Plasma-MIG Hybrid Welding Process for TATM700 Automotive Structural Steel

Literature Overview

This paper by Zhang Hongtao, Sang Jian, Wang Qichen, Teng Yao, and Zhang Wenjie, published in the Welding Journal (Vol. 40, No. 12, 2019, pp. 25–30), investigates the plasma-MIG hybrid welding process for TATM700 low-alloy high-strength steel used in automotive structural components, specifically beam steel with a thickness of 12.5 mm. The study examined different groove configurations and compared the plasma-MIG hybrid process against conventional multi-pass manual TIG welding with flux-cored wire. The research was conducted jointly by Harbin Institute of Technology (Weihai), CIMC Offshore Engineering Research Institute, and Weihai Donghai Shipbuilding Co., Ltd., and was supported by multiple funding sources including the Taishan Scholars Young Expert Support Program (tsqn20161062), HIT Research Innovation Fund (HIT.NSRIF.201707), Shandong Provincial Key R&D Program (2018GGX103032), and Shandong Provincial Natural Science Foundation (ZR2018MEE027).

Process Comparison: Plasma-MIG Hybrid vs. Manual TIG

The comparison between plasma-MIG hybrid welding and multi-pass manual TIG welding reveals significant differences in efficiency, weld quality, and mechanical properties. The plasma-MIG hybrid process combines the deep, narrow penetration of plasma arc welding with the higher deposition rate of MIG welding, creating a synergistic effect that enables single-pass or reduced-pass welding of 12.5 mm thick material. In contrast, the manual TIG process requires multiple passes with flux-cored wire to achieve complete penetration and fill the groove, resulting in significantly longer welding times and greater heat input.

The tensile strength reaching 95% of the base metal is a particularly important finding for automotive structural applications. In automotive design, the weld strength ratio (Weld Strength / Base Metal Strength) is a critical parameter that directly influences the design safety factor and material utilization. A 95% strength ratio is generally considered excellent for high-strength steel welds, indicating that the plasma-MIG hybrid process produces welds with minimal strength loss relative to the base metal.

Microstructural Analysis

The weld metal microstructure consists of a small amount of side plate ferrite and a large amount of acicular ferrite. This microstructure is highly desirable for high-strength steels because acicular ferrite provides an excellent combination of strength and toughness. The acicular ferrite morphology forms when the cooling rate is in the appropriate range (typically 5–50°C/s) and the weld metal composition contains sufficient grain-refining elements such as titanium, vanadium, or niobium.

The presence of side plate ferrite along the fusion boundary is a common feature in high-strength steel welds, particularly in regions of slower cooling rate. Side plate ferrite can reduce toughness if it forms a continuous network, but in this case, the limited amount of side plate ferrite combined with the dominant acicular ferrite results in good overall mechanical properties. The lower hardness observed compared to manual TIG welding is consistent with the faster cooling rate and more uniform heat input distribution of the hybrid process, which suppresses the formation of hard, brittle phases.

The observation that groove configuration has relatively little influence on the plasma-MIG hybrid weld quality is practically significant. In automotive production, where welding speed and process robustness are paramount, the ability to maintain consistent weld quality across different groove preparations reduces the need for precise machining and fit-up tolerances. This process robustness is a major advantage over conventional welding processes that are highly sensitive to groove geometry.

Engineering Practice and Manufacturing Implications

For automotive manufacturing engineers, the plasma-MIG hybrid process offers a compelling solution for welding high-strength structural steels like TATM700. The key advantages include: (1) higher welding speed and reduced cycle time, directly impacting production throughput; (2) lower distortion due to reduced total heat input, which reduces or eliminates the need for post-weld straightening operations; (3) consistent weld quality that is less sensitive to operator skill and groove preparation variations; and (4) mechanical properties that meet or exceed the requirements of automotive structural design codes.

The 12.5 mm thickness studied in this research represents a typical thickness for automotive structural beams and cross-members. In production settings, engineers should consider the process window for this hybrid technique, particularly the coordination between plasma arc and MIG arc parameters. The plasma arc typically operates at lower current (30–80 A) with higher current density, while the MIG arc operates at higher current (150–300 A) for deposition. The spatial relationship between the two arcs—whether the plasma arc leads, trails, or is coaxial with the MIG arc—significantly affects the molten pool dynamics and weld geometry.

The slightly lower hardness of the plasma-MIG hybrid weld compared to manual TIG may actually be beneficial in terms of weldability and fatigue performance. Lower hardness in the weld metal generally correlates with higher ductility and better resistance to hydrogen-induced cracking, which is particularly relevant for high-strength steels that are susceptible to cold cracking. The acicular ferrite-dominated microstructure also provides superior low-temperature impact toughness compared to microstructures dominated by side plate ferrite or martensite.

Study Insights and Reflections

The finding that groove configuration has minimal impact on weld quality is one of the most practically significant results of this study. In automotive manufacturing, where production speed and process flexibility are critical, the ability to use simpler groove preparations without sacrificing weld quality represents a substantial cost and time savings. This process robustness is characteristic of hybrid welding processes that combine high-energy-density heat sources, which create deeper, more stable molten pools that are less sensitive to geometric variations.

The comparison with manual TIG welding highlights an important consideration for process selection: while manual TIG welding may produce slightly higher hardness, the overall mechanical performance including tensile strength, bend performance, and welding efficiency clearly favors the plasma-MIG hybrid approach. In production environments, the total cost of ownership—including welding time, labor cost, distortion correction, and inspection requirements—strongly supports the adoption of hybrid welding processes for high-strength steel structural components.

This research validates the plasma-MIG hybrid welding technology as a viable and superior alternative to conventional welding methods for automotive-grade high-strength steels, offering the combination of process efficiency, quality consistency, and mechanical performance that modern automotive manufacturing demands.