[Welding] TIG Welding Aluminum - Larry Carley (eBook, 7 Pages)

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Download [Welding] TIG Welding Aluminum - Larry Carley (eBook, 7 Pages)


TIG Welding Aluminum, Larry Carley, Automotive Rebuilder, October 2000 The last thing you want to find when rebuilding a cylinder head is a crack. Yet cracks are common in many of today’s aluminum heads because of the thermal stresses experienced in today’s engines. Aluminum has over twice the coefficient of thermal expansion as cast iron, yet only about half of iron’s strength and rigidity depending on the alloy and heat treatment. To make matters worse, as the operating temperature goes up, aluminum’s strength comes down, increasing the risk of cracking in areas of high stress. Consequently, the combination of thinner castings, weaker materials and elevated operating temperatures in today’s engines means cracks are inevitable in many applications. According to some rebuilders, roughly 25% of all late model OHC aluminum heads are found to be cracked. With some the percentage is much higher. Almost 100% of older Ford Escort 1.6L heads are cracked! The high incidence of cracks in aluminum heads has created a shortage of "easy to rebuild" heads for various engine applications, driving up the price of good salvageable cores as well as reman heads. Even so, though cracks increase the cost of rebuilding a head they do not necessarily condemn the casting to the scrap pile. Many heads that would have been discarded in the past because of cracks are now being repaired because cores are either too hard to find or are too expensive. The availability and use of new aftermarket castings has also grown considerably in recent years because of the cracking problem. Depending on the specific head, many rebuilders still find it costs less to repair a cracked head than to replace it with a new one. Small cracks and pinholes in aluminum castings can often be repaired by pinning or stitching (see the August issue of Automotive Rebuilder, "What’s Hot In Cold Crack Repair," page 50). But for large cracks or other damage that requires filling in or building up missing metal, TIG (Tungsten Inert Gas) welding can save many heads that would otherwise have to be replaced. A matter of economics Most rebuilders want to reduce scrappage as much as possible, and will make every effort to repair a cracked or damaged head rather than buy another core or a new casting. Even so, the core value of the head ultimately determines whether it is more economical to repair it or replace it when cracks are found. If the core valve of the head is only $15 to $25, it doesn’t make economic sense to invest a lot of labor into welding it. But if the core value is $100 or more, welding may be a more economical choice than buying another core or a new casting. It all depends on the application and the extent of the damage. Ron Schmitt of Tri-State Cylinder Head Repair in Evansville, IN, rebuilds about 850 cylinder heads a month. "When we decide whether or not to weld a cracked head, we base our decision on the cost to repair the head versus the cost to replace it," explained Schmitt. "The cost to repair it includes the total time to clean, prep and weld the head, not just the time to weld it. We also have to consider the lost revenue of our time invested in the head because we are not able to use that repair time on other work in the shop." Schmitt said he has the ability to repair almost any head, but ends up junking a lot of heads because they’re not worth the effort. "We do a lot of time studies here to analyze our costs, said Schmitt. "To figure the average cost to weld a particular head, we may take 10 identical heads and track everything we do to fix those heads. Some might be easy to weld; others might be hard depending on the extent of the damage. "The time study includes analyzing how long every step in the process takes: locating the cracks, removing the seats and guides, grinding out the cracks, preparing and preheating the head prior to welding, welding and filling the cracks, remachining the head after welding and reinstalling the seats and guides. "We then average the time invested in all 10 heads to come up with an estimated cost to repair that particular type of head. This gives us a number that we can then compare against the head’s replacement cost to decide whether or not it is worth the effort to fix it." Schmitt said there is no magic number for deciding whether it’s more economical to weld or replace because it varies depending on the application. "If core availability is a problem, then we sometimes have to fix heads that we would otherwise replace. Other times, it makes more sense to replace the head than weld it," said Schmitt. TIG welding Also called Gas Tungsten Arc Welding (GTAW), TIG welding has saved many aluminum cylinder heads that would have otherwise reached the end of the road. The TIG welding process is different from ordinary arc welding and MIG (Metal Inert Gas) welding in that the electrode is not consumed. An electric arc between the tungsten electrode and metal provides the necessary heat to melt the aluminum, but the electrode itself does