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Er/Zr and Sc/Zr were added into 5xxx series alloy to prepare welding wires, and the welding wires with the different compositions were used to weld 6xxx extruded alloy. The effects of different alloying additions on the microstructure and properties of welded joints were investigated. The results show that Al3(Sc1-x, Zrx) particles in the Al–Mg-Sc-Zr alloy welding wire can be used as heterogeneous nucleation site, which can effectively refine the grain structure and reduce the flow of low melting point elements to the PMZ of the joint. Compared with Al–Mg and Al–Mg–Er–Zr welding wire, the grain refinement in WZ is improved by 20.5 % and 13.8 %, and that in FZ is improved by 35.0 % and 8.1 %. At the same time, the refined grain structure can effectively eliminate the stress concentration of the joint and reduce the probability of liquation cracks. The joint welded by Al–Mg-Sc-Zr wire possesses the highest ultimate tensile strength of 249 MPa, while the joints welded by Al–Mg-Sc-Zr wire and Al–Mg wire possess ultimate tensile strength of 159 MPa and 128 MPa, respectively.

The newly-developed Ni–28W–6Cr alloy is the structural material for the next-generation molten salt reactor (MSR). Due to the detrimental effects of welding hot cracks on the quality of welded structures, it is essential to study the hot cracking behavior and mechanism to achieve the qualified Ni–28W–6Cr alloy welded joint. This paper analyzes the characteristics of the dynamic cracking behavior of welding hot cracks using a high-speed camera. The microstructure, morphology, and cracking mechanism of hot cracks are studied. The results show that solidification cracks and liquation cracks occur in the welded joint. According to the propagation direction of the hot crack, there are longitudinal cracks and transverse cracks, respectively. The quantity and length of hot cracks increase with the laser power. The laser welded joint's hardness value fluctuates from 200 HV to 260 HV. The width of heat affected zone (HAZ) is approximately 0.5–1 mm, and there is a slight softening zone in the HAZ. Meanwhile, the hardness of the weld metal is the highest in the welded joint. The solidification cracks tend to initiate and crack along the solidification grain boundary, which is mainly related to the thermal stress caused by the welding process and the weak bonding strength of grain boundaries caused by the element segregation and precipitation at the grain boundary. Liquation cracking is mainly affected by welding thermal stress and element segregation at the grain boundary in the HAZ. However, the eutectic carbides play the role of inhibiting the propagation of liquation cracks.

High-strength Al–Zn–Mg–Cu aluminum alloys are widely used in large-scale aerospace parts. However, high crack sensitivity limits their development during additive manufacturing. The mechanism of crack formation in Al–Zn–Mg–Cu alloy parts manufactured by wire-arc additive manufacturing was studied through a combination of thermal stress simulations and microstructure analyses. Increasing number of deposition layers gradually expanded the partial melting zone, leading to heat accumulation. In addition, the equivalent stress at both sides was considerably higher than that at the center of the parts, and the equivalent stress at the bottom was the highest, which gradually decreased with increasing distance from the bottom. However, in the initial stage of the deposition, the area of the partial melting zone in the bottom was small; the crack sensitivity was low (no cracks were found). With further deposition, liquation cracks occurred 40–60 mm from the bottom and were initiated at sufficient stress and temperature. The increase in the deposition height increased the stress and crack propagation to form a macro crack.

The present study has evaluated the solidification and liquation cracks of GTD-111 superalloy by pulsed Nd:YAG laser techniques. For this purpose, samples were subjected to different temperatures under the conditions of heat treatment (1120, 1200 and, 1230 °C), pre-cooling (−30 and 0 °C), and pre-heating (100 and 200 °C). The Rosenthal equation was used for calculation of mushy zone (MZ), partially-melted zone (PMZ), and heat-affected zone (HAZ). The values of the temperature gradient (G) and welding cooling rate (G × R) were calculated using the Rosenthal equation. The results showed that by reducing the temperature of the sample before welding (from 200 to −30 °C), the G (from 3763.5 to 5821.5 °C/mm) and the G × R (from 31,237 to 48,310.2 °C/s) increased. Experimental test results were in agreement with Rosenthal's numerical results. Microstructural studies have shown that both solidification and liquation cracks are minimized by reducing the pre-cold temperature. This is attributed to the reduction in the segregation of alloying elements and the reduction in the volume of the weld pool. Further studies of the microstructure revealed that there was a correlation between the thickness of the grain boundary (GB) liquid and the formation of cracks. In this way, if the liquation material is from γ–γ′ eutectic and M3B2 boride, increasing the liquid film thickness (LFT) increases the sensitivity to cracking (Eq. (5) in the text), and if the liquation material is from γ′ phase and MC carbide, increasing the LFT reduces the sensitivity to cracking due to increased stress release.

Repair-welding of a cast Mar-M004 superalloy by gas tungsten arc welding was performed. Liquation cracks of the heat-affected zone (HAZ) in a Mar-M004 weldment were closely related to the presence of low-melting constituents along the solidified boundaries in the weld. The metal carbides (MC), M3B2 and M5B3 borides, Ni7(Hf,Zr)2 intermetallic compounds, and γ-γ′colonies were found at the interdendritic boundaries. Fine boride precipitates mixed with intermetallic compounds in lamellar form were more likely to liquate during repair-welding. The melting of borides and intermetallic compounds in 1180 °C/4 h treated samples confirmed the poor weldability of the Mar-M004 superalloy due to enhanced liquation cracking. In addition to boride formation, fractographs of liquation cracks revealed strong segregation of B element in carbides and intermetallics, which might further lower the solidus temperature of the repair weld.

The causes of liquation cracking in the heat-affected zone (HAZ) of a cast Mar-M004 superalloy weld were investigated. X-ray diffraction (XRD), electron probe microanalyzer (EPMA), and electron backscatter diffraction (EBSD) were applied to identify the final microconstituents at the solidification boundaries of the cast alloy. Fine borides and lamellar eutectics were present in front of some γ-γ′ colonies, which were expected to be liquefied prematurely during welding. The metal carbide (MC) enriched in Nb, Hf; M3B2 and M5B3 borides enriched in Cr and Mo; and lamellar Ni-Hf intermetallics were mainly responsible for the induced liquation cracking of the Mar-M004 weld, especially the MC carbides. Scanning electron microscope (SEM) fractographs showed that the fracture features of those liquation cracks were associated with the interdendritic constituents in the cast superalloy.