are used for small scale materials processing applications. The fast axial flow CO2 laser and the less widely used slow flow laser, are used for thick section cutting 1-15mm and deep penetration welding. While these lasers share the same active medium, they have important functional characteristics, which contribute to the wide range of CW (continuous wave) powers, pulse powers and pulse durations available from the CO2 laser.

The active medium in a CO2 laser is a mixture of carbon dioxide, nitrogen and (generally) helium. It is the carbon dioxide which produces the laser light, while the nitrogen molecules help excite the CO2 molecules and increase the efficiency of the light generation processes. The helium plays a dual role in assisting heat transfer from the gas caused by the electric discharge used to excite the gas, and also helps the CO2 molecules to return to the ground state.
Sealed Tube CO2 Lasers

Sealed Tube CO2 Lasers

These lasers are operated as conventional gas discharge lasers in the form of long narrow glass tubes, filled with the lasing gas mixture. Electrodes at either end of the tube provide the discharge current. A totally reflecting and partially transmitting mirror, usually made from polished metal and coated zinc selenide respectively, form the resonant cavity. The tube is sealed using Brewster angled windows. Fig.1, shows a schematic drawing of a sealed tube CO2 laser. As the electric discharge in the tube breaks down the CO2 , an ordinary gas mixture would stop working very quickly and so methods are provided to cause the CO2 to regenerate, either by addition of hydrogen or water or by the use of catalytic action. Several thousand hours of operation are possible with sealed tube CO2 lasers before the tube has to be cleaned and re-filled or replaced. DC and sometimes RF discharges are used with these lasers. CW power up to about 200W is available from these lasers with good beam quality. Pulsed power supplies can produce laser pulses lasting 0.1 - 1msecs with peak powers 5-10 times the CW power level.








Fig. 1 Sealed tube CO 2 laser schematic






Waveguide CO2 Lasers

The waveguide laser is an efficient way to produce a compact CO2 laser. It consists of (see Fig.2), two transverse RF electrodes separated by insulating sections that form a bore region. The lateral dimensions of the bore are a few millimetres, which propagates the beam in 'waveguide mode'. The tube is normally sealed with a gas reservoir separate from the tube itself. The small bore allows high pressure operation and provides rapid heat removal; both of which lead to high gain and high power output from a compact unit.








Fig. 2 Waveguide CO2 laser schematic









TEA CO2 Lasers

Discharge instabilities prevent operation of CW CO2 lasers at pressures above about 100mbar. Pulses in the nanosecond to microsecond duration range can be produced by passing a pulsed current transversely through the lasing gas. Such TEA (transversely excited atmospheric) lasers operate at gas pressures of one atmosphere and above in order to obtain high energy output per unit volume of gas. A transverse discharge from two long electrodes is employed (see Fig.3). Prior to application of the pulsed discharge, a form of pre-ionisation is used to ionise the space between the electrodes uniformly, thus allowing the discharge to proceed in a uniform fashion over the entire electrode assembly. The prime attractions of TEA lasers are their ability to generate short intense pulses and the extraction of high power per unit volume of laser gas. Pulse duration as low as a few tens of nanoseconds up to a few microseconds are possible. Pulse energies range from the millijoule region to 500Joules at pulse repetition rates from about 300Hz down to single shot.








Fig. 3 TEA CO 2 laser schematic








Optics for CO2 Lasers

Reflective mirrors - silicon with high reflectivity coatings, gold coated copper.
Lenses and windows - gallium arsenide and germanium (not transparent in visible region) and coated zinc selenide (orange in the visible region).
Wallplug Efficiency between 5% and 20%


Beam Diameter and Divergence

The shape and length of the laser cavity and nature of the resonator optics determine the beam diameter and divergence of the CO2 laser. Typical ranges are:

beam diameter (mm) beam divergence (mrads)
Sealed tube: 1 - 7 2 - 6
Waveguide: 1 - 2 3 - 10
TEA: 4 - 12 0.5 - 3

Diode lasers

by Paul Hilton



High power diode lasers (known as HPDL's) feature a very high electrical to optical power conversion efficiency coupled with a very compact size. With suitable 'focusing' optics, today's HPDL's are suitable for some materials processing applications. The laser diodes which drive the HPDL's are also being used to replace flashlamp pumping in solid state lasers. Diode lasers also exhibit very high wall plug efficiencies which can be greater than 30% on commercially available systems.

Diode lasers consist of a p-n junction within a multi-layer semiconductor structure. For powers greater than about 4W, the only commonly used manufacturing approach produces a diode laser bar about 10mm long, with emission of radiation confined to the narrow junction region (typically 1µm thick). Along the 10mm length, many thousands of single emitters, of the order 5µm wide, produce laser output with, because of diffraction, very large beam divergence. (See Fig.1). The resulting beam with its large angular spread is characteristic of semiconductor lasers, and, compared to other types of laser, presents a drawback in terms of focusability. The beam divergence is up to 90° perpendicular to the emitting line (known as the 'fast' axis) and about 10° along the emitting line (known as the slow axis).



