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Functionally graded material (FGM) is an appropriate response to high-performance and multi-functional applications. In this research, dual-wire plasma arc welding can be used to fabricate SS 316L-Inconel 625 FGM with a composition gradient of 50 wt.% by adjusting the volume fraction of welding wire delivered to the melt pool. The phase evolution, microstructure, composition, microhardness and tensile properties of different composition regions along the building direction were analyzed. The results show that good bonding in the bi-metallic interface region and defect-free microstructure. The microstructure along the deposition direction is mainly columnar and equiaxed dendritic structure, and the grain growth direction is mainly along the deposition direction. The existence of Laves phases is proved by EDS mapping and point detection. Due to remelting, an error is existed between the actual composition distribution and the designed discrete gradient. The microhardness value decreases first, and reaching a minimum at the bi-metallic interface of the 100–50 wt.% SS 316L, and then increase gradually (157 HV-208 HV). The ultimate tensile strength, yield strength and elongation are 554.12 ± 7.44 MPa, 340.79 ± 4.13 MPa and 26.65 ± 0.27%, respectively. From the feasibility study, the dual-wire plasma arc welding provides a novel additive manufacturing process for the FGMs.

Experimental investigation on controlled pulse keyholing plasma arc welding (PAW) assisted by ultrasonic vibration was carried out. Because of application of the ultrasonic vibration, the peak value of the pulse welding current was decreased by 7–20%, and the welding speed was increased by 17–27% with different base current durations, different current descent times, and different descending currents. It indicates that the ultrasonic vibration can make controlled pulse plasma arc welding achieve stable one-pulse-one-keyhole mode at a lower heat input and a faster welding speed. Under the same welding process parameters, the area of keyhole exit was increased by 38% and the deviation distance of keyhole exit was reduced by 34%. The above results prove that the ultrasonic vibration can reduce the average welding current and heat input, improve the welding efficiency, and enhance the keyholing ability of the plasma arc for controlled-pulse keyholing PAW.

Titanium alloys are widely applied in the aerospace field owing to their exceptional properties, but coarse grains can easily form during the welding process, seriously diminishing the mechanical properties of the welded joint. To address this problem, a pulsed current was introduced to refine the grain structure of titanium alloy welds. In this study, we investigated the effect of varying the pulsed current frequency on grain refinement in titanium alloy welds and explored the mechanism of grain refinement using the “overlap welding” method. The results showed that the grain refinement improved with increasing pulse current frequency, particularly at the original β and α grain boundaries. Compared to non-pulsed welding, the original β grain boundary decreased from 305 to 158 μm, and the maximum α grain size decreased from 93.236 to 46.693 μm when the pulse frequency was 100 Hz. Moreover, dendrite fragmentation was identified as the primary mechanism of grain refinement in pulsed plasma arc welding of the Ti6Al4V alloy. The repeated heating of the molten pool by application of the pulsed current caused the dendrite tip to remelt in the pasting region, promoting dendrite fragmentation.

As a complex thermo-physical process, the plasma arc welding (PAW) is easy to be unstable due to external interferences. Weld quality monitoring is important for intelligent robot PAW welding. Due to the different instability mechanisms, it is difficult to obtain high adaptivity and accuracy with features extracted in a single time window. In this paper, a novel feature extraction method based on sliding multiscale windows is proposed to improve model accuracy and calculation speed. A group of windows with different time widths are established to extract multiscale information. Windows slide throughout welding process and are synchronized on the timeline for feature correlation. The welding current and arc voltage are processed to extract features inside windows, including signal denoising by discrete wavelet transform (DWT) and dimension reduction by primary components analysis (PCA). Based on the feature vectors extracted from multiscale-windows, support vector machine (SVM) with radial basis function (RBF) kernel is used. The best window width is determined automatically by model training. The proposed method is used to predict weld quality for PAW in the field of shipbuilding. The results show that the model with multiscale feature extraction is helpful to improve prediction precision and recall ratio.

Homogeneous and heterogeneous PAW (plasma arc welding) experiments were conducted to obtain weld beads with tensile strength of 100% or more compared to that of the base metal in lap joint fillet welding of CP (complex phase) 780 MPa grade high strength steel sheet. Homogeneous and heterogeneous welding mean welding technic without and with welding wire in each. Common basic welding parameters of both homogeneous and heterogeneous PAW wire were applied, including welding speed of 21 cm/min, arc length of 3.5 mm, weaving amplitude of 3 mm, work angle of 30°, and Ar 99.99% shielding gas with 12 L/min. In homogeneous PAW, the optimized additional welding conditions were a welding current of 90 and 95 A, push angle of 20°, and weaving frequency of 5 Hz. The fracture strengths of all the samples were less than that of the base metal, and all fractures occurred at the weld bead fusion zone due to insufficient reinforcement. In heterogeneous PAW, the optimized additional welding conditions were a welding current of 125 A, push angle of 0°, weaving frequency of 4 Hz, and wire feed speeds of 1.53 and 1.38 m/min. Moreover, the welding wire was fed from behind the arc. The fracture strengths of all the samples were greater than that of the base metal, and fractures occurred at the base metal or fusion zone boundary. The reason for higher tensile in heterogeneous PAW wire is deduced as the large cross-section area of reinforcement provided by the welding wire comparing to homogeneous welding. It is well known that small radius of welding heat source clearly produces narrow heat affected zone. Therefore, the PAW produces relatively narrower heat affected zone, then resulted as fully enough strength of weldment. This is another reason of the current achievement.

