The effects of single and double pulse resistance spot welding on the microstructures of an advanced high strength automotive steel are presented in this work. The double pulse welding schemes partially remelt the primary weld nugget and anneal the area at the fusion boundary of the nugget. The effects of the annealing treatment on the segregation and the microstructure have been studied by electron probe microanalysis (EPMA) in combination with electron backscatter diffraction (EBSD). Results show that phosphorus has been redistributed at the primary weld nugget edge of the double pulse welds, while the mean block width and ellipticity of the prior austenite grains were smaller in welds subjected to double pulsing compared with single pulse weld. A favourable failure mode was obtained for the double pulse welds although behaviour did not correlate with the measured grain size.

This paper presents the effects of double pulse resistance spot welding (RSW) on the microstructural evolution, elemental distribution and mechanical properties of a 3 rd generation 1 GPa advanced high strength steel (AHSS). In order to investigate the effect of double pulsing, the steel was exposed to single and various double pulse RSW schedules. The first current pulse was applied to create the weld nugget, while the second current pulse generated a secondary weld nugget and annealed or (partial) re-melted the primary weld nugget, depending on the magnitude of the current. The effect of the second current pulse on the weld nugget and heat-affected zone characteristics was investigated using optical microscopy and electron probe microanalysis (EPMA). Optical and electron microscopy revealed that the secondary weld nugget is fully martensitic, showing a typical solidification microstructure, while the annealed zone reveals an equi-axed martensitic structure. EPMA results showed that elemental segregation has been considerably reduced in the annealed zone. Mechanical properties of the welds show that the AHSS studied is prone to weld metal failure for single pulse RSW. However, the double pulse RSW method can lead to significantly improved mechanical performance and favourable failure modes.

The effect of an automotive paint bake (PB) thermal cycle on the microstructural evolution and the mechanical properties of resistance spot welded advanced high strength steel is presented in this work. Mechanical behavior of the heat-treated welds reveals an increase in maximum cross-tension strength, displacement and subsequently energy absorption capability when 453 K (180°C)-20 minutes a bake thermal cycle is applied after welding. The microstructures of resistance spot welds with and without a PB heat treatment were characterized using scanning and transmission electron microscopy (TEM). TEM analysis revealed that the weld nugget and HAZ of the resistance spot welds consist of a martensitic microstructure. The microstructural analysis of the post-weld heat-treated samples shows the presence of e carbides in a martensitic matrix within the weld nugget and the HAZ. It is shown that the improved mechanical response of the paint-baked welds is associated with carbide precipitation during heat treatment.

In this paper, we describe the effects of mechanical loading on bcc-to-bcc phase transformations of an Advanced High Strength Steel during cooling. In-situ synchrotron diffraction was employed to measure time-temperature-load diffraction patterns. Calculations were made of the volume fractions of the phases, the transformation kinetics, and the austenite lattice parameter during cooling and simultaneous loading. In addition, volume fractions and lattice parameters of retained austenite at room temperature under different loading conditions were obtained. The results show that applying a load during cooling of the fcc phase significantly increases the volume fraction of a bcc phase before the start of the martensitic transformation. The kinetics of phase transformations were affected by the applied loads. The volume fraction and lattice parameter of retained austenite at room temperature vary in different samples and the highest retained austenite and the largest lattice parameter were obtained in the sample subjected to the highest load.

Flake Powder Metallurgy (FPM) is utilized for the processing of Al–Al2O3 composites. The effects of contents of 1-[Formula: see text]m-sized alumina (0, 3, 6 and 9 vol.%) on the microstructure, hardness, porosity and wear behavior of these composites are investigated. The as-received aluminum powder particles are milled in a planetary ball mill for different time durations (0.5, 1, 1.5 and 2 h), and the resultant flake powders are characterized by sieving, SEM, optical microscopy and XRD to determine their particle size, morphology and grain size. Al flakes and different amounts of Al2O3 powders are stacked into the mold cavity using a floating column filled with alcohol. Then the compacts are cold pressed at 750 MPa and sintered in a tube furnace at 655∘C for 60[Formula: see text]min. For comparison, reference samples from as-received aluminum powders are also fabricated. SEM studies showed a uniform distribution of alumina particles within the matrix of FPM-processed composites. These composites, despite their higher porosity, exhibited higher hardness levels and improved wear properties in comparison with the conventionally produced powder metallurgy (PM) counterparts. This is due to: (i) the special morphology of the flake powders that contributed to a more uniform distribution of alumina within the matrix and (ii) their smaller grain size due to work hardening that occurred during milling, which resulted in higher hardness values.

IN the original article there is an error in the first sentence of the abstract. bcc-to-bcc should be fcc to bcc. The corrected sentence is as follows:In this paper, we describe the effects of mechanical loading on fcc to bcc phase transformations of an Advanced High Strength Steel during cooling.

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