Kent Henry 
Former EASA Technical Support Specialist

Editor's Note: Part 1 of this two-part series was published March 2010. Part 2 was published April 2010.

Suppose two motors came in for an emergency repair. One of the mo­tors is a steel frame and the other is an aluminum frame. Both have broken feet that need to be welded back on to frame. Which motor would you prefer to weld? I believe that most would choose steel over aluminum. But that doesn’t have to be the case.

Part 1 of 2 
Welding aluminum requires a dif­ferent approach. An a unique and intimi­dating aspect of heating aluminum is the lack of visual feedback. If a torch is used to heat a steel or cast iron fan, the metal gradu­allychangescolor. Eventually, it will begin to glow red from an increase in temperature as it nears the melting point. If the same process were repeated on an aluminum fan cover, it would not change color from the increase in temperature. It may appear to darken slightly just prior to suddenly melting. The same is true while welding thin aluminum materials; there is no visual indication of the intense heat applied. The weld­ing arc is the only clue. See Figure 1. The comfort level in working with a metal that displays little if any visual feedback and such a sudden change of state can be unnerving.

Another unique characteristic of aluminum that when it is exposed to air, it does not rust like steel. Instead, it rapidly develops an oxide surface film that is so thin it is invisible without the magnification of a microscope. The theox­ide film has a melting point of around 3600°F(1980°C). Pure aluminum melts at 1220° F (660° C) while aluminum al­loysmeltaround 1020° F (550° C).

The disparity in melting temper­atures between the aluminum and the oxide filmmustbetak­en into account. If the oxide film is not removed prior to welding, the aluminum will melt but is unable to fuse with the adjacent alu­minum or any filler materials due to the oxide film barrier.

The end result is two-fold:1)There will be a marked me­chanical weakness due to incomplete bonding and; 2) if the part happens to be part of an electrical circuit, it will become a high-resistance connection. 

Preparation 
In a well-ventilated area such as a paint booth, rinse the part with a non-chlorinated solvent type degreaser to remove oils, greases, paint, and con­tamination. Acetone or alcohol can be used if you don’t have a degreaser. Be sure the cleaners have completely evaporated before you preheat or weld the part. Heating or welding with these chemicals present on the metal can release toxic gases or acids. Check the Material Safety Data Sheet (MSDS) for the degreaser or solvent used for the proper safety precautions. 

Immediately prior to welding, the surface must be cleaned with a stain­less steel wire brush to remove the oxide film. Purchasing smaller,dispos­able toothbrush style stainless steel brushes make an economical choice since they are relatively inexpensive and the fine bristles clean without transferring metal. If you want to reuse the brush be sure to tag or mark it for “aluminum only” so it does not be­comecontaminated.Since aluminum is a soft material, lightly brushing in one direction helps reduce the chance of embedding oxide in the surface metal. 

Preheating 
Typically aluminum does not re­quire preheating before welding. 

Thermal conductivity 
If someone talks to me about a heat sink, there is no doubt that I would visualize an aluminum-finned struc­ture and for good reason: aluminum has excellent thermal conductivity. 

Pure aluminum has about 5 times the thermal conductivity of steel. Consequently, it takes more voltage and current to weld aluminum than the same thickness of the steel. 

As aluminum is welded, the heat will rapidly transfer throughout the part. This can cause the initial weld bead to be somewhat cold and lack penetration unless the current is increased for a short period. 

A technique that some use is to weld for about 1” (25 mm), stop and immediately repeat welding over the first bead. This results in an improved starting bead as well as inducing enough heat to enable a consistent weld bead from this point forward. 

Welding       
Aluminum can be welded by three methods. 

• Gas Tungsten Arc Welding (GTAW), Tungsten Inert-Gas (TIG) 
• “Heliarc” - Trademark of Linde (helium was the first shielding gas used)
• Gas Metal Arc Weld (GMAW), also known as MIG Welding 

For this article we will focus on the basics of using the GTAW or TIG method. See Figures 2 and 3. The nozzle angle is about 15 to 20 degrees open to ward the direction of the weld, which is toward the filler rod hand. Note the filler rod is about 90 degrees from the torch centerline axis. This method requires a higher degree of hand and eye coordination and some practice. The torch has a tungsten electrode that is air or water-cooled and an enveloping inert gas provides a shield for the weld pool.The operator controls the torch so that the tungsten isabout1/8”to1/4”(3to6mm)from the surface of the weld pool.Care must be used so that the tungsten does not contact the weld pool or the filler rod or it will foul the electrode. 

