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High-Strength Low-Alloy (HSLA) steel exhibits excellent tensile strength and ductility, and it also possesses good welding performance due to its low carbon equivalent. However, welding defects always inevitably appear during the fusion weldingof HSLA steel. In this paper, the previous investigations on the microstructure of the joined HSLA steel by different fusion welding processes arereviewed, and the mechanical properties of the fabricated joint are analyzed. Also, the practicability of different fusion welding processes on HSLA steel has been systematically analyzed. Finally, the prospect of the welding of HSLA steel is expected according to the research status. Keywords: High-Strength Low-Alloy Steel; Fusion Welding; Welding Defect; Microstructure; Mechanical Properties

Martensitic stainless steels are widely used in industries due to their high strength and good corrosion resistance performance. Precipitation-hardened (PH) martensitic stainless steels feature very high strength compared with other stainless steels, around 3-4 times the strength of austenitic stainless steels such as 304 and 316. However, the poor workability due to the high strength and hardness induced by precipitation hardening limits the extensive utilization of PH stainless steels as structural components of complex shapes. Laser powder bed fusion (L-PBF) is an attractive additive manufacturing technology, which not only exhibits the advantages of producing complex and precise parts with a short lead time, but also avoids or reduces the subsequent machining process. In this review, the microstructures of martensitic stainless steels in the as-built state, as well as the effects of process parameters, building atmosphere, and heat treatments on the microstructures, are reviewed. Then, the characteristics of defects in the as-built state and the causes are specifically analyzed. Afterward, the effect of process parameters and heat treatment conditions on mechanical properties are summarized and reviewed. Finally, the remaining issues and suggestions on future research on L-PBF of martensitic precipitation-hardened stainless steels are put forward.

1045 carbon steel and 30CrMnSiNi2A high-strength low-alloy (HSLA) steel were joined successfully using inertia friction welding. The effects of the rotational speed on the microstructure and mechanical properties of the joints were investigated. With increasing rotational speed, the tensile strength first increased and then decreased. The weld samples with the highest tensile strength (713 MPa), largest elongation (15.3 %), and high impact toughness (28.8 J/cm2) were achieved at a rotational speed of 2200 rpm. Using a higher rotational speed (2800 rpm) increased the width of the heat-affected zone (HAZ) on the 1045 carbon steel side. Welding joints failed in the base metal and HAZ of the 1045 carbon steel for samples prepared at 2200 and 2800 rpm, respectively. The highest joint efficiency obtained was 97 %, while the lowest was 63 %. The hardness profiles also indicated that the weld joint had higher hardness than that of either of the two base metals. The highest hardness of the weld joint (581 HV) was observed for the 2200-rpm sample. The high strength and toughness of the weld joint were attributed to the phase transformation of lath martensite, bainite, ultrafine ferrite and cementite at the weld interface. Welding defects were observed in the 1000-rpm sample as the low heat input and inclusions at the weld interface caused incomplete metallurgical bonding, resulting in the fracture occurring at the weld seam.

In this study, an as-cast Mg–Gd binary alloy was bonded successfully via vacuum diffusion bonding at 500–560 °C in a closed graphite mould, and the microstructures and mechanical properties of the formed joints were analysed. It was found that Gd atoms diffuse to the bonding interface at temperatures lower than the eutectic temperature, with the resulting precipitated compounds improving the bonding strength. At temperatures higher than the eutectic temperature, the eutectic liquid produced at the interface gets squeezed into the residual voids, where it solidifies to form metallurgical bonds. Two types of grain boundary migration processes were observed at the interface in the joint formed at 550 °C. With an increase in the bonding temperature, the tensile strength of the joints first increased and then decreased, with the joint formed at 550 °C exhibiting a maximum efficiency of 88.3%. Casting defects in the base metals were eliminated during the bonding process with the use of a closed mould; this was also the reason that the joint produced at 550 °C showed a higher elongation than the as-cast alloy. However, the amount of the interfacial eutectic liquid increased sharply with the increase in the temperature. Thus, the joint formed at 560 °C showed a low elongation of 0.4% because a large number of microcracks formed in it owing to the solidification and shrinkage of the liquid phase.