This article is the conclusion of Welding Digest's three-part series on aluminum welding. Be sure to read Part 1 about aluminum's thermal and reactive properties, and Part 2 on melting and solidification.

Aluminum alloys are softer and weaker compared to steel and ferrous alloys. Lower strength of aluminum means cracks can open up at lower stresses compared to steel and ferrous alloys.

Aluminum alloys are strengthened differently than steel alloys. Aluminum alloys do not transition through a crystal or phase change, unlike carbon and alloy steels, which transform from austenitic to ferritic structures. Cold work or work hardening and precipitation hardening are the prevalent aluminum alloy strengthening mechanisms.

Figure 1: Ultimate tensile strength of various weldable aluminum alloys at elevated temperatures. Source: IEEE GS - Data from ASM Handbook
Aluminum alloys dramatically drop in strength at elevated temperatures, so the metal in the weld and heat-affected zone (HAZ) is inherently weaker. At 315° C, 6061 T6 aluminum retains only 10% of its room temperature tensile strength. In contrast, carbon steels are used in continuous service up to 370° C (per ASME BPVC) and certain austenitic stainless steels are used in continuous service up to 1150° C. Even when the metal in the weld and surrounding region is returned back to room temperature, the room temperature strength will be reduced due to annealing out of work hardening strains or overaging. Post heat treatment of a welded aluminum assembly can be used to increase strength of precipitation strengthened grades. The drawback is the risk of inducing more distortion when residual stresses are relieved during heat treatment.

Figure 2: Elevated temperature strength of 6061 aluminum and thermal stress in constrained workpiece. Source: IEEE GS

The thermal expansion coefficient (CTE, α) of aluminum is twice the CTE of steel, which means for workpiece with the same length (L) and temperature change ΔT, aluminum will have double the thermal strain [ΔL = αL(ΔT)] compared to steel. On the plus side, aluminum’s lower elastic modulus results in lower thermal stress [σ = α(ΔT)E] for the same temperature change. If the temperature change is high enough in a constrained part, then the thermal stress can exceed the ultimate tensile strength (UTS) resulting in cracking. Parts over constrained through fixturing and clamping can induce high thermal stress from arc heating, which can translate into high residual stresses stress on cooling. High residual stresses can result in cracking along the length of the weld bead (longitudinal cracking). The occurrence of longitudinal cracking can be reduced by increasing the weld bead size, avoiding constraints and preheating to reduce the ΔT and by reducing overall heat input.

Conclusion

The specific properties of aluminum create obstacles to forming good welds compared to steel such as surface oxide characteristics, the solubility of hydrogen in molten aluminum, solidification shrinkage, thermal expansion coefficient, low electrical resistivity, wide melting range of alloyed aluminum, low elevated temperature strength and high thermal conductivity.

Figure 3: Material properties of aluminum and steel influence welding processes. Source: AWS, NIST, NASA, ASM International, and Other References

While aluminum welding or joining can be difficult, many of these obstacles can be overcome to produce high integrity aluminum welds through application of:

• Proper workpiece preparation and cleaning methods
• Suitable welding equipment (square wave or AC GTAW; pulsed AC, reciprocating wire or short circuit gas metal arc welding [GMAW])
• Appropriate filler alloys selection for base metal and process
• Qualified aluminum welding procedure specifications (WPS)
• Compliance with applicable AWS, ASME, AMS and other codes and specifications (see AWS’s aluminum codes and specifications here)
• Aluminum welding qualified welders, weld inspectors and welding engineers
• Post treatments to enhance aluminum weld fatigue strength (toe grinding, shot peening)

While aluminum welding can be difficult, aluminum welds are indispensable in aerospace, automotive, ship and boat building, and electrical power distribution industries. For example, according to the Aluminum Association, welding is the preferred method of joining permanent connections in aluminum bus bars. Bolted joints will have inherent contact resistance whereas a welded joint maintain bulk conductivity. In addition, the contact resistance in bolted joints can increase over time due to thermal expansion and creep-induced loosening, as well as fretting wear.