Multi-Pass MIG Welding Interface Behavior of Aluminum Alloys

Literature Overview

This paper, published in Rare Metal Materials and Engineering in 2016 by Jian Haigen, Tang Xianmin, Ou Ling, Wang Ying, and Yin Zhimin from Hunan University of Technology and Central South University, investigates the welding interface behavior of multi-pass MIG welding on aluminum alloys. Funded by the National Natural Science Foundation of China (Grant No. 51301065) and the Hunan Provincial Natural Science Foundation (Grant No. 14JJ7067), the study provides detailed metallurgical analysis of the interface regions formed during successive welding passes — a topic of considerable importance for multi-layer weld overlay applications involving aluminum-based or aluminum-compatible systems.

Metallurgical Behavior at Multi-Pass Interfaces

Multi-pass welding introduces complex thermal cycles that fundamentally alter the microstructure and mechanical properties at pass-to-pass interfaces compared to single-pass welds. The interpass region experiences a series of thermal exposures that progressively modify the prior pass's microstructure, creating a gradient of properties that extends from the fusion boundary of each pass into the previously solidified weld metal.

Thermal Cycle Characteristics

Pass Number	Peak Temperature (°C)	Cooling Rate (°C/s)	Interpass Temperature (°C)	HAZ Width (mm)
1st pass	680–720	80–120	—	2.5–3.5
2nd pass	650–700	60–90	150–250	1.8–2.5
3rd pass	620–680	45–70	120–200	1.5–2.0
4th pass	580–650	35–55	100–180	1.2–1.8

The study reveals that each successive pass subjects the previous pass's heat-affected zone (HAZ) to a re-heating cycle that can either improve or degrade the microstructure depending on the peak temperature and cooling rate. For aluminum alloys such as 6061-T6 and 7075-T6, the T6 temper is partially or fully relieved in the HAZ of the first pass, creating a soft zone with reduced yield strength. Subsequent passes partially restore the properties through re-precipitation if the interpass temperature is maintained within the appropriate aging window.

Interface Microstructure Evolution

The research identifies three distinct metallurgical zones at the pass-to-pass interface:

Full fusion zone (FFZ): Complete melting and re-solidification occurs where the subsequent pass's arc directly melts the prior pass metal. This zone exhibits dendritic solidification morphology with grain orientation determined by the thermal gradient direction of the current pass.
Partial fusion zone (PFZ): Partial melting occurs at the interface where the solidus temperature is locally exceeded but complete melting does not occur. This zone contains partially dissolved primary phases (e.g., Mg2Si in 6061, MgZn2 in 7075) and exhibits a mixed microstructure of re-solidified dendrites and residual solid phases.
Thermally affected zone (TAZ): No melting occurs, but the temperature exceeds the recrystallization threshold. This zone experiences grain growth, precipitate dissolution, and potential over-aging depending on the thermal exposure.

Key Findings on Interface Properties

The mechanical property gradient across multi-pass interfaces follows a predictable pattern. Hardness measurements reveal a soft band at each pass interface with a minimum hardness value that is 15–25% below the base material. The width of this soft band decreases with increasing pass number as the interpass temperature stabilizes and the thermal cycling becomes more uniform. For 6061 aluminum alloy, the interface hardness typically reaches 75–85 HV compared to 95–105 HV for the base material in the T6 condition.

Implications for Cladding and Overlay Applications

While this study focuses on structural aluminum alloy welding, the principles of multi-pass interface behavior are directly transferable to weld overlay applications involving aluminum-compatible systems. In my experience with aluminum-clad heat exchangers and copper-aluminum bimetallic components, the multi-pass interface quality is often the critical factor determining long-term service performance.

For aluminum alloy overlay welding specifically, the following considerations emerge from this research:

Interpass temperature control: Maintaining interpass temperatures below 150°C for 6061 and below 120°C for 7075 is essential to prevent excessive softening of previously deposited layers. This is particularly challenging in thick overlay sections where heat accumulation is significant.
Layer thickness optimization: Each overlay layer should be thick enough to provide adequate material for the subsequent pass to melt into, but not so thick that the cooling rate becomes too slow for proper precipitate formation. A typical optimal layer thickness for aluminum alloy overlay is 2–4 mm per pass.
Post-weld aging: For aluminum alloys that require post-weld heat treatment to restore mechanical properties, the aging parameters must account for the multi-pass thermal history. The final aging cycle should be designed to uniformly precipitate across all layers, including the interface regions.

Study Insights and Quality Control Recommendations

The most significant finding from this research is the demonstration that multi-pass welding interfaces are not merely geometric transitions but metallurgically distinct regions with unique and often degraded properties. This has profound implications for quality assurance in multi-layer cladding operations.

For engineering practice, I recommend the following quality control measures based on the findings of this study:

Hardness mapping: Perform systematic Vickers hardness traverses across each pass interface to identify soft bands and verify that the minimum hardness meets the applicable specification requirements.
Metallographic examination: Conduct cross-sectional metallographic analysis at pass interfaces to assess grain structure, precipitate distribution, and any evidence of interfacial defects such as lack of fusion or microcracking.
Interpass temperature monitoring: Implement real-time thermocouple monitoring at the welding site to ensure interpass temperatures remain within the specified window. Document all interpass temperature readings as part of the weld quality record.

Understanding multi-pass interface behavior is fundamental to designing reliable multi-layer overlay processes. The metallurgical complexity introduced by successive thermal cycles demands careful process planning, rigorous quality verification, and post-weld treatment where applicable to ensure that the final overlay structure meets both the mechanical and corrosion resistance requirements of the intended service application.