Friction stir welding/processing of metals and alloys: A comprehensive review on microstructural evolution

Smithells Metals Reference Book

The correlation of microstructural characteristics and toughness of the simulated coarse grained heat affected zone (CGHAZ) of low carbon bainitic steel was investigated in this study. The toughness of simulated specimens was examined by using an instrumented Charpy impact tester after the simulation welding test was conducted with different cooling times. Microstructure observation and crystallographic feature analysis were conducted by means of optical microscope and scanning electron microscope equipped with electron back scattered diffraction (EBSD) system, respectively. The main microstructure of simulated specimen changes from lath martensite to coarse bainite with the increase in cooling time. The deterioration of its toughness occurs when the cooling time ranges from 10 to 50 s compared with base metal toughness, and the toughness becomes even worse when the cooling time increases to 90 s or more. The MA (martensite–austenite) constituent is primary responsible for the low toughness of simulated CGHAZ with high values of cooling time because the large MA constituent reduces the crack initiation energy significantly. For crack propagation energy, the small effective grain size of lath martensite plays an important role in improving the crack propagation energy. By contrast, high misorientation packet boundary in coarse bainite seems to have few contributions to the improvement of the toughness because cleavage fracture micromechanism of coarse bainite is mainly controlled by crack initiation.

Microstructural characteristics and toughness of the simulated coarse grained heat affected zone of high strength low carbon bainitic steel

Hybrid welding, using the combination of a laser and an electrical arc, is designed to overcome problems commonly encountered during either laser or arc welding such as cracking, brittle phase formation and porosity. When placed in close contact with each other, the two heat sources interact in such a way as to produce a single high intensity energy source. This synergistic interaction of the two heat sources has been shown to alleviate problems commonly encountered in each individual welding process. Hybrid welding allows increased gap tolerances, as compared to laser welding, while retaining the high weld speed and penetration necessary for the efficient welding of thicker workpieces. A number of simultaneously occurring physical processes have been identified as contributing to these unique properties obtained during hybrid welding. However, the physical understanding of these interactions is still evolving. This review critically analyses the recent advances in the fundamental understa...

Problems and issues in laser-arc hybrid welding

The effect of large heat input on the microstructure and corrosion behaviour of simulated heat affected zone in 2205 duplex stainless steel

The prior-austenite grain size in the heat-affected zones of single-pass butt welded joints in thick HSLA-100 steel plates was evaluated. The grain size was related with the temporal history of the location, which was obtained using an existing thermal model based on Gaussian heat source distribution for two different heat-input conditions. For a heat-input of 10 kJ/cm the grain size in the coarse grain heat-affected zone was 80 μm corresponding to a peak temperature of 1450 °C, but falls sharply to 40 μm within a distance of 0.5 mm from the fusion line. At a higher heat-input of 40 kJ/cm the grain size is 130 μm for the same peak temperature and the coarse grain heat-affected zone is 1.1 mm wide. The austenite grain size significantly influences the microstructure and properties of the heat-affected zone.

Effect of heat-input on austenite grain size in the heat-affected zone of HSLA-100 steel

Characterization of intermetallics in aluminum to zinc coated interstitial free steel joining by pulsed MIG brazing for automotive application

Continuous cooling transformation (CCT) diagrams for HSLA-80 and HSLA-100 steels pertaining to fusion welding with heat inputs of 10 to 40 kJ/cm, and peak temperatures of 1000 °C to 1400 °C have been developed. The corresponding nonlinear cooling profiles and related γ → α phase transformation start and finish temperatures for various peak temperature conditions have been taken into account. The martensite start (M
 s
 ) temperature for each of the grades and ambient temperature microstructures were considered for mapping the CCT diagrams. The austenite condition and cooling rate are found to influence the phase transformation temperatures, transformation kinetics, and morphology of the transformed products. In the fine-grain heat-affected zone (FGHAZ) of HSLA-80 steel, the transformation during cooling begins at temperatures of 550 °C to 560 °C, and in the HSLA-100 steel at 470 °C to 490 °C. In comparison, the transformation temperature is lower by 120 °C and 30 °C in the coarse-grain heat-affected zone (CGHAZ) of HSLA-80 steel and HSLA-100 steel, respectively. At these temperatures, acicular ferrite (AF) and lath martensite (LM) phases are formed. While the FGHAZ contains a greater proportion of acicular ferrite, the CGHAZ has a higher volume fraction of LM. Cooling profiles from the same peak temperature influence the transformation kinetics with slower cooling rates producing a higher volume fraction of acicular ferrite at the expense of LM. The CCT diagrams produced can predict the microstructure of the entire HAZ and have overcome the limitations of the conventional CCT diagrams, primarily with respect to the CGHAZ.

Mahadev Shome

Papers

Effect of heat-input on austenite grain size in the heat-affected zone of HSLA-100 steel

Characterization of intermetallics in aluminum to zinc coated interstitial free steel joining by pulsed MIG brazing for automotive application

Continuous cooling transformation diagrams applicable to the heat-affected zone of HSLA-80 and HSLA-100 steels

Effect of nugget size and notch geometry on the high cycle fatigue performance of resistance spot welded DP590 steel sheets

Microstructure and impact toughness of reheated coarse grain heat affected zones of API X65 and API X80 linepipe steels