JOINING EXOTIC METALS

NASCAR purists prefer that the sport remain focused on the skills of the drivers and less on the advancement of technology. In an effort to maintain a level playing field, many contend that emerging technologies be left to racing circuits that feature the cars.

Ironically, the move to further improve NASCAR’s safety has ushered in a host of new technologies, many of which unintentionally also boost performance. Where to draw the line, however, is a debate likely to continue for years. All of that aside, NASCAR strictly regulates which materials can be used for each specific component. Balancing safety and performance, technology unavoidably advances and follows closely in the more advanced footsteps of Formula One racing and the aerospace industry.

But while the issue remains in dispute, race team fabricators nonetheless work to stay ahead of approaching technologies. On the forefront of the latest debate are materials such as Inconel 625 and chromoly 4130. Both are finding their way onto more racing “stockcars” every day, as the sport’s ruling body sees the need and then grants permission.

Over the past few years, the trend in materials has moved many NASCAR components from carbon steel to stainless steel. Today that trend is shifting again, from stainless steel to Inconel. Many race shop welders are trying their hand at titanium in anticipation that it, too, will be allowed throughout the cars. Titanium is now limited only to certain engine parts that are rarely welded, but race shop welders foresee a widening use that could be only a few years away.

NEW MATERIALS
Inconel is a trademarked name for a family of high-strength nickel, chromium and iron alloys that have exceptional anti-corrosion and heat resistant properties. This nickel-based “super alloy” is, in many ways, a vast improvement over stainless steel.

Inconel has been used in a variety of hi-tech applications, including military vehicle exhaust ducts, submarine propulsion motors, underwater cables, heat exchangers and gas turbine shroud rings. The material also has been used in Formula One and Champ Car exhaust systems for years. But more recently, several Winston Cup racing teams have incorporated Iconel 625 into ultra-light and highly durable exhaust systems and headers.

Race teams continuously push the limits of ground clearance restrictions in an effort to lower the car’s center of gravity and increase aerodynamics and handling. In doing so, they often abuse the vehicle’s undercarriage exhaust systems that sometimes scrape along the track during hard turns and throw a plume of sparks behind the car. Because of this, teams are sometimes forced to repair or even replace exhaust systems. Using a much stronger material such as Inconel can extend the life of exhausts and limit needed repairs. As an added benefit, Inconel’s ability to resist vibration and cycle heat through exhaust tubing has actually helped to increase the horsepower in some cars.

Chromoly is an abbreviation for chromium-molybdenum steel. It comprises a range of low-alloy steels that have been used for things such as bicycle frames and racecar roll cages. It is not as lightweight as aluminum alloy, but it has the advantages of high-tensile strength and malleability. It is easily welded, considerably stronger and more durable that standard 1020 steel tubing. Chromoly has found its way onto many NASCAR suspension systems and chassis extensions, where heavier, weaker carbon steel was previously used. Its use is growing wider all the time.

Titanium is a metallic chemical element that is strong, lustrous and corrosion resistant. It is used in strong, light-weight alloys, most notably with iron and aluminium. NASCAR currently only allows titanium for a limited number of engine parts, but racing teams anticipate that wider use is not far off. Titanium is noted for its high strength-to-weight ratio. It is a light, strong metal with low density, which when pure is quite ductile, especially in an oxygen-free environment, and it is relatively easy to work. Commercially pure grades of titanium have a tensile strength equal to that of high strength low-alloy steels, but are 43 percent lighter. Titanium is 60 percent heavier than aluminium, but more than twice as strong as 6061-T6 aluminum alloy — making it ideal for certain racecar components.

WELDING
The massive steel cage-like chasses of NASCAR vehicles are made of carbon steel and are generally welded using GMAW. But nearly all of the cars’ components are meticulously GTAW welded. Weight-to-strength ratios are a primary concern in racecar welding. So the goal for both GMAW and GTAW welding is to create a strong, deeply penetrating weld, while minimizing the use of filler metal. An ideal racecar weld is flush to the material being joined. Bulging beads generally are unacceptable.