not melt or become part of the weld. An inert shielding gas (usually argon, but sometimes a mixture of helium and argon) is used to protect the molten weld puddle from atmospheric contamination and oxidation, and a separate filler rod is used to add metal as needed. No flux is needed with this process. With TIG welding, an alternating current (AC) power source is generally used. Aluminum forms an oxide layer on the surface, which can contaminate the weld. An AC power source switches the polarity of the arc between positive and negative. When the polarity of the arc is positive, current flows from the work surface towards the electrode producing a cleaning effect that blasts the oxide away from the surface. When the arc switches back to negative, energy flows from the electrode to the work surface producing heat that melts the metal. By increasing or decreasing the duration of the positive or negative phase of the AC cycle, the cleaning and welding characteristics of the arc can be modified. Some TIG welders have a "squarewave" control feature that allows the electrode negative (EN) portion of the AC cycle to be increased up to 90 percent. By increasing the negative phase the AC cycle provides greater heat penetration, faster welding and narrows the weld bead. It also allows the use of a smaller diameter tungsten electrode to more precisely direct the heat in a confined area. Reducing the negative phase of the cycle produces greater cleaning action to remove heavier oxidation, and reduces heat penetration when welding thin castings. TIG welders Two companies that provide TIG welding equipment suitable for welding cylinder heads include Lincoln Electric, Cleveland, OH (216-481-8100), and Miller Electric Mfg. Co., Lithonia, GA (800-426-4553). Carl Peters and Dave Male at Lincoln Electric say rebuilders shouldn’t consider anything less than a 275 amp AC/DC TIG welder for repairing cylinder heads because lower amperage units may not have enough penetration for welding heavier castings. Lincoln’s 275-amp AC/DC TIG welder, that includes a torch and gas bottle holder in the package, sells for $3,327. Its larger 355-amp AC/DC squarewave welder package, which also comes with a torch and gas holder, is $4,982. Miller Electric has two TIG welders of interest to rebuilders: the 300 amp Dynasty DX ($4,605), and the 375 amp Aerowave ($8,121). Equipment changes According to Mike Sammons, welding engineer at Miller Electric, the difference between a high-end AC/DC TIG welder built in 1994 and one built in 1999 resembles the difference between a Ford Escort and a BMW 735i. "The first one is a quality product that serves a basic function, while the latter promises to be the ultimate driving machine," he says. Most of the significant TIG welding advances focus on new ways of manipulating the AC wave form. These advances largely resulted from using "inverter-based" power sources, which permit controlling the arc in ways never before possible, i.e., extending the balance control, adjusting output frequency and independently controlling current in each half of the AC cycle. Sammons says there are no hard rules about setting balance control, but the typical error involves over-balancing the cycle. Too much cleaning action (electrode positive or EP) causes excess heat build-up on the tungsten. This creates a very large ball on the end of the tungsten electrode. Subsequently, the arc loses stability and the operator loses the ability to control the direction of the arc and the weld puddle. Arc starts begin to degrade as well. On the other hand, too much electrode negative results in too much penetration and a scummy weld puddle. If the puddle looks like it has black pepper flakes floating on it, add more cleaning action (EP) to remove oxides and other impurities. Inverter-based welders let operators adjust the welding output frequency from 20 to 250 Hz. Conventional welders have a fixed output of 60 Hz. Decreasing frequency produces a broader arc cone, which widens the weld bead profile and better removes impurities from the surface of the metal. It also transfers the maximum amount of energy to the work piece, which speeds up applications requiring heavy metal deposition (as when filling a large crack or void in a casting). Increasing frequency produces a tight, focused arc cone. This narrows the weld bead, which helps when welding in tight spots. The operator can direct the arc precisely at the crack and not have the arc dance from side to side. And, very importantly, operators can use a pointed tungsten, which further improves arc control and bead shape. A good starting point for general welding, says Sammons, would be 80 to 120 Hz. These frequencies will be comfortable to work with, increase control of the arc direction and boost travel speed. For a fillet weld application with full penetration in the weld without putting too much amperage in the metal, increase the frequency to 225 to 250 Hz. For build-up work, start at 60 Hz and adjust lower from there. "Independent amperage control of the EN and EP portions of the AC cycle, which is only possible with Miller’s Aerowave welder, allows the operator to direct more or less energy into the