How to Control Distortion in Thick Aluminum Welding

Aerospace, shipbuilding, rail transit, special vehicles, and large equipment increasingly require lightweight structures with high strength, long life, and corrosion resistance. Aluminum alloys are important structural materials because they combine high strength, low weight, good ductility, and good corrosion resistance.

U.S. research on high-strength aluminum began in the 1940s. In 1956, strain-hardened 5083 Al-Mg-Mn plate was trial-produced. Because its combined performance was limited in some high-strength applications, heat-treatable 7039 Al-Zn-Mg alloy followed in the early 1960s. Its strong susceptibility to stress-corrosion cracking, however, made prolonged seawater or salt-spray exposure unsuitable. During the 1980s, the United States, the former Soviet Union, and others developed a new generation of high-strength alloys. The U.S.-developed 2519-T87 Al-Cu alloy offers high strength, balanced mechanical properties, stress-corrosion resistance, and weldability.

New-generation high-strength alloys, particularly Al-Cu 2xxx alloys, provide greater structural strength and improved stress-corrosion resistance for large structures in demanding environments. Welding still causes joint softening and a low joint-strength coefficient. Large precision structures additionally require weld distortion to be held below one-third of that produced by conventional welding.

Product and Process Challenges

The principal difficulties are severe joint softening and a low strength coefficient; a refractory surface film of Al₂O₃ with a 2,060°C melting point; the high energy input required by thick high-strength material; susceptibility to hot cracks and pores; and distortion caused by high thermal conductivity and a large coefficient of linear expansion. Distortion becomes especially difficult to control above 20 mm thickness, so measures must be technically sound and practical for shop-floor production.

Distortion-Control Measures

1. Select the Welding Process

Thick aluminum plate conducts heat rapidly and favors a process with high power density. Tests of single- and twin-wire MIG, electron-beam welding, friction-stir welding, and laser-MIG hybrid welding identified twin-wire welding as the production process.

Single-wire MIG used low-frequency compound pulses. Pool vibration intensified stirring, promoted heterogeneous nucleation during solidification, refined the weld structure, and improved strength and ductility through electromagnetic stirring. The joint met or exceeded the tensile target. Tests established a favorable combination of travel speed, current, and low-frequency modulation pulse.

High-speed twin-wire welding produced a high-strength joint. Comparison of high-current two-pass and low-current four-pass twin-wire MIG led to high-current multipass parameters combining their advantages. Joint tensile strength was 296 MPa, or 62% of the base metal, and impact toughness was 13 J/cm², or 42% of the base metal.

Three approaches were compared: conventional welding with a Y groove, welding with an X groove, and vibration welding with an X groove. The X groove reduced distortion to one-fifth of the conventional result, while vibration welding reduced it still further.

2. Calculate and Optimize the Welding Sequence

For a large structure with many seams, number each weld and define an initial sequence from the structural features. A structure with 16 principal welds, for example, initially used 1, 3, 4, 2, 5, 6, 7, …, 16. Finite-element analysis can compare candidate sequences and identify one with lower distortion and a more favorable stress distribution.

First analyze the pass sequence within an individual multipass seam, because it changes both the magnitude and distribution of residual stress. Establishing a low-stress pass sequence before shop welding helps limit the final residual stress.

Then optimize the overall order of the seams. Changing the opening sequence from 1, 3, 4, 2 to 1, 3, 2, 4 substantially altered the stress distribution. The second sequence reduced the area above 300 MPa, which can improve resistance to stress-corrosion cracking.

3. Use Vibratory Stress Relief to Stabilize Stress and Dimensions

Dynamic stress is a key parameter in vibratory stress relief. The exciter applies a dynamic stress corresponding to cyclic alternating load. When this stress combines with the original residual stress and causes local or overall plastic deformation, residual stress can relax, become more uniform, and decrease, while the metal matrix becomes more resistant to deformation.

The general criteria are σ_dynamic + σ_residual ≥ σ_s, where σ_s is the yield stress required for plastic deformation to propagate and produce macroscopic deformation; σ_dynamic/σ_residual = 0.1–0.3; and the dynamic-stress amplitude should reach one-third to two-thirds of the component’s working stress. Within a suitable range, greater dynamic stress releases more strain and relieves stress more effectively. Excessive dynamic stress can damage the workpiece or reduce fatigue life.

Tests used a WZ-86A vibratory stress-relief unit manufactured by Hailun Vibratory Stress Relief Equipment Co., Ltd. The process held weld residual stress within 0.3σ_s.

The trials established and implemented the product-specific vibratory stress-relief parameters.

Control Result

A production-compatible welding process, comparative trials of groove forms, parameters and sequences, and vibratory stress relief reduced residual stress and distortion without sacrificing mechanical or corrosion performance. The optimized process held distortion within one-third of the conventional welding result, meeting design and manufacturing requirements for large high-strength structures.

In addition to aerospace, ships, rail transit, and large machinery, the method can support selected special vehicles, lightweight protective structures, and other high-strength aluminum components.