Laser cladding provides advanced surface treatment capabilities for enhancing the properties of components. However, its effectiveness is often challenged by the formation of hot cracks during the cladding process. This study focuses on the formation mechanism and inhibition of hot cracks in a novel cobalt-based alloy (K688) coating applied to 304LN stainless steel via laser cladding. The results indicate that hot crack formation is influenced by liquid film stability, the stress concentration, and precipitation phases. Most hot cracks were found at 25°–45° high-angle grain boundaries (HAGBs) due to the high energy of these grain boundaries, which stabilize the liquid film. A flat-top beam, compared to a Gaussian beam, creates a melt pool with a lower temperature gradient and more mitigatory fluid flow, reducing thermal stresses within the coating and the fraction of crack-sensitive, high-angle grain boundaries (S-HAGBs). Finally, crack formation was significantly inhibited by utilizing a flat-top laser beam to optimize the process parameters. These findings provide a technical foundation for achieving high-quality laser cladding of dissimilar materials, offering insights into optimizing process parameters to prevent hot crack formation.

The preparation of rhenium powder by a hydrogen reduction of ammonium perrhenate is the only industrial production method. However, due to the uneven particle size distribution and large particle size of rhenium powder, it is difficult to prepare high-density rhenium ingot. Moreover, the existing process requires a secondary high-temperature reduction and the deoxidization process is complex and requires a high-temperature resistance of the equipment. Attempting to tackle the difficulties, this paper described a novel process to improve the particle size distribution uniformity and reduce the particle size of rhenium powder, aiming to produce a high-density rhenium ingot, and ammonium perrhenate is completely reduced by hydrogen at a low temperature. When the particle size of the rhenium powder was 19.74 µm, the density of the pressed rhenium ingot was 20.106 g/cm³, which was close to the theoretical density of rhenium. In addition, the hydrogen reduction mechanism of ammonium perrhenate was investigated in this paper. The results showed that the disproportionation of ReO₃ decreased the rate of the reduction reaction, and the XRD and XPS patterns showed that the increase in the reduction temperature was conducive to increasing the reduction reaction rate and reducing the influence of disproportionation on the reduction process. At the same reduction temperature, reducing the particle sizes of ammonium perrhenate was conducive to increasing the hydrogen reduction rate and reducing the influence of the disproportionation.

DZ951G is a novel developed nickel-based directional solidified superalloy with an incipient high melting point and low density. Compared with DZ417G superalloy, DZ951G superalloy has a higher ultimate tensile strength. At intermediate temperatures, the plasticity and strength were both markedly improved, and an obviously anomalous yield behavior could be observed where the yield strength reached its maximum at 760 °C. Below 600 °C, two competitive modes of dislocations shearing γ′ particles existed, in which one was the formation of stacking faults and another was a/2 dislocations shearing. At intermediate temperatures, a transitional phase between shearing γ′ particles and bypassing appeared, and the fracture translated from brittle fracture into ductile fracture. Exceeding 900 °C, bypassing of dislocations was operated under thermal activation. Moreover, short continuous stacking faults still existed at 760 °C. Finally, the various dislocation configurations were rationally illuminated and explained with the intrinsic connection of mechanical properties.

Solidification segregation behavior and homogenization heat treatment processes of a difficult-to-deform superalloy for use at 850 °C were studied. Additionally, the effect of carbon content on homogenization, the thermal treatment process, and pore growth behavior within the alloy were discussed. Our results revealed that Al, Ti, and Nb elements are distributed in the interdendritic space, while W and Mo elements are distributed in the dendrite. There is a significant quantity of γ-γ′ eutectic and MC carbide precipitates in the interdendritic space. Notably, for the alloy containing 0.1 wt% C, a homogenization heat treatment at 1200 °C for 48 h can effectively eliminate the segregation and undesirable phases. As carbon content increased, the γ-γ′ eutectic phases diminished, and the homogenization time decreased accordingly. In this context, pores are smaller and more dispersed, which may enhance alloy forging properties.

Ni-based single crystal(SC) superalloys are preferential materials for manufacturing blades and vanes of gas turbines,due to their superior mechanical properties resulting from the absence of grain boundaries.However,as the component geometry becomes more complex and the content of refractory elements increases gradually,the forming frequency of stray grains increases significantly leading to the component rejection during directional solidification(DS) of Ni-based SC superalloys. This becomes now one of the major problems encountered during DS and single crystal growth.In the interest of saving the actual manufacturing cost,therefore,the alloys with weak tendency of stray grain formation should be first applied.However,there are still no effective method to quantitatively evaluate the stray grain formation ability of a certain Ni-based SC superalloy.Thus,it is quite necessary to design a new method to do so.In this study,the two model samples with different platform geometries are first designed to investigate quantitatively the tendency of stray grain formation,used to summarize the ability of stray grain formation of different alloys and reveal the mechanism of stray grain formation within platforms.The first model sample with the same platform height but length increasing by degrees along the solidification direction,is used to quantitatively characterize the stray grain formation tendency of different superalloys by using its platform length of stray formation occurring first in time.The second model sample with the same platform length but height decreasing by degrees along the solidification direction,is used to quantitatively characterize this tendency by using the platform height of stray formation occurring first in time.The experimental results show that it is easier for stray grains to nucleate and grow within the platform region with either platform length increasing or platform height reducing.The stray grain formation within outside platform is prior to that within inside platform.This tendencies for the first,second and third generation SC superalloys,however,are different:the first is the weakest,following by the second,and the third is the strongest.Furthermore,the formation of stray grains is dominated by undercooling.The melt undercooling at platform edges is larger than within platform insides and increases gradually with either platform length lengthening or platform height reducing.When the undercooling at platform edges exceeds the critical nucleation undercooling,the stray grain would be able to nucleate and overgrow quickly into the platform insides.