work piece, as well as take heat off the tungsten," says Sammons. "For example, when welding a thick piece of aluminum, the operator can put 250 amps of EN into the work and only 60 amps of EP into the tungsten. This provides faster travel speeds, faster feed of filler rods, deeper penetration, and the potential to eliminate preheating. Some companies have cut production time by up to twothirds using this technology," said Sammons. Sammons says independently increasing EN amperage while maintaining or reducing EP amperage also provides the following: narrows the arc cone, nearly eliminating the etched zone at the toes of welds; lets the operator use a smaller diameter electrode to make narrower welds, e.g., using a 3/32½ diameter to weld at 280 amps EN; may allow the use of straight Argon in place of Argon/Helium, a costly mix which produces more heat. With the new technology directing heat into the work, not the tungsten, straight Argon alone may suffice. Independently increasing EP amperage while maintaining or reducing EN amperage produces a wider arc cone, wider bead and shallower penetration. No guidelines for setting independent EN and EP current values have been established because it is such new technology. However, like balance control, don’t start out with huge variations. Start with practice pieces and experiment to find the values that work best for a particular application. Frequency control is another area where advances have been made. The AC arc in a traditional, rectifier-based TIG welder is prone to stumbling, wandering and outages, says Sammons, all of which lead to poorer weld quality. These symptoms usually occur during the EN to EP transition because the welder does not have enough voltage to drive through the zero amp range and then reestablish the arc at the electrode, or because the welder cannot transition through the zero amp range quickly enough. To improve arc starts and arc stability, a traditional TIG machine superimposes high frequency (HF) on the AC sine wave to form a path for the arc to follow as it crosses the zero amp range. As a result, one control switch on the front panel typically lets the operator select between "HF off," "HF start only" or "HF continuous." Unfortunately, welders generate HF at 1.2 MHz, a frequency close to that of AM radio. As a result, HF interferes with electronics such as that used in CNC machines, computers and other electronically controlled equipment that may be in the shop. Using an inverter-based AC TIG welder, says Sammons, which uses microprocessor controls to quickly switch between EP and EN, can minimize such problems. An inverter eliminates the need for continuous high frequency to maintain the arc, as well as and the cost of purchasing a separate high-frequency module. It can also minimize or even eliminate the need for HF on start-up. Sammons says Miller’s Dynasty DX only uses high frequency for 10ms during start up. Another difference between conventional TIG welders and inverter-based TIG welders is weight, size and portability. A conventional TIG welder transforms power with a large iron core wrapped with copper and/or aluminum wire. This makes them heavy. To handle the current used to weld thicker sections of metal, a TIG welder’s transformer must weigh 200 to 400 lb. or more. By comparison, inverters operate on the principle that increasing the current frequency reduces the core size and number of wire turns. Before the current reaches the transformer, an inverter boosts the line frequency to 20 to 100 kilohertz. This allows the transformer to be very small and compact (just 5 lbs. on some welders). Power usage is another variable to consider. Conventional AC/DC TIG machines operate only on single-phase power. To weld at 250 amps (230 V primary) requires 92 amps of primary power (and 66 amps with power factor correction). So if you want to add additional TIG welders, it may require making expensive modifications to your electrical system. Inverters, on the other hand, can use either single- or three-phase input power. An inverter using three-phase power, welding at 300 amps (230 V primary) requires just 37 amps of primary current. If 460 V primary current is available, welding at 300 amps requires only 18 amps of primary current making it more power efficient. Welding variables A basic, professional quality TIG welder with AC output lets the operator adjust four variables: amperage, balance control, shielding gas pre-flow time and gas post-flow time. The welder will have either potentiometer-type control knobs or touch panels to control these variables. "Pre-flowing" the shielding gas serves two functions. It purges the immediate weld area from contaminants in the surrounding atmosphere, and it aids with arc initiation. For non-critical applications, Sammons says setting the pre-flow control timer at one second should sufficiently purge the weld area. For critical applications, increase pre-flow time to six seconds or more (a typical TIG welder offers a 0 to 10 second range). "Post-flowing" the shielding gas protects the weld puddle, at the end of the weld, while it cools through temperature ranges where the weld becomes more susceptible to