The present study investigated the main microstructural characteristics of dissimilar welding between AISI 316L and AISI 444 stainless steel. In the welds, 25 to 50 kN axial forces were applied, varying the rotation speed between 350 and 450 rpm and keeping the welding speed constant at 1 mm/s. Three types of joint configurations were used: The first type was the butt joint, and the other two types were overlapped joints. Under all conditions, relevant microstructural changes were not observed between base metal (BM) and heat-affected zone (HAZ). In the thermomechanically affected zone (TMAZ) and stir zone (SZ) of the butt joint, the increase in the axial force and the rotational speed shows austenitic and ferritic grains that are poorly refined at the top and more deformed grains with intense refining at the bottom. In the overlap joints, the positioning of AISI 316L steel, with low SFE and easier recrystallisation at the top of the joint, provided a more negligible difference in deformed and recrystallised grains between the upper and lower TMAZ and SZ, where AISI 444 steel is positioned. In none of the welded conditions is it possible to observe areas of excessive grain growth and formation of undesirable phases such as chi, sigma and Laves despite these phases being predicted in the phase diagram of these steels in the temperature range reached in these dissimilar FSW welds. Therefore, it is possible to produce dissimilar joints between AISI 444 and AISI 316L steels by the FSW process with excellent grain refinement, absence of defects and undesirable phases and high cohesion between the two materials.
Corrosion behavior of FS-welded AZ31B magnesium alloys under aggressive environments remains a major concern. In the present investigation, AZ31B magnesium alloy plates of 6 mm thickness were joined using the friction stir welding process, followed by post-weld heat treatment (PWHT) at 350 °C for 1 hour and furnace cooling. The microstructural evolution of the weldment was examined using optical microscopy, scanning electron microscopy, and transmission electron microscopy. Further investigations were carried out to evaluate the pitting corrosion, electrochemical impedance behavior, exfoliation corrosion, and stress corrosion resistance of the weldment in chloride-containing environments. The results revealed that the stir zone of the as-welded FS joints exhibited superior corrosion resistance compared to the base metal due to the formation of fine dynamically recrystallized grains and a homogeneous distribution of Al–Mn precipitates. PWHT further enhanced the corrosion resistance by promoting precipitate homogenization, relieving residual stresses, and stabilizing the passive oxide layer. The present study highlights the significant role of post-weld heat treatment in enhancing the corrosion resistance and environmental sensitivity of AZ31B magnesium alloy friction stir welds for structural applications operating under aggressive service conditions.
WAAM is one of the most promising 3D metal fabricating technologies for medium to large size parts due to higher deposition rates, less cost, reduced lead times and material savings. In this study, the effect of the wire diameter 1.2 mm (LDW- Low diameter wire) and 1.6 mm (HDW-High diameter wire) on 304L stainless steel fabricated using CMT-WAAM process. However, the effect of wire diameter indicates a significant role in thin-walled structure and affects the material properties and process stability. Microstructure of 304L thin-wall consists of large number of columnar grains in austenite (γ) dendrites. Thermal influence of the subsequent layer caused δ phases to turn into lathy and vermicular morphologies in γ dendrites. EBSD results show well-aligned columnar dendrite crystals with a dominant (001) texture along the building direction. Micro-hardness values are higher at bottom region due to the variation in microstructure and complex thermal history; the average hardness of various deposit wall was 225 HV. Tensile properties of horizontal direction indicate higher UTS and YS values as compared to vertical direction samples, and confirm the anisotropy in mechanical properties. Furthermore, fracture surface of tensile samples of both directions specified ductile fracture due to more dimples formation. This study understands the importance of welding consumables on microstructure and mechanical integrity of WAAM 304L and offers significant potential for tunable performance. Our findings provide mechanistic insights into understanding and highlight the essential role of wire diameter behavior in the WAAM stainless steels.
Over the centuries, Planet Earth has been visited by aliens. Alien rocks, that is, from deep space—many owing their origin to the big-bang event, which occurred over 4 billion years ago at the beginning of time, or more precisely the beginning of our universe as we know it today. These wanderers of the Cosmos have made long journeys to reach us. Most of them experienced a fiery finish as they plunged through our atmosphere—to leave a crater, become buried in an icefield, to disintegrate in the atmosphere and spread themselves over large land areas, or to be lost in an ocean. The size of these “resident aliens” varies from monoliths weighing thousands of kilograms to dust particles of micron and microgram size. Both extremes are often difficult to find. The larger fragments have relied, on ancient stories of events in the atmosphere. These recounted experiences have sometimes been handed down over generations before “solid evidence” is discovered on, or found buried in the terrain, and identified as extra-terrestrial in origin. Capturing dust particles is a relatively recent endeavor and relies on the Earth’s magnetic field to concentrate them, and focus their trajectory toward the polar regions. Scientists with specially designed “traps,” or through sifting in sediment and deep snow layers, have successfully captured a rewarding haul of these micron and sub-micron-sized particles, especially at the South Pole [1].