Welding chromoly is very similar to welding carbon steel. What racecar welders have come to learn in the past five years or so since they started using this alloy is that industrial standards for chromoly generally do not apply to thinner gauges of the metal typically used on racecars. Textbook explanations often assume that the material is thicker than .120 in, and they recommend preheating chromoly 300 deg F to 400 deg F. But because racecars use a thinner material, preheating is unnecessary.

Removing oxygen from the inside of an Inconel tube and replacing it with argon, or “purging,” is recommended but yet unproven to yield improved results for chromoly. Some top race welders purge the air when welding chromoly, while others do not. The difference is virtually indistinguishable.

TOP TEN FACTS ABOUT TIG WELDING CHROMOLY 4130
Yes, you can TIG weld 4130 tubing up to .120 in wall thickness easily with the techniques and procedures described below. These procedures apply to typical sporting applications such as experimental airplanes, racecar frames, roll cages, go-carts, bicycles and motorcycle frames. The suitability of these techniques and procedures must be evaluated for your specific application.

Q. Can I weld 4130 using the TIG process?
A. Yes, 4130 chromoly has been TIG welded in the aerospace and aircraft industries for years. As with all welding, proper procedures and techniques must be followed.

Q. Do I need to pre-heat?
A. Thin wall tubing less than .120 in applications do not typically require the normal 300 deg F to 400 deg F pre-heat to obtain acceptable results. However, tubing should be around 70 deg F or above before welding.

Q. What filler material should I use?
A. Although there are several good filler materials, ER80S-D2 is one you should consider. This material is capable of producing welds that approximate the strength of 4130. ER-70S-2 is an acceptable alternative to ER80S-D2, as is ER70S-6, although weld strength will be slightly lower.

Q. When I use ER70S-2 filler material, do I sacrifice strength for elongation?
A. Yes. The filler material, when diluted with the parent material, will typically undermatch the 4130. However, with the proper joint design, such as cluster or gusset, the cross-sectional area and linear inches of weld can compensate for the reduced weld deposit strength.

Q. Why is 4130 filler metal not recommended?
A. 4130 filler typically is used for applications where the weld will be heat-treated. Due to its higher hardness and reduced elongation, it is not recommended for sporting applications such as experimental airplanes, racecar frames and roll cages.

Q. Can I weld 4130 using any other filler materials?
A. Some fabricators prefer austenitic stainless steel fillers to weld 4130 tubing. This is acceptable provided 310 or 312 stainless steel fillers are used. Other stainless steel fillers can cause cracking. Stainless steel filler material is typically more expensive.

Q. Do I need to heat treat (stress relieve) 4130 after welding?
A. Thin wall tubing normally does not require stress relief. For parts thicker than .120 in, stress-relieving is recommended. 1,100 deg F is the optimum temperature for tubing applications. An oxy/acetylene torch with neutral flame can be used. It should be oscillated to avoid hot spots.

Q. Do I need to pre-clean 4130 material?
A. Remove surface scale and oils with mild abrasives and acetone. Wipe to remove all oils and lubricants. All burrs should be removed with a hand-scraper or de-burring tool. Better welding results from clean materials.

Q. Do I need to back-purge 4130 material?
A. Backpurging is not normally necessary, although some fabricators do. It will not hurt the weld and may actually improve the root pass of some welds.

Q. Should I quench the metal after I finish welding?
A. Absolutely not. Rapid quenching of the metal will create problems such as cracking and lamellar tearing. Always allow the weld to slow cool.

INSIGHTS INTO INCONEL AND TITANIUM
Inconel 625 is a more difficult material to weld than chromoly or standard steel. The puddle does not reach the same level of fluidity during the weld as carbon steel. The puddle is more difficult to manipulate, and it is harder to maneuver across gaps. Because of this, certain precautions should be followed. Proper shielding on the top and bottom of the welds in any nickel-based alloy is very important to avoid porosity and reduced ductility. An Inconel tube must be purged of its inside air and replaced with argon before welding.

