The high temperature tensile properties of fiber laser-welded Mg-Gd-Y-Zr alloy in the as-welded and heat-treated state were systematically studied. The elevated temperature tensile properties of as-welded and heat-treated joints declined slightly from room temperature to 200 °C. With the temperature reaching 300 °C, the strengths of the two types of joints plummeted. High temperature tensile properties can be enhanced significantly by the β′ precipitates in the fusion zone and heat-affected zone after heat treatment. The ultrafine grains and unstable divorced eutectic in the fusion zone of as-welded joints deteriorated the tensile properties at 300 °C. The deformation mechanism of the heat-treated samples at 300 °C is the dislocation slip with void growth at the grain boundaries due to the dissolution of precipitates.

•The incomplete fusion and porosity during multipass laser welding are investigated.•The effect of heat input on dendrite arm spacing in the fusion zone is discussed.•The precipitation behavior of Nb-rich precipitates in the fusion zone is explained.The defects and microstructure in the fusion zone of multipass laser welded joints with Inconel 52M filler wire are investigated for nuclear power plants. Experimental results indicate that the incomplete fusion forms as the deposited metals do not completely cover the groove during multipass laser welding. The dendritic morphologies are observed on the inner surface of the porosity in the fusion zone. Many small cellular are found in the zones near the fusion boundary. With solidification preceding, cellular gradually turn into columnar dendrites and symmetrical columnar dendrites are exhibited in the weld center of the fusion zone. The fine equiaxed grains form and columnar dendrites disappear in the remelted zone of two passes. The dendrite arm spacing in the fusion zone becomes widened with increasing welding heat input. Nb-rich carbides/carbonitrides are preferentially precipitated in the fusion zone of multipass laser welded joints. In respect to high cooling rate during multipass laser welding, element segregation could be insufficient to achieve the component of Laves phase.

The segregation and liquation crackings in the heat-affected zone (HAZ) beside Inconel 52M overlays of multipass laser-welded joints are investigated for nuclear power plants. The results indicate that Nb-rich precipitates are distributed in chains and some aggregate together as coarse particles in the local regions at the interface. With increasing heat input, the transition width becomes widened and the fraction of the precipitates at the interface significantly increases. Closely associated to Nb segregation, liquation crackings occur along columnar dendrites in the HAZ beside Inconel 52M overlays. Due to no deformation coordination of the columnar dendrites with Nb-rich segregates in Inconel 52M overlays, liquation crackings occur in thermal cycle of multipass laser welding. The enrichment of Nb element in the interdendritic regions increases favorable factors of liquation crackings at grain boundaries.

•Thick plate was successfully welded by high-power laser welding in a horizontal (2G) butt joint configuration.•A U-shaped groove fabricated on the thick plate greatly increased the weld penetration and decrease the weld width.•The wire tip located in the center of the laser spot produced a smooth and continuous surface of the weld bead.•The columnar grains of the weld metal grown in different orientations contributed to a higher tensile strength.An innovative laser technique has been developed for welding 16 mm thick 304L stainless steel in the horizontal (2G) butt joint configuration. Autogenous laser welding (ALW) was applied to join the thick plate with a square groove and zero air gap, and laser welding assisted with a cold wire (LWACW) was used to fill the unfilled joint. The dimension of the square groove was optimized to achieve full penetration joint with smooth reinforcement at the back side. The wire feed process was also optimized to produce a sound weld appearance. The microstructure and mechanical properties were observed and evaluated by optical microscope, electron backscatter diffraction, microhardness measurement and tensile test. The results showed that the square groove contributed to produce a joint with deeper penetration and narrower width. The relative position of weld wire with respect to the laser spot had a great influence on the weld appearance. Sound appearance with defect-free joint was successfully produced for the 16 mm thick plate in the horizontal position. The microstructure and microhardness of the top and middle samples are different, resulting in the different tensile strength and fractured location.Download high-res image (323KB)Download full-size image

Thick-section steel has been widely used in many heavy industries. Traditionally, very thick steel plates could be welded by using submerged arc welding and other welding processes. However, there were more or less drawbacks in these welding methods. Laser welding, a high-energy density welding method, is being considered for such structures to improve the production efficiency and reduce the residual stresses of the joints. In this study, butt joints with narrow gap were welded using a high-power CO2 laser. The effect of welding parameters including the relative position between the laser beam and the filler wire, welding speed, and the distance from the intersection of the beam and wire to root of the groove on the weld bead geometry and welding defects was studied. Additionally, high-speed photography was introduced in the experiment as an efficient method to record the total process of welding, especially the transfer of molten drop. The study found that when the beam was focused on the center of the groove, the filler wire could be melted successfully even though it would tremble slightly during welding process. The optimized distance from the intersection of the beam and wire to groove root was 3 mm. Later, butt weld joints of 70-mm-thick steel plate without lack of fusion can be obtained under optimized welding parameters.

The study investigated laser welding with filler wire to determine the effects of wire feeding modes and the height of intersection of the wire and the beam (H) on weld geometry and droplet transfer. Droplet transfer behavior was investigated using high-speed imaging. The results showed that the wire melting changed with wire feeding. The leading wire melted completely and the welding depth and width were greater due to higher levels of energy available to heat the plate in the weld pool. The transfer mode changed to liquid bridge transfer and globular transfer with increased H value. With increasing H, the frequency of droplet transfer and the droplet size varied. At 2 mm H, the droplet transfer was very stable resulting in optimum welding.

