Effect of MIG Welding Process on Liquation Cracking in 6xxx Aluminum Alloys

Literature Overview

This paper, published in Light Alloy Fabrication Technology (2020, Vol. 48, No. 8, pp. 61-65), investigates the liquation cracking behavior in 6xxx series aluminum alloy extruded profiles welded by MIG process. The research was conducted by engineers at Liaoning Zhongwang Aluminum Deep Processing Co., Ltd., a major aluminum profile manufacturer. The study examines 2 mm wall thickness 6082 and 6061 aluminum alloy profiles, employing PT (penetrant testing), inverted metallurgical microscopy, universal testing machine, and direct reading spectrometer for comprehensive characterization.

Core Technical Content

6xxx Aluminum Alloy Characteristics

The 6xxx series aluminum alloys (6061, 6082) are Al-Mg-Si alloys that derive their strength from the precipitation hardening response of the Mg₂Si phase. Key welding-relevant characteristics include:

Precipitation hardenable (T6 temper available)
Moderate to high hot cracking susceptibility
Significant liquation cracking tendency in the HAZ
Strengthening primarily from Mg₂Si precipitates

Liquation Cracking Mechanism

The study provides a clear mechanistic explanation for liquation cracking in thin-wall 6xxx alloy profiles:

Pre-existing low-melting-point eutectics: The T6 temper base metal contains fine Mg₂Si precipitates at grain boundaries and within grains
Thermal cycle effect: During welding, the coarse grain zone (CGZ) of the HAZ is heated to temperatures where grain boundary eutectics partially melt
Liquid film formation: Molten low-melting-point material accumulates along grain boundaries, forming continuous liquid films
Stress-driven cracking: Welding thermal stresses and residual stresses act on these liquid films, causing intergranular cracking
Crack propagation: Cracks propagate along the weakened grain boundaries in the CGZ

Quantitative Analysis of Crack Susceptibility

Factor	Effect on Liquation Cracking	Mechanism
Yield-to-tensile ratio	Higher ratio increases susceptibility	Greater constraint stress during solidification
Wall thickness	Thinner walls increase susceptibility	Higher cooling rates, greater thermal stress gradients
Heat input	Higher heat input increases susceptibility	Larger CGZ, more extensive eutectic melting
Alloy composition	Higher Mg and Si content increases susceptibility	More low-melting eutectic phases

Process Optimization Strategies

The authors propose several effective countermeasures:

Reduce yield-to-tensile ratio: Select base metal in a lower temper condition (e.g., T4 or O temper) to reduce constraint stresses
Increase wall thickness: Thicker profiles reduce thermal stress gradients and lower cooling rates
Reduce heat input: Lower welding current, higher travel speed, or use of pulsed MIG to minimize CGZ extent

Test Results Summary

Test Method	Key Finding
PT (Penetrant Testing)	Surface liquation cracks detected on both 6082 and 6061 welds
Inverted Metallurgical Microscopy	Cracks located in CGZ of HAZ, intergranular morphology
Universal Testing Machine	Reduced tensile strength in cracked joints
Direct Reading Spectrometer	Confirmed alloy composition and filler metal dilution

Engineering Practice Integration

Welding Procedure Specification for 6xxx Thin-Wall Profiles

Parameter	Recommended Value	Rationale
Welding current	80-120 A (pulsed)	Minimize heat input while maintaining penetration
Travel speed	300-500 mm/min	Reduce thermal cycle duration
Wire feed speed	4-6 m/min	Balance deposition with heat input
Shielding gas	Pure Ar or 98% Ar + 2% O₂	Stable arc, reduced porosity
Preheat temperature	0-100°C	Reduce thermal gradients without excessive grain growth
Interpass temperature	Below 150°C	Prevent additional eutectic melting in previous HAZ

FMEA for Liquation Cracking Prevention

Potential Failure Mode	Severity	Occurrence	Detection	Mitigation
Liquation cracking in CGZ	9	6	4	Reduce heat input, optimize preheat
Hot cracking in weld metal	8	5	3	Select appropriate filler (ER4043/ER5356)
Insufficient penetration	6	4	5	Increase current, optimize technique
Porosity	5	3	4	Clean surfaces, maintain gas flow

Study Insights and Reflections

This work provides valuable practical guidance for welding thin-wall 6xxx aluminum alloy profiles, which are widely used in automotive, architectural, and transportation applications. The identification of the yield-to-tensile ratio as a key factor in liquation cracking susceptibility is particularly insightful, as it connects the base metal mechanical condition directly to weldability. This finding has implications for procurement and material specification: specifying profiles in a lower temper condition may significantly improve weldability at the cost of some base metal strength.

The emphasis on reducing heat input as a primary countermeasure aligns with the fundamental metallurgical understanding that liquation cracking is governed by the extent and temperature of eutectic melting in the CGZ. However, the challenge is balancing this with adequate penetration and weld quality, particularly for thin-wall profiles where excessive heat input causes burn-through and excessive heat input causes liquation cracking.

For production environments, the study highlights the importance of:

Process qualification through systematic parameter optimization
Non-destructive testing (PT) as a critical quality gate for detecting liquation cracks
Material condition control as a weldability factor, not merely a mechanical property specification
Welder training and technique consistency to maintain optimized parameters

The findings have direct relevance to current industry challenges in welding aluminum alloy profiles for lightweight structures, where reducing wall thickness for weight savings must be balanced against weldability constraints. The work demonstrates that even modest reductions in wall thickness can significantly increase liquation cracking susceptibility, requiring compensating adjustments in welding parameters and material condition.