Powers of the order 80W and higher can be achieved from one diode bar. For high power applications, combining the power from several diode bars is required. For materials processing applications, the semiconductor material is based on InGaAs on a GaAs substrate (940nm) or InGaAlAs on a GaAs substrate (808nm). Both these wavelengths are invisible to the eye.

As a result of the rather unusual beam characteristics of the diode laser and the added complication of increasing power by adding diode bars, several different possibilities exist for beam manipulation to achieve the required power densities for material processing applications. It would appear that this is the area in which one 'diode laser' supplier may be distinguished from another.

A 3kW (highest currently available commercially is 6kW) diode laser (including beam focusing) is smaller than a shoebox and its control, power supply and cooling system is the size of a two drawer filing cabinet. As a result, a clear division can be seen between those manufacturers who would place the laser directly on the arm of a robot say and those who favour fibre optic beam delivery to a focusing head (the latter very similar to that required for a Nd:YAG laser). The approach to beam shaping and focusing is therefore different for these two cases. Two of these design configurations, suitable for material processing applications are described below. Lenses for beam shaping with diode lasers are usually manufactured from glass or fused silicon.

Individual Beam Shaping (IBS) Diode Laser

This system uses sophisticated optics to combine the beam from three individual diode bars mounted as can be seen in Fig.2a. In addition, special diodes are used where the emitting zone is confined to 5 areas 500µm wide with a centre to centre spacing of 1.5mm. This design is the basis of improved beam quality which permits the generation of focused spots about 0.25 x 0.6mm². With its output power of about 150W, the power density ~10 5W/cm², is sufficient for conduction welding of metals.









Fig. 2a IBS diode laser schematic
















Optical Fibre Delivered Diode Laser

This laser also uses a complex optical system designed to minimise the spot size from a single bar so that the beam can be launched down a silica optical fibre. After fast axis collimation with a micro lens, the slow axis is chopped into small beamlets by a special diamond machined mirror. A set of prisms then compresses these beamlets together before collimation with a cylindrical lens and final focusing via spherical optics. Fig.2b, shows how the combination of diamond machined mirror, prisms and lenses, produces a 0.8mm diameter beam for launching into the fibre. Using this configuration, a 35W single bar device can produce a power density of about 7 x 10³ W/cm².











Fig. 2b Fibre delivered diode laser schematic







New Developments
Diode laser technology continues to develop at a fast pace. Its limitations continue, however, to be available spot size. Much effort has gone into this and most of the higher power (1kW+) diode lasers now use the technique of wavelength coupling in order to maximise power in a small spot.

Use of excimer lasers for materials processing

by Dave Taylor


Excimer lasers are characterised by short wavelengths, high intensities and short pulse durations. These characteristics mean that a single photon is capable of breaking a chemical bond. The majority of laser materials processing techniques are essentially thermal processes in which absorption of a large number of photons heats the material to enable cutting, welding or surface modification operations to be performed.

Excimer lasers were first demonstrated in 1975, some time after many of the other laser sources, and are now fairly well established in their niche applications. The term 'excimer' stands for 'excited dimer', where 'dimer' refers to a diatomic molecule such as O 2 or N 2 . This is not strictly a correct term, as the two atoms that make up the molecules used in excimer lasers can be different. The most important molecules are rare gas halides such as F 2 , ArF, KrCl, KrF, XeCl and XeF. These do not exist in nature but can be produced by passing an electrical discharge through a suitable gas mixture. This means that excimer lasers generate ultraviolet energy over a range of wavelengths, depending on the gas mixture used (e.g. 157nm for F 2 and 351nm for XeF).

Typical average output powers are in the range from less than 1 watt up to around 200 watts. This is two orders of magnitude less than the more traditional Nd:YAG or CO 2 lasers which operate in the infrared part of the spectrum. The high intensity beam of an excimer laser is the product of pulse energy (10 - 1000mJ), spot size (governed by focusing optics) and pulse duration (around 10ns).

Applications of excimer laser are primarily in machining of materials such as plastics, paper, ceramics, glasses, crystals, composites and biological tissue. When illuminated with an excimer laser, the relatively weak organic bonds are broken down. This creates a pressure rise and subsequent shock wave that removes material, with little heat transfer to the surrounding material, in a process called 'ablation'. This processing is usually most efficient when carried out using a mask with an image of the required feature. Excimer laser machining is used for its precision, producing features down to approximately 40µm in resolution, but with virtually no heat affected zone.








Excimer laser processing using step-and-repeat mask projection








Some research has been carried out in welding and cutting of sheet metals, but showed no significant advantages over CO 2 or Nd:YAG laser which are available in much higher average powers. Excimer lasers do also lend themselves to more 'niche' applications, such as surface modification of metals and glass for strengthening adhesive bonding and smoothing of machined surfaces to increase wear resistance of components such as camshafts and pistons. As well as increasing wear resistance, this process is being studied as a means of increasing corrosion resistance through production of a thin, amorphous layer on the surface of the material.