The operator should select an AC output from the power supply. AC power works well since the electrode positive (EP) por­tionoftheACcycle flows current from the tungsten elec­trode to the metal; this helps dissipate any remaining ox­ide film from the surface. The other half of the wave is the electrode nega­tive (EN) portion of the cycle, which provides the actual welding heat.

Modern inverter type power sup­plies offer square wave output that enhances the welding process. For example, a 40% EP and 60% EN ratio typically works well for many appli­cations. See Figure 4. 

The greater the amount of EP per cycle, the more cleaning action at the weld area and the bead will be wide with shallow penetration. See Figure 5. The greater the amount of EN per cycle the less cleaning at the weld resulting in longer EP times,thus increased penetration and heat concentration and a somewhat narrower weld bead. See Figure 6. These inverter power supplies offer the operator greater control over the process. Some machines offer an auto balance setting of the EPand EN cycles. See Figure 7. 

To repair a crack, bevel grind halfway through the thickness at the crack. Put a few tack welds on that side and then flip it over. Repeat the same level and tack weld process on the backside. Now that the crack is stabilized, flip it back over and fully weld the first side along the crack and then flip it over again to fully weld the backside of the crack. If you can’t access both sides of the crack, the best alternative is to grind a deeper level on the exposed side, being sure you leave about 20 to 25% of the thickness of the material in the bottom of the crack to support your weld; otherwise, the weld will blow through. If you blow through, you may have to use a piece of steel as a backer plate to support the area as you weld.

Part 2 of 2 

For aluminum, use of pure argon gas for shielding provides the best cleaning and penetration up to around athicknessof1/2”(13mm). For thicker aluminum, argon may be mixed with helium(typically25%heliumand75% argon). The introduction of helium provides an increase in heat that per­mits thicker materials to be welded. As a general rule of thumb, adjust the gas flow meter to around 15 to 20 CFH(cubic feet per hour),or7.1to9.4 liter/min (liters per minute). 

Electrodes 
Theelectrodesareavailableinvari­ous composites from pure tungsten-to-tungsten hybrids. Pure tungsten erodes faster than all other electrodes; however, they are the least expensive. The electrodes for aluminum are color-coded. SeeTable 1.Electrodes are available in various diameters from0.02” to 0.25” (0.5 mm to 6.5mm) using the smallest diameter electrode for the weld current works best. If the elec­trode is too small for the current, you will feel it quiver in the torch as you weld. If you are unsure, check with your welding supplier for the proper electrode diameter based on the weld current required and the type of torch being used.

A balled tip electrode is suggested for welding aluminum on sine-wave and square-wave TIG welders. To ball the end of the tungsten, strike an arc with the recommended AC amper­age for a given electrode diameter and the ball should form on the end of the tungsten. See Figure 8. Adjust the electrode so that the tip of the ball extends about 1/4” (6.5 mm) beyond the nozzle of cup.

If the tungsten becomes fouled, it will smoke, change color, and sputter, leaving black soot surrounding the weld bead. If the tungsten gets fouled from contactor weld splatter,cut about 1/2” (13 mm) from the end to remove the fouling.The best way to shorten the tungsten is with a grinder. For safety, always wear eye protection and a res­pirator when grinding the electrodes. Be sure the grinding wheel is clean or the tungsten can get fouled from the debris in the wheel surface. 

Nozzles or cup size 
The nozzle or cup inside diameter should be around three times the size of the diameter of the electrode outside diameter. See Figure 9 . The nozzles are numbered for size; each step is 1/16” (1.59 mm). For example, a #5 nozzle is 5/16” (7.9 mm) ID, and a size 7 nozzle is 7/16” (11.1 mm) ID. 

Amperage 
A hand or foot current controller should be used to allow remote control of the weld current. See Figure 10. Most welder power supplies require that you set the controls to remote amperage control. If not, you may get the full amperage set on the power supply whenever the control is depressed. Be sure to check the opera­tor manual for the proper settings.  

