A review on the multi-scaled structures and mechanical/thermal properties of tool steels fabricated by laser powder bed fusion additive manufacturing

The laser powder bed fusion(LPBF) process can integrally form geometrically complex and high-performance metallic parts that have attracted much interest,especially in the molds industry.The appearance of the LPBF makes it possible to design and produce complex conformal cooling channel systems in molds.Thus,LPBF-processed tool steels have attracted more and more attention.The complex thermal history in the LPBF process makes the microstructural characteristics and properties different from those of conventional manufactured tool steels.This paper provides an overview of LPBF-processed tool steels by describing the physical phenomena,the microstructural characteristics,and the mechanical/thermal properties,including tensile properties,wear resistance,and thermal properties.The microstructural characteristics are presented through a multiscale perspective,ranging from densification,meso-structure,microstructure,substructure in grains,to nanoprecipitates.Finally,a summary of tool steels and their challenges and outlooks are introduced.

A correlative multidimensional study ofγ’precipitates with Ta addition in Re-containing Ni-based single crystal superalloys

The microstructural evolution in Re-containing Ni-based single crystal superalloys with different Tantalum(Ta)content(2 Ta,5 Ta and 8 Ta in wt%)was investigated.Ta addition significantly affected theγ’precipitate morphology,γ/γ’lattice misfit and microstructural stability during long-term aging.Results showed that the partitioning behaviors of solutes were enhanced by Ta addition,meanwhile,the reversal partitioning behavior of W was triggered which partitioned fromγ’precipitate to matrix.The elemental concentration redistribution caused variations in lattice misfit from positive to negative,the values of lattice misfit were measured to be 0.16%for 2 Ta alloy,then decreased to-0.07%for 5 Ta alloy and negatively increased to-0.23%for 8 Ta alloy.These variations in the lattice misfit were reflected on the transition ofγ’morphology from round-cornered cuboidal shape to cuboidal with sharp corners,accomplished with increasing shape parameter ratioη.Consequently,the optimalγ’shape could be obtained at lattice misfit of approximately 0.3%.Theγ’coarsening investigation at 900℃(up to 2000 h)indicated that Ta addition was beneficial for improving the microstructural stability by reducing the coarsening rate and interfacial energy,accompanied by the enhanced capability of resistingγ’coalescence.By incorporating the calculated interfacial energy,computational modeling,Thermo-Calc and PrecipiCalc,were employed to elucidate theγ’kinetic pathways,the simulation results agreed with experiments,indicating that the model and parameters were reasonable.Additionally,it was found that there was no overlap betweenγ’nucleation and coarsening when theγ/γ’interfacial energy increased to a critical value.