•Different quantities of precipitates are present at different location of grain.•The network-distributed Mg24(Gd,Y)5 restricts grain growth.•Segregation of Zr affects migration of grain boundaries.The microstructure evolution in the fusion zone of laser-welded Mg-Gd-Y-Zr alloy during solution and aging treatment is investigated. The morphology of the Mg24(Gd,Y)5 in the divorced eutectic at the grain boundary transforms from a continuous network to disconnected and fragmentized islands and then to spheroidal particles before complete dissolution during the solution treatment at 430 °C. During the subsequent aging treatment at 225 °C, the precipitation sequence in the fusion zone follows the order of supersaturated solid solution (SSSS) → βʺ(D019) → βʹ(cbco) → β1(fcc) → β(fcc). High-density precipitates are present at the original grain boundaries of the fusion zone from the welded structure but there are less precipitates in the interior of the original grains. The grain growth during the solution treatment at 430 °C comprises the slowly increasing stage, rapidly increasing stage, and stable stage. The network-distributed Mg24(Gd,Y)5 impedes migration of the grain boundaries, restricts grain growth in the first slowly increasing stage, and segregation of zirconium near the grain boundaries also affects migration of the grain boundaries.

The laser-induced plasma is an important physical phenomenon during the process of penetration welding with high-power CO2 laser. To study the effect of assisting gas in the laser-induced plasma behavior and characteristics of weld formation, different configurations of heat input and side assisting gas were arranged to investigate the plasma behavior, and plasma images were recorded using high speed camera. By analyzing plasma images, spectrum intensity, and static pressure, the results have showed that the application of side assisting gas suppresses the laser-induced plasma significantly and improves the utilization of laser power, in which a sound weld quality is obtained. The combination of side nozzle used in the study with a gas flow rate around 20 L/min, the mixed gas composed of helium and argon in which helium accounted for 75%, and the position of side nozzle in which x and y were set at around 0 mm, h was set at around 6 mm could bring good controlling effect on plasma and well weld seam profile as well as deeper penetration. The reference value provides a promising solution to improve the absorption and efficiency of laser power during CO2 laser welding using side assisting gas.

In this paper, friction stir processing (FSP) with B4C particles (B4Cp) is used to improve the surface modification of 6061 aluminum alloy. Optical microscopy, scanning election microscopy, and energy-dispersive X-ray analysis have been performed to investigate the microstructure and the distribution of B4Cp. Wear resistance and microhardness were evaluated in detail. It is observed that the increasing number of FSP passes causes a more uniform distribution of B4Cp. The homogeneous distribution of B4Cp was observed in the weld zone, which significantly improved the wear resistance and microhardness of the surface composite layer as compared to those of the as-received Al alloy.

The current study investigates the overlays deposited by laser beam (LB) and gas tungsten arc (GTA) cladding with Inconel 52M filler wire in nuclear power plants. The effects of the deposition methods on the cross section profile, microstructures, and mechanical properties of both overlays are studied using optical microscope, scanning electron microscope (SEM), tensile and impact test and microhardness measurements. Experimental results show that LB cladding with higher wire feed rate improves the deposition efficiency as compared with GTA cladding. The microstructure of the LB clads consists of cellular and columnar dendrites, and Nb-rich spherical particles are precipitated in the interdendritic regions, while the equiaxed dendrites are dominant in the GTA clads. The microhardness of the GTA clads is 20–30 HV higher than that of the LB clads. Furthermore, the tensile strength, toughness, and elongation of the LB clads decrease in comparison to the GTA clads. The deep penetration of the LB clads resulting in a wave shape of each pass and high dilution does not benefit the mechanical properties as compared with the GTA clads.

The effects of heat input on the quality of laser-welded Mg-rare earth alloy NZ30K were studied. Using a 15-kW high-power CO2 laser, the microstructure and mechanical properties of welded joints under different heat inputs had been analyzed and tested. It is found that the welding heat input plays an important role in laser welding of NZ30K. Good welded joint without macroscopic defects can be obtained using the proper heat input. With the increasing heat input, welding penetration gets deeper, the width of the heat-affected-zone becomes larger, and the distribution of precipitates changes concentration. Tensile tests display that the ultimate tensile strength (UTS) of the welded joint tends to increase at first with the increasing heat input. After the welded joint gets full penetration, the UTS remains almost the same, although the heat input is increased.

The welding process of Mg-rare earth alloy Mg-3Nd-0.2Zn-0.4Zr (NZ30K) was studied using 15 kW high-power CO2 laser, the microstructure and performance of the typical welded joints had been analyzed and tested. There is no softening zone according to the microhardness test of the welded joint. The microstructure of the fusion zone consists of α-Mg-phase and β-phase (Mg12Nd).The results show that Mg-rare earth alloy NZ30K can be well joined with the high power laser and the welded joint has good performance.

The weldability of a 9.5 mm thick Mg-Rare earth alloy NZ30K using 15 kW high power CO2 laser was studied. The microstructure and mechanical properties of the typical welded joints had been analyzed and tested. When using the right laser welding parameters, good weld forming can be obtained. The microstructure of the fusion zone is small equiaxed grains. There is no softening zone to be observed according to the micro-hardness distribution across the welded joints. The results show that the thick Mg-Rare earth alloy NZ30K plate can be welded by the high power CO2 laser with good weld quality.

For energy saving more and more high performance materials are to be applied. Welding of high performance materials or cladding of high wear resistance materials are critical research points for their wide application. In the paper laser welding of a 1000 MPa complex phase steel and a novel TiB2 reinforced aluminum metal matrix composite were investigated, where the weld formation, microstructure and mechanical properties of welded joints were discussed. Further a laser clad wear-resistant coating with a new designed Fe-based powder was introduced in term of microstructure and wear resistance.