Flux Core Arc Welding (FCAW) Advantages & Disadvantages

Marty Rice is a welding instructor at a high school career center in Texas. He is the author of Arc Welding 101, which appears in each issue of Practical Welding Today®, and is an honorary member of the Iron Workers Local 263.

Advantages
When using FCAW, a welder does not have to stop and change rods as in SMAW. That means longer beads with fewer restarts, high weld deposit, and more production. This means less chance of defects in the restart area.
The process uses DCEP and produces deep fusion with a good weld appearance. In addition, smaller-diameter wires can be used in all positions.
FCAW can be used with or without shielding gas; if you use shielding gas, carbon dioxide is very cheap. (Other gases, such as 75/25, also can be used.) The flux contains oxidizers, so the base metal needs minimum cleaning before a weld is made. Postweld cleanup is a breeze because the slag chips off very easily.
On top of that, the wire stickout with FCAW is a lot longer than with GMAW (about ½ to ¾ inch), so welders can see and control the puddle much better. It couldn't be any better if it welded itself--which it can if it is set up for automatic welding. I usually can have a student welding satisfactorily the first day with FCAW (and GMAW, for that matter)

Disadvantages
Fumes! FCAW puts out more smoke than a Houston barbecue joint. If you are using FCAW in a shop, you really should have a strong point-of-contact ventilation system. If not, your lungs are going to be full of welding fumes, and that just isn't healthy.
Other than the fumes, the only other disadvantage is that FCAW usually is used only on mild steel. It has limited uses for cast iron and stainless, but mild steel is all I've ever seen it used on.
In the Field
The only time I used FCAW in the field was on column splices in which one column was stacked on the one below it. This arrangement uses gusset plates where the bolts are attached, and the column itself is beveled for a groove weld.
The included angle (the sum of both column angles where the beveled edges meet) usually is very wide, which leaves a large amount of welding to be done. These are zero-defect welds that are X-rayed for soundness. Stick welding takes entirely too long for these angles and requires too many restarts, which increase the chance of defects.
With FCAW, one welder sits in a basket on one side of the column while another welds the opposite side. This puts the same amount of heat on each side, eliminating any distortion. It makes for good, continuous beads with little time lost. And in construction, time equals money.

Nd:YAG laser welding

by Paul Hilton

The Nd:YAG laser is one of the most versatile laser sources used in materials processing. The relative robustness and compactness of the laser and the possibility for the 1.06 micron light it produces to be transmitted to the workpiece via silica optical fibres, are two features which contribute to its success. Nd:YAG lasers were first commercialised operating mainly in pulsed mode, where the high peak powers which can be generated were found useful in applications such as drilling, cutting and marking. These pulsed lasers can also be utilised for welding a range of materials. More recently, high power (up to 10kW), continuous wave (CW) Nd:YAG lasers have become available. The Nd:YAG crystals in these lasers can be pumped either using white light flashlamps or, more efficiently, using laser diodes. The latter methods are used to produce high quality beams, which can be focused to smaller spots (and therefore produce higher power densities) than the flashlamp pumped lasers. Because of the possibility of using fibre optic beam delivery, these lasers are often used in conjunction with articulated arm robots, in order to work on components of complex shape.

Because of the wide range of applied power and power densities available from Nd:YAG lasers, different welding methods are possible. If the laser is in pulsed mode, and if the surface temperature is below the boiling point, heat transport is predominantly by conduction and a conduction limited weld is produced. If the applied power is higher (for a given speed), boiling begins in the weld pool and a deep penetration weld can be formed. After the pulse, the material flows back into the cavity and solidifies. Both these methods can be used to produce spot welds. A seam weld is produced by a sequence of overlapping deep penetration 'spot' welds or by the formation of a continuous molten weld pool. For the former, once the energy input is sufficient to ensure that the weld does not solidify between pulses, the 'keyhole' type weld normally associated with CO 2 laser welding can be formed. Pulsed laser welding is normally used at thicknesses below about 3mm. Higher power 4-10kW CW Nd:YAG lasers are capable of keyhole type welding in materials from 0.8mm (car body steel) to 15mm (ship steel) thickness.

Nd:YAG laser welding is used commercially on a wide range of C-Mn steels, coated steels, stainless steels, aluminium alloys, titanium and molybdenum. The low heat input welding offered by Nd:YAG lasers is utilised in the electronics, packaging, domestic goods and automotive sectors, and significant interest has been shown more recently, particularly for the high power CW lasers, in the shipbuilding, oil and gas, aerospace and yellow goods sectors. Important R&D issues involve development of high power lasers of better beam quality, use of distributed energy in the beam focus, weld quality maintenance for both thick and thin sections and weld classification.

The principal risks involved in Nd:YAG laser welding are: optical (the beam can burn the skin or damage the retina if focused by the eye), electrical, and fume generation. A current application issue is safe use of Nd:YAG lasers in anything other than a fully opaque (to the Nd:YAG laser wavelength) enclosure, such as might be found in a shipyard for example. (twi.org)