Another rule of thumb is for each 0.001 inch (.025 mm) of part thickness, aluminum requires about 1 ampere minimum of AC welding power. Set up the current controller to provide the desired amperage at 75% of full range. This allows a technician some reserve capability if the need arises during the welding process.

Observe the machine’s nameplate for the rated duty cycle. The duty cycle is the percentage of time in a 10-min­ute period at a given amperage that it may be used without over-heating the machine. For example, if the rating is a 40% duty cycle at 150 amperes, then the welder can weld for 4 minutes at 150 amperes. He must then stop and allowthemachinetorestfor6minutes while the internal cooling fans cool the machine. 

Filler rod 
Filler rod must be added to the weld pool while welding. The type of filler rod depends on the type of aluminum being welded. For most alloys, a 4043 filler rod will yield good results. 

Typically, the filler rod diameter should be smaller than the metal thickness being welded. Keep in mind the smaller the filler rod, the harder it is to feed to the weld. The glove on the hand that feeds the filler rod needs to allow some degree of dexterity or it will be difficult to keep a proper feed rate to the weld pool. If the glove is too stiff, your hand and forearm will tire quickly, making it difficult to produce a quality weld. Check with your welding supply firm for their recommendations. 

Safety 
Wearing the proper personal protective equipment (PPE) is an easy way welders can help protect themselves from injury.  

Checklist of tips 
 Here is a basic checklist of tips, safety items, and settings to review prior to welding:

• Use the proper PPE. 
• Be sure the part has been de­greased and just brushed with a stainless steel brush. 
• Whenever possible, situate the part in a flat, horizontal posi­tion for welding. 
• An auto shading welding hood provides better safety from arc flash since the technician can see with the hood closed. Al­ways wear safety glasses under the weld hood. 
• Check the torch electrode so that it extends about 1/4”(6.5 mm) from the nozzle.  
• Attach the welder ground cable to the part being welded. The high-frequency AC can dam­age electrical circuits in nearby equipment if ground to the part being welded is interrupted.    
• Select AC polarity. 
• Set the high frequency to con­tinuous. 
• Set the amperage to about 1amp per thousandth of inch of thickness. For material thicker than about 1/4” (6.5 mm), the amperage requirements may become nonlinear and you will need to do some testing to find the proper amperage once you have completed this checklist. 
• Set the amperage control switch for remote control and connect a hand or foot operated amper­age controller. 
• Open the shielding gas tank valve. If you have a water-cooled torch, energize the cooler system and power up the welder.  
• While holding the torch safely away from you, depress the am­perage controller and use your opposite torch hand to adjust the shielding gas so the flow meter reads 15 to 20 CFH (cubic feet per hour), 7.1 to 9.4 liter/min (liters per mintue). 
• The shielding gas should con­tinue to flow for a few seconds after releasing the controller to cool the tungsten. 
• Never weldon equipment or parts that contain electronic components. The high frequency from the welder can damage electronics. 

Close your welding hood. With the torch in the dominant hand and the filler rod in the weaker hand, begin welding. The weld should move in the direction of the filler rod hand, so start welding on the part at the side closest to the dominant hand. Gradually increase the amperage with the controller to begin melting the aluminum. Keep the filler rod in the shielding gas plume, but not directly in the arc. The aluminum will shine just as it is ready to add some filler rod. At this point, begin a rhythmic movement about every second of stepping forward (toward the filler rod side of the weld) around 1/8” (3 mm) each step. After each step you should have a short pause. When the aluminum shines,push the filler rod to the weld pool, retract the filler rod, as you step forward and repeat. Every second, the torch should step forward to­ward the filler rod. Using the old rule to count off seconds of 1001, 1002, 1003 seems to help me. The dominant hand leads and the weaker hand follows; Place the torch tip around 1/8” (3 mm) above the piece and depress the amperage control part of the way.The high frequency will start an arc and the part will smoke as the arc cleans the weld area. 

One advantage in the service center is that there is usually a good supply of scrap alu­minum parts to use to practice welding in order to become an expert. 
it’s just like dancing, but with your hands. 

Once the technique is mastered, you should achieve a consistent weld that many say looks like a stack of dimes. See Figure 11. One advantage in the service center is that there is usuallyagoodsupplyofscrapalumi­num parts to use to practice welding in order to become an expert. 

Categories: Service center equipment

Tags: Service center equipment Welding

Rate this article:

No rating

Print