Comparative Mechanical Properties of Dissimilar Steel LMHW and MIG Welded Joints

Literature Overview

This 2019 study published in the Transactions of the China Welding Institute by Zhou Shujun and colleagues from Nanjing University of Aeronautics and Astronautics examines the mechanical properties of dissimilar steel joints produced using two distinct welding methods: low-melting-point hot wire (LMHW) welding and conventional metal inert gas (MIG) welding. The research was supported by a Jiangsu Provincial Science and Technology Project, and the industrial collaboration with Yangzhou Dongsheng Automotive Parts Co., Ltd. underscores its practical orientation toward automotive structural component fabrication.

Core Technical Content

Dissimilar steel welding presents unique metallurgical challenges when joining steels with different carbon equivalents, hardenability, and thermal expansion characteristics. The LMHW process is a specialized variant of hot-wire TIG (HW-TIG) or hot-wire MIG welding that employs a filler wire with a lower melting point than the base metals. This technique was originally developed for joining dissimilar metals such as aluminum to steel, but its application to dissimilar steel combinations represents a relatively novel approach.

The fundamental principle of LMHW is that the low-melting-point filler wire melts preferentially under the heat input, creating a dilution-resistant bond zone while minimizing the formation of hard and brittle phases in the heat-affected zone (HAZ). In contrast, conventional MIG welding relies on the melting of both base metals and filler to form the weld metal, which can lead to significant dilution and the formation of martensitic phases in high-carbon or high-alloy steels.

Metallurgical Analysis and Process Comparison

The metallographic examination of LMHW joints typically reveals a distinct three-layer structure: base metal, a thin bond layer composed primarily of the low-melting-point filler, and the weld metal cap. The bond layer thickness in LMHW joints is generally in the range of 0.1–0.5 mm, which is thin enough to maintain ductility but thick enough to ensure reliable metallurgical bonding. The microstructure of this bond layer is predominantly fine-grained ferrite with dispersed carbides, avoiding the formation of coarse acicular ferrite or martensite that would compromise toughness.

In MIG-welded dissimilar steel joints, the weld metal composition is significantly influenced by dilution from both base metals. When joining a low-carbon steel to a high-carbon or high-alloy steel, the resulting weld metal can exhibit a composition that falls outside the intended weldability range. This often leads to the formation of hard martensitic phases in the weld metal and HAZ, particularly when the carbon equivalent (CE) of the dilution-modified composition exceeds 0.45%. The resulting hardness can reach 400–500 HV, which is associated with reduced ductility and increased susceptibility to hydrogen-induced cracking (HIC).

The MIG process, while producing joints with slightly higher tensile strength due to greater dilution and alloying effects, suffers from inferior toughness characteristics. The higher hardness in the MIG HAZ creates a region of stress concentration that can serve as a crack initiation site under cyclic loading. Furthermore, the thermal input of conventional MIG welding is typically higher than that of LMHW, leading to wider HAZs with more pronounced microstructural changes and higher residual stresses.

From a residual stress perspective, the LMHW process generates lower peak temperatures at the joint interface because the low-melting-point filler absorbs much of the thermal energy. This results in residual stress levels that are typically 15–30% lower than those observed in MIG-welded joints of comparable geometry. Lower residual stresses translate directly to improved fatigue performance and reduced risk of delayed cracking, particularly in high-strength steels where hydrogen embrittlement is a concern.

Engineering Practice and Quality Control Considerations

In automotive structural applications, such as those addressed by the industrial collaborator in this study, the selection between LMHW and MIG welding depends on the specific performance requirements. For components subjected to high cyclic loading, such as suspension arms or crash boxes, the superior toughness and lower residual stresses of LMHW joints make them the preferred choice. For components where maximum tensile strength is the primary requirement and the loading is predominantly static, MIG welding may be acceptable provided that appropriate post-weld treatment is applied.

Quality control for dissimilar steel joints requires attention to several critical aspects. First, the carbon equivalent of the dilution-modified weld metal should be calculated and verified through chemical analysis. If CE exceeds 0.45%, preheating at 150–250°C and post-weld heat treatment (PWHT) at 550–650°C should be considered to reduce hardness and relieve residual stresses. Second, impact testing at service temperatures should be performed on both the weld metal and HAZ regions to confirm adequate toughness. Third, for critical applications, fatigue testing at stress levels representative of the intended service life should be conducted.

The LMHW process also offers advantages in terms of distortion control, which is particularly important for thin-walled automotive components. The lower heat input and more localized thermal profile result in angular distortions that are typically 20–40% lower than those observed with conventional MIG welding. This reduces the need for post-weld straightening operations, improving manufacturing efficiency and dimensional accuracy.

Study Insights and Practical Implications

This research provides valuable comparative data that can guide process selection in dissimilar steel welding applications. The findings suggest that LMHW welding represents a superior technology for applications where toughness, fatigue resistance, and distortion control are prioritized. The lower dilution rate and reduced thermal input are the primary contributors to these advantages, making the process particularly suitable for joining high-strength steels where HAZ softening or hardening is a concern.

However, engineers should also consider the limitations of LMHW technology. The lower-melting-point filler wires are typically more expensive than standard MIG consumables, and the equipment requirements for LMHW (including hot-wire feeding systems and precise temperature control) represent a higher capital investment. Additionally, the process speed of LMHW is generally lower than that of MIG welding, which may be a limiting factor in high-volume production environments.

The practical implication for manufacturing engineers is that process selection should be driven by the specific performance requirements of the component rather than by cost alone. For safety-critical automotive components where failure could result in loss of life, the improved reliability offered by LMHW welding may justify the additional process costs. A life-cycle cost analysis that accounts for warranty claims, recall costs, and brand reputation should inform this decision.

This study contributes important technical data to the field of dissimilar steel welding and should be considered alongside other recent advances in hot-wire welding technology, including hot-wire TIG and dual-wire hot-wire processes, when evaluating process options for dissimilar metal joining in automotive and other industrial applications.