Challenges and considerations in joining exotic materials
The welding itself is only half the story
Every day Voss Aerospace faces challenges that vary as much as the materials its welders join and fabrication processes they use.
The company fusion- and resistance-welds 300 and 400 Series stainless steel, CRES A286, nickel alloys 625 and 718, titanium, and aluminum to produce sheet metal and machined V-retainer couplings, mated flanges, band clamps, and strap assemblies.
With expertise in seven metal groups in fusion welding—including gas metal arc welding (GMAW) and gas tungsten arc welding (GTAW)—and three metal groups in resistance welding exotic materials, the company fabricates products used in intelligent bombs and missiles; tanks, tactical vehicles, and troop carriers; tactical bombers, jet fighters, and helicopters; and construction equipment.
But to fabricate these products, the company has to perform other processes too—weld preparation, fixturing, documentation, accreditation, and quality control.
"Fusion welding success with these materials requires very systematic procedures," said Michael Schleckman, the company's quality manager. "The fact that we are working with different materials at the same time may seem chaotic, but it is not. We have procedures that govern how we process every material and each order. Requests for quotation are reviewed to make sure they match our capabilities. If they do, we prepare the proper weld procedure in advance of making the quote. The key is your advance planning. If you don't do it, then no matter how good you are, success will be elusive."
The company's fusion welders must have military and aerospace certifications for all three weld classes. Most customers don't specify a weld class unless it's a critical part. In such instances, a B (semicritical) or C (noncritical) weld class is assigned based on the company's understanding of the part's intended use. Aerospace Class B indicates a semicritical application in which a failure of the weldment would cause reduced efficiency of the system but not cause system loss or personnel endangerment.
Critical Aerospace Class A welds are specified whereby a weldment's failure would cause system loss, loss of a major component, loss of control, unintentional release of critical stores, or personnel endangerment.
Based on the assigned weld class, a weld procedure specification (WPS) is prepared. The WPS is an integral part of the permanent work documentation. For military and aerospace work, everything must be documented and traceable. This includes the raw material source, weld rod type and source, welding gas used to shield oxygen, and the type of trailing gas cup used to purge the area. Such shielding is important in welding titanium and nickel and nickel-based alloys to prevent absorption of oxygen. Additionally, such shielding is important to protect the welder from inhaling gases produced by the welding process.
For all military and aerospace work, sample parts are made to check and qualify the weld procedure. To qualify the weld procedure for Class A critical welds, the sample parts must be checked and tested by an independent lab. On acceptance, all preparations to weld that component are set up and documented.
Layout of the welding bench also is a part of the procedures that govern the fusion welding process for a given order. The welding wire used is just as important as the metal being welded. The procedure ensures that only the correct weld wire is allowed on the bench.
Aerospace and military fusion welding standards require the welder to have and maintain the appropriate weld class certifications. In addition to continuing education, recertification is required every five years.
"To successfully secure these certifications for exotic materials requires considerable tutoring, practice, and patience as the skill level for exotic materials is both high and demanding," said Wayne Wicks, chief welder. Requalification also is required when a welder has not welded with a given process for three months, or if there is a specific reason to question the ability of a welder to meet the requirements for qualification in a given process.
Resistance welding qualification differs from fusion welding qualification. With RW it's a matter of first qualifying the welding machine. Once the machine is qualified, different operators can weld parts without the need for individual operator certifications. Cross-section weld samples that are polished and acid-etched are used in weld quality checks. Weld quality is determined using a combination of micro- and macromagnification inspections to examine the weld nugget.
Metals and Their Differences
Like the processes used to join various metals, the materials themselves present an assortment of challenges that require knowledge and skill when welding them.
Titanium. Cleanness, coverage, and contamination are constant issues with titanium. In addition, welders have a four-hour start-to-finish working time frame to prepare, clean, and weld a titanium part. After four hours, air causes a hard oxide film to form on the outside surface. To weld titanium with a hard oxide surface requires a higher melting temperature to burn it off, with the potential of oxide particles becoming contaminants in the weld joint. Simply, titanium has to be perfectly clean before welding. To clean titanium and remove oxides and lubricants, a flash nitric hydrochloric acid etch is used with a water rinse.
Chlorine-based cleaning solvents cannot be used with titanium because the material is like a sponge. It quickly absorbs the chlorine, which can cause material cracking and potential failure down the road. Also, because of its chlorine content, the wash water cannot be city water. The water has to be filtered to ensure absolute purity and cleanness.
In welding titanium shapes, it's important to file or machine the sheared edges to remove material that may have become embedded. Filing or machining also provides a straight weld to edge.
Aluminum. Because hard film oxides form on the surface, aluminum also requires a special cleaning procedure.
Cleaning can be done using chemicals or stainless steel wire brushing. Welding aluminum that has mass or weight can cause heat to dissipate from the joint. Precautions are necessary to help reduce that effect, including preheating the heavy sections or using a different shielding gas.
Nickel and Nickel-based Alloys. Nickel and nickel-based alloys present a different challenge: The weld puddle is sluggish.
To compensate, different gases need to be used to increase the heat at the joint. Some nickel-based alloys require close attention to weld settings. Alloys that are crack-sensitive may require additional filler wire to the settings. This reinforces the weld joint as it cools and can help stop cracks from forming.
Stainless Steel. The stainless steel (Group IIa) puddle has to be protected from the atmosphere during welding. Chromium oxide is a refractory material that melts well above the melting point of stainless. The back and the joint should be shielded with the proper gas to keep oxides from forming.
GTAW frequently is used to join precipitation-hardened stainless steels (Group IIb). Parts generally are less than 0.250 inch thick. Argon usually is the shielding gas. PH stainless steel is similar to conventional stainless because both require shielding gas at the back and root of the joint during welding.
Accreditation is another consideration important for welding exotic materials. Accreditations from the National Aerospace & Defense Contractors Association Program (NADCAP) are the standards for the aerospace industry and apply to industry at large as well.
One such NADCAP accreditation is AS 7110 for welding and brazing. The audit for welding and brazing was developed by a task group of experts in the field from each of the member companies that previously conducted their own on-site audits. While it has an aerospace standard (AS) designation, it does incorporate all of the military and special customer requirements for welding and brazing.
AS 7110 is a check list, not a weld procedure. It's a thorough audit of a company's welding and brazing capabilities. For the first five years, it's an annual audit. After five audits, the length between audits is extended to 18 months.
Currently, the NADCAP accreditation is a U.S. requirement. However, it is expected that it will be global in scope in a matter of time simply because European companies that make parts for U.S. industry now have to meet the NADCAP requirements.
Voss Aerospace, a division of Voss Industries, 2168 W. 25th St., Cleveland, OH 44113-4172, 216-771-7655, fax 216-771-2887, www.vossind.com.
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