In this study, bypass-current plasma arc welding (BC-PAW) was innovatively proposed to join aluminum AA 5083 alloy. A comparative analysis between simulation and experimental results was performed to further explore the effects of bypass-current on the thermal behaviors, residual stress, and distortion of as-received weld. The results show that adding bypass-current in plasma arc welding is beneficial to the welds, and it helps to reduce the heat input into the workpiece and subsequently improve the weld thermal concentration. In addition, due to the bypass-current induced less heat flow density, the residual stress of the as-received welds can be reduced maximumly to 22 MPa, contributing to a significant improvement in the weld distortion. The research work develops a new route to the welding field, and has a significant contribution to further welding aluminum process improvement and optimization.

T2 copper was joined to 1060 pure aluminum by a novel welding method, that is, an ultrasonic-assisted plasma arc Welding–Brazing process. The impacts of ultrasonication on the interfacial microstructure and weld formation were studied. Optical microscopy and scanning electron microscopy were used to observe weld formation and the resulting microstructure. The elemental distributions and phase formation of the alloy were identified by energy dispersive spectrometry and x-ray diffraction. A tensile test machine was used to determine the shear strength of the weld lap joint. The results showed that ultrasonic vibration significantly improved the wetting and spreading ability of Al on the Cu substrate and changed the morphology of the intermetallic compound (IMC) layer. Ultrasound assistance can also significantly break the continuous IMC layer, and some bulk-shaped IMC phases were found in the weld. In studies of the mechanical performance of the weld joint, the shear strength did not increase linearly with ultrasonic power. When the ultrasonic power was 1400 W, the maximum shear strength of the welded joint reached 86.31 MPa, and the shear strength was 67.2% higher than that of the ultrasonic joint made without increasing the power.

In the keyhole variable polarity plasma arc weldng (VPPAW) process at horizontal position, the metal driven by gravity gathered on one side of the molten pool, and the weld formation is difficult, especially for thick workpiece welding. A specially designed experiment to analyze the influence of gravity on weld formation and a novel nozzle structure with side holes was proposed to generate a novel non-axisymmetric side-compressed plasma arc and redistribute arc pressure. The arc shape and pressure distribution were studied, and the ratio of difference for arc pressure in different directions Rp was introduced to evaluate the effects of non-axisymmetric side compression for the plasma arc. The results indicate that the non-axisymmetric distributed side holes reshape the plasma arc both in the EN and EP phases. The pressure of the non-axisymmetric side-compressed plasma arc decreases relatively strongly in one direction (direction b) and relatively weakly in the other direction (direction a). Rp is significant at 1 mm to 5 mm from the arc center, with a relatively large Rp within this range. The compression effect is enhanced with an increase in welding current or plasma gas flow rate, and Rp increases from 24% to 49% as the plasma gas flow rate increases from 2 L/min to 4.5 L/min. Specially designed validation experiments confirm that the new plasma arc significantly affects the weld formation in keyhole VPPAW process. An aluminum alloy workpiece with 8 mm thickness and no groove preparation was welded by the novel plasma arc in a horizontal welding position, and the weld is well formed.

This study comparatively investigated how the metallographic and mechanical properties of Armox 500T armor steel plates joined with Hybrid Plasma Arc Welding (HPAW), Gas Metal Arc Welding (GMAW), and Cold Metal Transfer Arc Welding (CMTW) methods were affected by welding methods. In this context, the Armox 500T welded specimens were examined by radiographic test, optical microscope (OM), Scanning Electron Microscope-Energy distribution spectroscopy (SEM-EDS), hardness, tensile, bending and notched impact tests. OM and SEM microscopy investigations showed that while sufficient fusion occurred between the weld metal and the substrate material in HPAW and GMAW methods. It was determined that the microstructures of the specimens welded with HPAW and GMAW were mostly composed of bainitic structure, in addition to a small amount of martensitic structure while, the microstructure of CMTW welded specimen consist of austenitic, martensitic and partially δ-ferrite structure. Choosing a carbide-forming wire, it was ensured that the hardness values of the weld metal and heat affected zone (HAZ) regions of the specimens combined with all three welding methods were higher than the base metal. The highest tensile strength was obtained in the HPAW welded specimen, while the highest impact notch strength was obtained in the CMTW welded specimen. All of the welded specimens failed within the HAZ and the fracture type is similar to the intergranular fracture type. The mechanical properties results of the specimens combined with three different welding methods have shown that HPAW is the most suitable welding method for joining Armox 500T armor steel.

Pulsed current micro plasma arc welding (MPAW) is one of the most widely used welding processes in sheet metal manufacturing industry. In any fusion arc welding process, the weld bead geometry plays an important role in determining the mechanical properties of the weld and hence quality of the weld. Moreover, the geometry of weld bead involves several simultaneously multiple quality characteristics such as front width, back width, front height and back height, which must be closely monitored, controlled and optimised. This paper presents the optimization of the pulsed current MPAW process by using the grey relational analysis considering the aforementioned quality characteristics. The specific targets are maximum front width and back width, minimum front height and back height. Experiments were performed under different welding conditions such as peak current, base current, pulse frequency and pulse width using Inconel 625 sheets of 0.25 mm thick. A response surface method (RSM)-based central composite design (CCD) experimental design is used to conduct experiments. Optimal welding parameters were determined by the grey relational grade obtained from the grey relational analysis. Optimal results have been verified through confirmation experiments.