Spot welding, particularly resistance spot welding (RSW), is a critical joining process in automotive industry. The development of advanced high strength steels for applications in automotive industry is accompanied with a challenge to better understand the physical and mechanical metallurgy of these materials during RSW. The present paper critically reviews the fundamental understanding of structure–properties relationship in automotive steels resistance spot welds. The focus is on the metallurgical characteristics, hardness–microstructure correlation, interfacial to pullout failure mode transition and mechanical performance of steel resistance spot welds under quasi-static, fatigue and impact loading conditions. A brief review of friction stir spot welding, as an alternative to RSW, is also included.

Critical review of automotive steels spot welding: process, structure and properties

Current state of Fe-Mn-Al-C low density steels

Metal matrix nanocomposites (MMNCs) are thosemetal matrix composites where the reinforcement is of nanometer dimensions, typically less than 100nm in size. Also, it is possible to have both the matrix and reinforcement phases of nanometer dimensions. The improvement in mechanical properties of MMNCs is attributed to the size and strength of the reinforcement as well as to the fine grain size of the matrix. Spark plasma sintering has been used extensively over the past years to consolidate wide range of materials including nanocomposites and was shown to be effective noneconventional sintering method for obtaining fully dense materials with preserved nanostructure features. The objective of this work is to briefly present the spark plasma sintering process and review published work on spark-plasma-sintered metals and metal matrix nanocomposites.

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Spark plasma sintering of metals and metal matrix nanocomposites: a review

Thermomechanical processing of advanced high strength steels

This paper describes the mechanical behavior of the 6061-T6 aluminium alloy at room temperature for various previous thermal histories representative of an electron beam welding. A fast-heating device has been designed to control and apply thermal loadings on tensile specimens. Tensile tests show that the yield stress at ambient temperature decreases if the maximum temperature reached increases or if the heating rate decreases. This variation of the mechanical properties is the result of microstructural changes which have been observed by Transmission Electron Microscopy (TEM).

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Effects of heat treatments on the microstructure and mechanical properties of a 6061 aluminium alloy

The current aim in the development of third-generation steels for lightweighting automotive applications is to increase strength keeping at least the same formability as current steel concepts. In this philosophy, an optimal concept would be one that brings, in addition, a lower density. For this purpose, low-density steels have been designed with important aluminum additions obtaining density reductions of 8–10% or higher in comparison with low-carbon steels. At the levels required for lightweighting, aluminum introduces complex phenomena in steels. Here, some of the effects of aluminum in phase stability, CALPHAD-type modeling, and microstructure development are described, the latter in relation with mechanical properties. Finally, the potential of two families of lightweight steels for automotive applications is assessed by comparison with a steel currently present in automotive structures.

Low-Density Steels: Complex Metallurgy for Automotive Applications

The heat affected zones (HAZ) of two Dual Phase steels spot welds, DP450 and DP980, were investigated experimentally with a Gleeble 3500 thermomechanical simulator. The thermal cycles experienced locally were identified by finite element analysis of the resistance spot welding process and their evolution with an increasing sheet thickness was highlighted within a usual range of [1.0–3.0] mm. The cooling rates are significantly lower in the case of thick sheets and this promotes the occurrence of diffusional phase transformations. HAZ microstructures and constitutive behaviours could be characterized with the Gleeble specimens. Experimental simulations were run in the subcritical and the coarse grain temperature ranges (700°C and 1200°C respectively) in order to address the main transformations for both steels. DP450 shows little sensitivity to the subcritical thermal cycles while DP980 exhibits significant softening. This is in accordance with their respective base metal martensite content and the occurrence of martensite tempering. On the contrary, DP450 microstructures and mechanical properties are strongly sensitive to the investigated range of coarse grained HAZ thermal cycles while the evolution is less pronounced in the case of DP980, which is related to their respective hardenability.

HAZ Microstructures and Local Mechanical Properties of High Strength Steels Resistance Spot Welds

Pure iron nanopowders have been sintered using the spark plasma sintering technique. Fully dense materials can be achieved by a simple route. The influence of process parameters on density and hardness has been studied. The samples exhibit high hardness characteristic of nanostructured iron. This is confirmed by grain size measurements by X-ray diffraction and TEM. Moreover, when submitted to compression, the sinter exhibits high maximal strength with non-negligible ductility.

Spark plasma sintering of pure iron nanopowders by simple route

Study of the Influence of a Low Copper Addition and of an Excess of Silicon on the Precipitation Kinetics and on the Precipitation Sequence of Al-Mg2Si Alloys

Véronique Massardier-Jourdan

Papers

Low-Density Steels: Complex Metallurgy for Automotive Applications

HAZ Microstructures and Local Mechanical Properties of High Strength Steels Resistance Spot Welds

Spark plasma sintering of pure iron nanopowders by simple route

Study of the Influence of a Low Copper Addition and of an Excess of Silicon on the Precipitation Kinetics and on the Precipitation Sequence of Al-Mg2Si Alloys

Study of the Role Played by Nitrogen on the Deep-Drawing Properties of Aluminium Killed Steel Sheets Obtained after a Continuous Annealing