Tight fitup is essential. Gaps require more welding manipulation and create increased chance for error. Because of this, race welders use more tacks to further improve fit up. Clean the material more thoroughly to avoid contamination, porosity and subsequent cracking. Winston cup welders use 100 percent argon shielding and ERNiCrMo-3 filler metal (AWS specification of A5.14) for Inconel 625.

Today, many NASCAR teams either buy or build Inconel headers and then weld them to their cars. Regardless of the arrangement, all teams that use Inconel headers need to weld them in some way ? whether to simply to install them or to repair or modify them. An Inconel header can cost $5,000 to $7,000, so it is important that the work is done correctly. However, the thinness of the material ? generally about .035 in or smaller ? allows welders just one pass, and that weld must be done correctly. There are no subsequent passes that can compensate for inferior workmanship. Welds on racecars are not cut out and reworked. Everything must be perfect.

Nearly pure titanium and most titanium alloys can be welded in a number of processes, but for racecars, GTAW will be preferred. Welding titanium is similar to welding stainless steels and nickel alloys, such as Inconel. Welding Titanium requires thorough surface cleaning and the correct use of shielding gas surrounding the entire weld. The puddle will have low fluidity, and molten titanium reacts readily with oxygen, nitrogen and hydrogen.

Exposure to these elements or surface contaminants can adversely affect the weld and lead to cracking. Heat-affected areas must be shielded until temperatures dip below 800 deg F. Shielding a titanium puddle with 100 percent argon is preferred, but some argon/helium mixtures are also effective. Adding to the debate of using titanium on racecars is the fact it cannot be welded to most other metals because of formation of brittle metallic compounds that lead to cracking.

WELDING TECHNOLOGY
Many NASCAR rules were designed to help race teams maintain relatively low operating costs. The average NASCAR car today costs roughly $185,000, which is about the same price for the transmission on a Formula One car. The idea of maintaining costs fits well with the notion of advancing technology as the need for safety dictates, but it is also mindful of the opinion that stockcar racing should not be a sport of dollars. Introducing new materials such as titanium and Inconel might have been avoided in the past in part because welding such materials was difficult and required very skilled craftsman, additional time and more money.

Today, however, welding these new materials has become considerably more commonplace with recent advancements in welding equipment, furthering NASCAR’s likelihood to allow them on racecars. In particular, inverter-based welding technology has become widely used because it allows operators to set software-driven welding parameters to a specific material, such as Inconel or titanium.

Inverter-based welding power sources operate at frequencies above 20 kHz, versus traditional power sources that operate at a line frequency of 50 Hz or 60 Hz. Some of the advantages an inverter has are smaller magnetic components, such as chokes and transformers, a higher efficiency and a faster response to the welding arc. Inverter power sources were first introduced to the industry in the early 1980s. The initial attraction was their small size and portability. Now inverters are designed for many different processes, including SMAW, GTAW, FCAW and SAW welding.

For race welders, inverters provide optimum arc characteristics for very specific processes, such as welding thin Inconel ? an uncommon practice most other places. Inverters are software programmable so that operators can manipulate the output weld characteristics. Certain inverters use an embedded software program that provides the ability to customize waveform output. Operators can choose from a predefined set of programs and manipulate the parameters of that program to best fit a given application.

WINNING WELDS
New welding technology fits well with racecar fabricators who relentlessly seek new ways to improve the quality of their work and innovate different techniques unique to high-performance vehicles. For the vast majority of NASCAR welders, safety is the absolute predominant concern, knowing that their welds must hold up to 200 mph turns, G-forces and collisions. Drivers’ lives are on the line every time they climb into one the many cars built entirely by hand.

But the weld shops are just as competitive as the drivers and pit crews who run the races each Sunday. They weld to win, and as safety drives the technological advancement of NASCAR, it is certain that performance will follow close behind.

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Dennis Klingman is a manager of technical training for the Lincoln Electric Company , 22800 Saint Clair Avenue, Cleveland, OH 44117-8542, www.lincolnelectric.com . He teaches an annual class on advanced motorsport welding to some of the top teams in NASCAR, including Joe Gibbs Racing, Ganassi Racing and Penske Racing.