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In this study, a heat source model designed explicitly for rectangular laser heat sources was developed to simulate the temperature field during the laser hot wire cladding process. The influence of the temperature field on the solidified microstructure was investigated. The difference in morphology and size of solidified microstructure was mainly caused by temperature gradient and cooling rate. The grain morphology in the edge region was mainly columnar crystal with small size. The grain morphology in the central area was mainly equiaxed crystals with the larger size. This study provided a reference for understanding the characteristics of the solidification microstructure and its formation during laser hot wire cladding process.

The aim of this research was to develop a high-efficiency and high-material-utilization additive manufacturing technology using the hot-wire laser method. In this study, the optimization of process conditions using a combination of a high-power diode laser with a relatively large laser spot hot-wire system was investigated. The effects of welding parameters such as the laser power, process speed, and wire feeding rate (wire feeding speed/process speed) on the bead appearance evaluation and the cross-sectional characteristics ( e.g. effective width, effective height, maximum height, and near net shape rate) were studied in detail. The process phenomena of the three-layer and multi-layer deposition were investigated by in-situ observation via a high-speed camera. Energy density input and wire feeding rate were found to be dominant parameters influencing both the stability of phenomena and bead appearance. With the increase of process speed, the effective width decreases, the effective height, maximum height, and the near net shape rate increases. Additionally, all measured values of the wire feeding rate of 30 improve compared with the values of the wire feeding rate of 20. The near net shape rate increased and the effective width over 10mm of three-layer deposition for the laser spot width of 11 mm was obtained with suitable process parameters. The defect-free 15-layer wall modeling of more than 50 mm in height, 8 mm in width, and 250 mm in length was obtained with high efficiency using the optimum conditions by the hot-wire laser method.

Laser hot wire cladding is an efficient and energy-saving additive manufacturing process, which is used in coating, repair, and 3D printing. In this study, 308L stainless steel wire was deposited on the low carbon steel substrate using Laser hot wire cladding technology. The microstructure evolution and microhardness changes in the heat-affected zone of the substrate and the cladding layer near the laser cladding interface were studied in detail by using optical microscopy, electron backscatter diffraction and microhardness tests. The phase transformations and the relationship between microstructure and mechanical properties of the cladding layer were discussed. Owing to the dual effect of temperature gradient and cooling rate, the grain morphology in the central region of the cladding layer is equiaxed, while the edge region is mostly columnar dendritic morphology along the structural direction. There is a dilution zone composed of single austenite near the interface of the cladding layer. In the cladding layer, the ferrite-austenite solidification mode is dominant, and the austenite-ferrite solidification mode is also found in local areas. The carbon migration process from the substrate to the cladding layer promotes the austenite-ferrite mode. The heat-affected zone microstructure consists mainly of ferrite and tempered bainite, and slatted martensite can be observed near the interface. The electron backscatter diffraction results demonstrate the existence of martensite in the heat-affected zone. The texture is more apparent in the cladding layer, while the heat-affected zone contains more residual strain and higher dislocation density. The average grain size in the heat-affected zone region is about 4 μm, while the average grain size in the cladding layer is above 14 μm. The hardness of the heat-affected zone near the interface is the highest due to numerous factors, such as grain size, microstructure transformation and dislocation density. The yield strength increases with the decrease of grain size and texture strength, demonstrating the correlation between microstructure and mechanical properties.

Laser hot wire cladding (LHWC) technology has been proved to be a very efficient laser cladding manufacturing process due to its advantages of fast deposition rate, high material utilization rate, and less environmental pollution. In this study, LHWC technology was used to prepare multiple layers of 308L stainless steel coatings on ordinary mild carbon steel substrates. The microstructure of the weldment section was evaluated by optical microscopy (OM). The results showed that the heat-affected zone (HAZ) of the sample was small, with a height of 1.58 mm and an area of 14.28 mm2. The metallurgical bonding between the substrate and coating was good. Because of the temperature gradient and grain growth rate, the grain morphologies in the different coating regions were different. From the bottom to the top, the grain morphologies were planar crystals, columnar dendrites, and equiaxed grains. Electron backscatter diffraction (EBSD) was used to obtain crystallographic information on the entire section of the sample. The interior of the cladding layer was mainly composed of columnar grains growing along the structural direction, most of which were coarse grains. The matrix and the HAZ consisted of fine equiaxed grains, and the crystal orientations were randomly distributed without obviously preferred orientations. Hardness analysis performed that the hardness of the HAZ was the highest, and the matrix had the lowest hardness. The hardness was higher in the layer-to-layer fusion zone and then gradually decreased with an increase in the cladding height inside each layer. The tensile test results illustrated that the elongation of the cladding layer (44.8 %) was higher than that of the HAZ (32.1 %) and the matrix (20.4 %) because the tensile direction of the tensile sample in the cladding layer was parallel to the laser scanning direction, which improved the tensile performance. The average grain size of the HAZ was the smallest, owing to the effect of fine grain strengthening, and the tensile strength was the highest(585 MPa). According to the tensile fracture profile, the sample matrix, HAZ, and the cladding layer exhibited ductile fractures.

