Research Papers Welder's Corner

Welding Metallurgy of Duplex Stainless Steel during Resistance Spot Welding

Source: Welding Journal Research Paper by S.H. Arabi, M. Pouranvari, and M. Movahedi

Abstract: This paper investigates the metallurgical and mechanical response of 2304 duplex stainless steel — as an interesting candidate for automotive body­in­white applications — to resistance spot welding. The results showed the high cooling rate associated with the resistance spot welding process suppressed the postsolidification ferrite­austenite transformation leading to improper ferrite­austenite phase balance with a reduced volume fraction of austenite and consequent precipitation of chromium­rich nitrides. The effects of welding current, as the key parameter determining the weld heat input, on the austenite volume fraction and precipitation are discussed. The minimum fusion zone hardness corresponding to the highest austenite volume fraction and minimum volume fraction of nitrides was obtained when the cooling rate was the lowest. The failure mode transition, peak load, and energy absorption of the welds were investigated. It was found that the fusion zone size is the key factor controlling peak load and energy absorption of the welds. Mechanical properties of the duplex stainless steel spot welds were compared with those of other automotive steels.

Introduction: Due to the continued rising vehicle safety and crash requirements and the need for weight reduction in the automotive industry, the use of ultrahighstrength steels in body-in-white applications is rapidly increasing. The ferrite-martensite dual-phase steels, transformation-induced plasticity (TRIP) assisted steels, and martensitic steels are the most common advanced high-strength steels (AHSSs) that are implemented in today’s car-body design (Refs. 1–3).

The introduction of advanced highstrength steels in the automotive industry is accompanied with the challenge of their weldability. Automotive structural assemblies use groups of spot welds to transfer load through the structure during a crash. Additionally, spot welds can act as fold initiation sites to manage impact energy (Ref. 4). Vehicle crashworthiness, which is defined as the capability of a car structure to provide adequate protection to its passengers against injuries in the event of a crash, largely depends on the integrity and the mechanical performance of the spot welds (Ref. 5). Spot weld failure during a crash is a critical issue for crashworthiness, stiffness and noise, vibration and harshness performance of the vehicle. Therefore, the quality, performance, and failure characteristics of resistance spot welds are important for determining the durability and safety design of the vehicles (Refs. 6–8). Through an examination of past research (Refs. 9–20), it is recognized that resistance spot welding (RSW) of AHSSs has several challenges, including the following:

1) Complex phase transformation in the weldment: The strength/ ductility properties of AHSS are governed by their sophisticated, designed microstructure. However, the stability of AHSS base metal microstructure is significantly affected by weld thermal cycle (Ref. 21). This produces significant property (strength and toughness) mismatch among the fusion zone (FZ), heat-affected zone (HAZ), and base metal, which in turn affects the load-bearing capacity and failure behavior of the AHSS spot welds. The two important phase transformations in AHSS welds are as follows:

• Martensite formation in both fusion zone and coarse-grain heat affected zone (Refs. 9–12), which can induce an adverse effect on the weld failure characteristics during some loading conditions (e.g., peel and cross-tension tests) (Refs. 13–15).

• Tempering of the base metal martensite in the subcritical HAZ, which creates softening compared to the base metal. The HAZ softening phenomenon can take place during welding of martensite-containing AHSS (e.g., dual-phase steels and martensitic steels) (Refs. 16–18). This phenomenon can reduce the loadWELDING RESEARCH SEPTEMBER 2017 / WELDING JOURNAL 307-s SUPPLEMENT TO THE WELDING JOURNAL, SEPTEMBER 2017 Sponsored by the American Welding Society and the Welding Research Council Welding Metallurgy of Duplex Stainless Steel during Resistance Spot Welding Solidification and postsolidification solid­state phase transformation during resistance spot welding of 2304 duplex stainless steel were investigated BY S. H. ARABI, M. POURANVARI, AND M. MOVAHEDI ABSTRACT This paper investigates the metallurgical and mechanical response of 2304 duplex stainless steel — as an interesting candidate for automotive body­in­white applications — to resistance spot welding. The results showed the high cooling rate associated with the resistance spot welding process suppressed the postsolidification ferrite­austenite transformation leading to improper ferrite­austenite phase balance with a reduced volume fraction of austenite and consequent precipitation of chromium­rich nitrides. The effects of welding current, as the key parameter determining the weld heat input, on the austenite volume fraction and precipitation are discussed. The minimum fusion zone hardness corresponding to the highest austenite volume fraction and minimum volume fraction of nitrides was obtained when the cooling rate was the lowest. The failure mode transition, peak load, and energy absorption of the welds were investigated. It was found that the fusion zone size is the key factor controlling peak load and energy absorption of the welds. Mechanical properties of the duplex stainless steel spot welds were compared with those of other automotive steels. KEYWORDS • Duplex Stainless Steel • Resistance Spot Welding • Phase Balance • Phase Transformations • Mechanical Performance Arabi Paper 201722 September 2017.qxp_Layout 1 8/10/17 9:30 AM Page 307 bearing capacity of spot welds compared to the strength expected from the initial base metal microstructure (Refs. 19, 20).

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