oxidation, cracking and contamination. As a rule of thumb, use one second of post-flow time for every 10 amps of weld output (0 to 50 seconds is an average TIG machine’s capability). The larger the puddle, the longer the required post-flow time. The post-flow also cools the tungsten and protects it from oxidization. Some TIG welders have "sequencer controls" which provide added control over the start current, start time, crater time and final current. The start controls (current over time) allow the operator to have a hotter or cooler start in comparison to the welding amperage. Thick aluminum castings, which act as a giant heat sink, benefit from hot starts because the extra amperage on start-up helps form a puddle more quickly. Thin sections, at risk from melting or warping, benefit from a cool start. The "start current" control knob lets the operator set the starting amperage at any point in the machine’s output range, while the "start time" control knob adjusts a timer, usually from 0 to 15 seconds. As soon as the operator triggers the amperage contactor/control device, i.e., foot pedal or fingertip control, the weld output equals the full preset start value (as opposed to manually ramping up to a value width). Once the timer times out, amperage control returns to the foot pedal or fingertip device. "Crater time" control ramps down from weld current to final or minimum current over time; time is typically adjustable from 0 to 15 seconds. "Final current" control lets the operator select the final amperage as a percentage (0 to 100) of the welding amperage. These features help slowly cool the weld, which prevents crater cracking, a common problem with aluminum. Activated by a contact switch on the torch, crater time and final current control simulate slowly letting off the foot control. Welding guidelines Though opinions may differ about specific details, we’ve put together 12 steps to TIG welding based on various AERA technical presentations and input from experienced TIG operators. Following these guidelines should allow you to repair aluminum cylinder heads successfully: 1) Clean heads thoroughly. Some experts prefer thermal cleaning because it bakes all the oils and resins out of the casting. But if you use thermal cleaning, don’t get the head too hot — see #5). Bead blasting may be needed after thermal cleaning to remove carbon and other residue. A stainless steel brush can also be used to clean the surface (use the brush for aluminum only or you may contaminate the weld). 2) Pressure test the head to check for cracks and porosity leaks. 3) If repairs are needed, remove guides and seats. 4) Fully grind out all cracks. Milling may be less messy than die grinding, where possible. 5) Preheat the head to 400° to 500° F for up to two hours before welding — unless the casting is a heat-treated alloy, in which case keep the temperature under 275° F. Overheating a heat-treated aluminum alloy can anneal (soften) the casting. The same caution applies when welding a heat-treated head, too. Limit the weld time to about five minutes or less to avoid overheating the casting. Once the head is at temperature, it will retain heat for 15 to 20 minutes. Using shielding around the weld area to reduce drafts and retain heat will improve heat retention and extend your welding time. If the heat’s temperature drops too low, put it back in the oven and reheat it back to your working temperature. 6) Use an AC TIG welder with at least 250-amp capacity. A water-cooled torch and foot amp control are also recommended. 7) Use 15-20 cfm of argon shielding gas while welding, or a 25/75 mixture of argon/helium for faster welding. 8) Use a 9X to 11X eye shield with facemask to protect the operator’s eyes. Gold tint shields provide better visibility than green tint. 9) The type of electrode used will depend on the type of equipment. Pure tungsten with a ball tip works well with most TIG welders, but a sharp tipped 2% thorated tungsten electrode can be used with inverter-based equipment. When welding cracks, use wave balancing to clean and float contaminants out of the weld puddle. Do not allow the tungsten electrode to contact the base metal or filler rod. Start the arc using high frequency holding the electrode about 1/8½ from the surface. Maintain a consistent arc length equal to about one electrode diameter from the surface. 10) Use ER 4043, ER 5356 or other compatible alloy filler rod to add metal as needed. Store filler rods in a sealed container to minimize oxidation and possible weld contamination. When filling cracks, keep the filler rod in the weld zone so the shielding gas can protect it from oxidizing. Hold the filler rod at a 15 to 20° angle from the workpiece, creating a 90° angle between the filler rod and tungsten electrode. 11) Put the head back in the oven after welding, allow temperature to stabilize, then shut the oven off and allow the head to slowly cool for several hours back to room temperature. This will minimize the risk of thermal stress causing new cracks to form. 12) After machining the head and installing seats and guides, pressure test the head to check for leaks.
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