In order to reduce the welding heat input of low carbon steel and optimize the welding deformation and other defects, the triple-wire indirect arc welding (TW-GIA welding) method was used, and the heat transfer mechanism of TW-GIA was discussed based on the infrared temperature measurement system and high-speed imaging measurement results. The results show that TW-GIA welding has a significant advantage of small heat input, and can realize single-pass welding without obvious welding deformation. The width of the heat affected zone was smaller than that of MIG welding. Unlike traditional arc welding, the heat input of TW-GIA welding gradually decrease with the increase of welding height. Heat transfer mode changed from arc heat convection + molten pool heat conduction to only molten pool heat conduction when the welding height increased from 4 mm to 24 mm. The grain size of TW-GIA fusion zone and heat affect zone was small, and the widmannstatten structure was eliminated. The average microhardness of TW-GIA weld zone reached 289.8 HV, the maximum tensile strength was 472 MPa, and the elongation was 26.5%.

Under the action of welding thermal cyclic load, the internal peritectic structure of austenitic stainless steel weld is coarse columnar crystal, and its orientation is anisotropic in different areas of the weld. The nondestructive testing of austenitic stainless steel welds is conducted using method of ultrasonic array for the ultrasonic scattering caused by coarse columnar crystals and the sound beam deflection caused by different grain orientations. A signal analysis method based on decomposition of the time-reversal operator is developed for noise reduction processing of ultrasound full matrix data. Using ray tracing method, the ultrasonic wave propagation path determination method in anisotropic media was investigated and applied to the correction of beam deflection for ultrasonic array total focus imaging of austenitic stainless steel welds. Nondestructive testing of austenitic stainless steel welds was conducted using ultrasonic array, and the results showed that the time-reversal operator-based decomposition method was effective for the suppression of scattering noise in the detection signal and highlight the echoes from defects, which can improve the signal-to-noise ratio of full-focus imaging by 10 dB, moreover the beam deflection correction can improve the accuracy of defect localization in total focus imaging of ultrasonic arrays.

Thermal-mechanical simulation of direct energy deposition (DED) is an effective method to predict the residual stress and deformation and optimize deposition process parameters. The simulation accuracy depends on input parameters which are difficult to be directly and accurately measured in most cases. This paper, taking heat source parameters as examples, proposes an inverse identification method based on support vector machine and genetic algorithm, and the inversely-identified heat parameters are further used to improve the thermal-mechanical model accuracy of real DED applications. Firstly, simulation errors of simple single-track deposition models under different heat source parameters are obtained. Secondly, the relationship between the heat source parameters and simulation errors is established using support vector regression, and the optimized initial heat sources are identified using genetic algorithm. Thirdly, a forward-inverse closed loop is applied to narrow the ranges of heat source parameters for more precise parameter identification. Finally, a thermal-mechanical model for a turbine blade using the optimal parameters is constructed to further verify the proposed method. The results show that the optimal heat source parameters extracted from single-track deposition models can be extended to accurately predict the thermal and mechanical results of a turbine blade DED case, which provide a practical method for DED process optimization (e.g. distortion compensation) in industry applications.

3 mm thick 7050-T7451aluminum alloy joint was obtained by friction stir welding, and the two-stage aging treatment was performed at 121 ℃×5 h + 163 ℃×27 h after welding. Microstructure, hardness profiles and stress corrosion sensitivity of the joint were measured and studied. The results indicate that through the two-step aging, the grain size is coarsened, the age-hardening precipitates and PFZ become wider at the same time, which results in the discontinuous grain boundary; the microhardness of the FSW joints decreased, but the heat-affected zone significantly narrowed, which increased the uniformity of the microhardness of the FSW joints; and the two-stage aging effectively reduced the stress corrosion sensitivity of the FSW joints. The joints with aging treatment were not broken after 60 days, however all the joints without aging treatment were broken within 1 day.

Dissimilar material friction welding process between TiAl alloy and GH3039 was studied. Based on the tensile properties of the joint, the welding parameters were optimized preliminarily. The microstructure and mechanism of the welded joint and the composition change of the weld zone were analyzed by scanning electron microscopy (SEM) and transmission electron microscopy (TEM). The results indicate that the friction welding of dissimilar material between TiAl alloy and GH3039 is feasible, under the action of thermo-mechanical coupling in friction welding process, the plastic deformation of thermo-mechanical affected zone on the side of GH3039 is larger than that of TiAl alloy; The alloy elements on both sides of the friction welding interface between TiAl alloy and GH3039 have diffused, a complex organizational structure of the layered intermetallic compounds has been formed. The fracture of welded joint broken in TiAl alloy base metal belongs to typical brittle fracture.