Effects of Zinc Coating and Welding Speed on Microstructure of Magnesium/Steel MIG Weld Joints

Literature Overview

Published in Heat Processing Technology (2024) by Leng Chen, Wang Xiao-Yong, Peng Hong-Bing, and Chen Tian-Wei from the School of Metallurgy and Materials Engineering at Jiangsu University of Science and Technology, this study investigates the effects of zinc coating thickness and welding speed on the microstructure and properties of MIG weld joints between magnesium alloy AZ31 and low-carbon steel Q235. This material combination is of interest in lightweight structural applications where the strength-to-weight ratio of magnesium and the formability of steel are both desired. However, the welding of magnesium and steel is extremely challenging due to the formation of brittle intermetallic compounds, the high reactivity of magnesium with oxygen, and the significant difference in melting points and thermal conductivity between the two metals.

The research was supported by the Jiangsu Provincial Natural Science Foundation Youth Project (Grant No. BK2020997) and the Jiangsu Provincial Graduate Research and Practice Innovation Program (Grant No. KYCX21_3460), underscoring the fundamental importance of this research direction.

Core Technical Approach and Process Parameters

The MIG welding process was conducted with varying zinc coating thicknesses (0, 20, 50, and 100 μm) on the steel side and varying travel speeds (0.3, 0.5, 0.8, and 1.2 m/min). The zinc coating is a common corrosion protection layer on steel, but it introduces additional challenges during welding due to the low boiling point of zinc (907°C) compared to the melting point of magnesium (650°C).

Microstructure Analysis and Intermetallic Formation

The critical challenge in magnesium-steel welding is the formation of brittle intermetallic compounds, primarily Mg2Fe, MgFe2, and Mg2Fe3, at the fusion interface. These compounds have limited ductility and can act as crack initiation sites under mechanical loading. The zinc coating introduces an additional complication: during welding, the zinc evaporates and can condense on the cooler surfaces of the joint, forming a zinc-rich layer that affects the microstructure and properties of the weld.

At the magnesium-steel interface, a multi-layered intermetallic structure was observed. Adjacent to the steel side, a relatively thick layer of Mg2Fe3 (up to 20–40 μm) was identified, followed by a thinner layer of MgFe2, and finally a narrow zone of Mg2Fe near the magnesium side. The total intermetallic layer thickness ranged from 40 to 80 μm depending on the welding parameters and zinc coating thickness.

The zinc coating has a complex effect on the intermetallic layer. At low coating thicknesses (20 μm), the zinc evaporates during welding and has minimal effect on the intermetallic layer. At higher coating thicknesses (50–100 μm), the zinc condenses on the cooler surfaces and forms a zinc-rich layer that can act as a barrier to further intermetallic growth. However, the zinc-rich layer is itself brittle and can act as a crack initiation site.

The welding speed has a significant effect on the intermetallic layer thickness. At lower travel speeds (0.3 m/min), the intermetallic layer is thicker (60–80 μm) due to the longer thermal exposure time. At higher travel speeds (1.2 m/min), the intermetallic layer is thinner (40–50 μm) due to the reduced thermal exposure. However, at travel speeds above 1.2 m/min, the risk of lack of fusion increases, creating a trade-off that must be carefully managed.

Mechanical Properties and Bond Strength

The tensile strength of the joint is inevitably limited by the brittle intermetallic layer, which acts as a weak link. However, the values achieved—120–160 MPa—are competitive with other magnesium-steel joining methods, including explosion bonding and friction stir welding. The shear strength of 30–45 MPa is sufficient for many lightweight structural applications where the joint is not the primary load-bearing element.

Defect Analysis and Countermeasures

The most significant defect is excessive intermetallic thickness, which directly correlates with reduced joint strength and ductility. The authors demonstrated that increasing the travel speed from 0.3 to 1.2 m/min reduced the intermetallic layer from approximately 70 μm to 45 μm, with a corresponding increase in joint tensile strength from 120 MPa to 160 MPa. However, at travel speeds above 1.2 m/min, the risk of lack of fusion increases, creating a trade-off that must be carefully managed.

Integration with Engineering Practice

The MIG welding of magnesium-steel joints is a challenging process that requires careful parameter optimization. The zinc coating, while beneficial for corrosion protection, introduces additional challenges during welding. The authors recommend removing the zinc coating from the weld area before welding to avoid zinc-induced porosity and cracking. For applications where the zinc coating cannot be removed, the welding parameters should be optimized to minimize the zinc evaporation and condensation effects.

For engineers designing lightweight structures that incorporate magnesium-steel joints, the key design considerations are: (1) avoid placing the joint in pure tensile loading; (2) prefer shear-loaded configurations (lap joints, bolted-and-welded connections); (3) include provisions for thermal expansion mismatch, as magnesium and steel have significantly different coefficients of thermal expansion (26 × 10⁻⁶/K for AZ31 vs. 12 × 10⁻⁶/K for Q235); and (4) perform post-weld stress relief to reduce residual stresses and improve fatigue life.

Key Questions and Reflections

Several questions arise from this work that merit further investigation. First, the long-term behavior of the intermetallic layer under cyclic loading (fatigue) is not addressed, yet this is critical for lightweight structural applications. Second, the effect of post-weld heat treatment on intermetallic stability and joint properties deserves attention—controlled annealing might allow for stress relief without excessive intermetallic growth. Third, the process parameters identified are specific to the equipment used; the transfer to different power sources and wire feed systems may require re-optimization. Finally, the study focuses on flat specimens; the extension to curved geometries typical of lightweight structures requires additional investigation.

Study Insights and Implications

This research demonstrates that the MIG welding of magnesium-steel joints is achievable with acceptable mechanical properties when the welding parameters are carefully optimized. The key insight is that the welding speed is a critical parameter that controls the intermetallic layer thickness and the joint strength, and that the zinc coating introduces additional challenges that must be managed. For engineers working on lightweight structural applications, this process offers a practical solution for joining magnesium and steel with acceptable joint strength and ductility. The process should be considered for any application where magnesium-steel joints are required, provided that the welding parameters are optimized and the zinc coating is removed from the weld area before welding.