The coupling of a hot tensile test with a novel in situ observation technique using a high-speed camera was investigated as a high-accuracy quantitative evaluation method for ductility-dip cracking (DDC) susceptibility. Several types of Alloy 690 filler wire were tested in this study owing to its susceptibility to DDC. The developed test method was used to directly measure the critical strain for DDC and high temperature ductility curves with a gauge length of 0.5 mm. Minimum critical strains of 1.3%, 4.0%, and 3.9% were obtained for ERNiCrFe-7, ERNiCrFe-13, and ERNiCrFe-15, respectively. The DDC susceptibilities of ERNiCrFe-13 and ERNiCrFe-15 were nearly the same and quite low compared with that of ERNiCrFe-7. This was likely caused by the tortuosity of the grain boundaries arising from the niobium content of around 2.5% in the former samples. Besides, ERNiCrFe-13 and ERNiCrFe-15 indicated higher minimum critical strains even though these specimens include higher content of sulfur and phosphorus than ERNiCrFe-7. Thus, containing niobium must be more effective to improve the susceptibility compared to sulfur and phosphorous in the alloy system.

Surface melting by gas tungsten arc (GTA) welding and overlaying by cold metal transfer (CMT) brazing using low melting point filler wire were investigated to develop a repair process for cracks in worn cast steel of steam turbine cases. Cr-Mo-V cast steel, operated for 188,500 h at 566 °C, was used as the base material. Silver and gold brazing filler wires were used as overlaying materials to decrease the heat input into the base metal and the peak temperature during the welding thermal cycle. Microstructural analysis revealed that the worn cast steel test samples contained ferrite phases with intragranular precipitates of Cr7C3, Mo2C, and CrSi2 and grain boundary precipitates of Cr23C6 and Mo2C. CMT brazing using low melting point filler wire was found to decrease the heat input and peak temperature during the thermal cycle of the process compared with those during GTA surface melting. Thus, the process helped to inhibit the formation of hardened phases such as intermetallics and martensite in the heat affected zone (HAZ). Additionally, in the case of CMT brazing using BAg-8, the change in the hardness of the HAZ was negligible even though other processes such as GTA surface melting cause significant changes. The creep-fatigue properties of weldments produced by CMT brazing with BAg-8 were the highest, and nearly the same as those of the base metal owing to the prevention of hardened phase formation. The number of fracture cycles using GTA surface melting and CMT brazing with BAu-4 was also quite small. Therefore, CMT brazing using low melting point filler wire such as BAg-8 is a promising candidate method for repairing steam turbine cases. However, it is necessary to take alloy segregation during turbine operation into account to design a suitable filler wire for practical use.

Solidification cracking occurs as a result of thermal strain and solidification behavior that is determined by welding conditions, especially welding speed. Brittle temperature range (BTR), an important index for evaluating solidification cracking, is used to quantity the effect of welding speed on susceptibility to solidification cracking. However, measurement of BTR using Trans-Varestraint test during laser welding has rarely been studied. The purpose of this work is to investigate the effect of welding speed on solidification cracking susceptibility in type 310S stainless steel using Trans-Varestraint test during laser welding. Solidification cracking and ductility-dip cracking were distinguished by observing fracture surfaces. Compared with traditional Trans-Varestraint test carried out for gas tungsten arc welding, the number of solidification cracks and total crack lengths during laser welding were lower; however, the number density of solidification cracks and total crack length per bead width during laser welding were higher. Both of these values had a tendency to first increase and then decrease slightly with increasing welding speed: the maximum values occurred at approximately 1.5 m/min. Temperature profiles at 0.2 and 1.0 m/min during laser welding were measured by an optical-fiber radiation thermometer combined with in-situ observation using a high-speed camera. BTR was measured using the center crack length along the heat flow in the welding direction. BTR at 1.0 m/min was less than that at 0.2 m/min during laser welding because the maximum crack length appeared at the side of molten pool at welding speeds greater than 1.0 m/min.

Shop primer is a predominant cause of pits and blowholes during fillet welding using a conventional CO2 arc welding process. The hot-wire laser welding process for fillet welds employs a reflected laser beam incident on a molten pool for melting the base metal, and produces a weld bead with an extremely low dilution of base metal. In this study, weld-defect prevention in hot-wire laser welding as applied to steel plates coated with shop primer was investigated by examining the vaporization of the shop primer at the molten pool front and beneath the stiffener at the root region. A melting technique employing a reflected laser beam incident on the molten pool surface can efficiently remove the shop primer at the molten pool front during welding. The quantity of several gases and metallic vapors emitted during decomposition of the shop primer at a root was reduced markedly during hot-wire laser welding under extremely low base metal dilution. The above features of the hot-wire laser welding process achieved sound fillet welds without pits and blowholes.

The objective of this research was to develop a single-pass vertical welding process for thick steel plates using hot-wire laser welding to reduce the heat input. A laser diode with a large rectangular beam spot and twin hot-wire feeding system were used. Welding phenomena were investigated using stationary and weaving laser irradiation with small and large groove widths. Single-pass vertical welding for thick steel plates with a low heat input and low dilution could be achieved. A critical energy density existed for each welding speed at which fusion of the groove surface could commence. The energy density affected fusion of the groove surface compared with welding speed. Weaving laser irradiation made it possible to maintain a high energy density and accommodate the large gap variation.