Dongyun Zhang

Bio: Dongyun Zhang is an academic researcher from Beijing University of Technology. The author has contributed to research in topics: Selective laser melting & Microstructure. The author has an hindex of 10, co-authored 19 publications receiving 641 citations.

Effect of standard heat treatment on the microstructure and mechanical properties of selective laser melting manufactured Inconel 718 superalloy

Abstract: Inconel 718 superalloy has been fabricated by selective laser melting technology (SLM). Its microstructure and mechanical properties were studied under solution+aging (SA) standard heat treatment, homogenization+solution+aging (HSA) standard heat treatment and as-fabricated conditions. Precipitated phases and microstructures were examined using OM, SEM, TEM and X-ray analysis methods. The fine dendrite structures with an average dendrite arm spacing of approximately 698 nm accompanying some interdendritic Laves phases and carbide particles can be observed in the as-fabricated materials. After standard heat treatments, dendrite microstructures are substituted by recrystallization grains, and Laves phases also dissolve into the matrix to precipitate strengthening phases and δ particles. The test values of all specimens meet Aerospace Material Specification for cast Inconel 718 alloy, and the transgranular ductile fracture mode exists for the three conditions. The strength and hardness of heat-treated SLM materials increase and are comparable with wrought Inconel 718 alloy, whereas their ductility decreases significantly compared with the as-fabricated material. This is because of the precipitation of fine γˊ and γ〞strengthening phases and needle-like δ phases. For the as-fabricated alloy, the formation of finer dislocated cellular structures that develop into a ductile dimple fracture shows excellent ductility. Due to dislocation pinning from γˊ and γ〞strengthening phases and the impediment of dislocation motion caused by the needle-like δ phases, the ductility of the SA materials decreases and causes a transgranular fracture, compared with the as-fabricated samples.

Comparison of microstructures and mechanical properties of Inconel 718 alloy processed by selective laser melting and casting

Abstract: The paper comparatively investigates the microstructures and mechanical properties of Inconel 718 superalloy manufactured by selective laser melting (SLM) and casting. The finite element analysis (FEA) method is used to simulate the temperature fields during SLM and casting processes. Driven by ultra-high temperature gradient and ultra-fast cooling rate during SLM process, the fine grains (average grain size of 48 µm) and dispersed fine precipitation in SLM-ed sample even after HSA (homogenization + solution + aging) and HA (homogenization + aging) heat treatment significantly enhance its mechanical properties, which far exceeds that of casting with average grain size of 1300 µm, and is comparable to that of forging. The microstructure of casting with coarse irregular Laves phases, acicular δ precipitates and globular carbides in the interdendritic zones after HSA heat treatment and some defects existed possibly result in premature failure of tensile samples. The microstructure without δ phases but only some globular carbides in the grain boundary of SLM-ed sample after HA heat treatment possesses higher mechanical properties than that after HSA heat treatment, in which there is only some finer needle-like δ phase and few carbides are precipitated in the grain boundaries. The analysis shows the large amounts of δ phase precipitated in the matrix will deteriorate the plasticity of SLM-ed IN718 superalloy, the appropriate reduction of the δ phase will improve the strength and plasticity of material simultaneously.

Effect of δ phase on high temperature mechanical performances of Inconel 718 fabricated with SLM process

Abstract: The paper studies comparatively on the microstructures and mechanical performances at elevated temperature of SLM-fabricated Inconel 718 alloy in as-build, SHT 980, SHT 1080, SHT 980 + 1080 heat treated conditions. The morphology and distribution of δ phase in the γ matrix are regulated by pre-designed heat treatment schemes, their effects on tensile properties and stress rupture properties at elevated temperature are discussed. The results show that the specimens of SLM-fabricated Inconel 718 alloy in SHT 980 + 1080 heat treated conditions possess the optimum mechanical performances at elevated temperature, in which the morphology and distribution of δ phase in the γ matrix are the key factors in determining their high temperature performances. The excessive δ phases within grains and along grain boundaries can easily lead to dislocation piling up in the process of tensile test at elevated temperature, causing local stress concentration and microcrack generation in the matrix, further initiating premature failure of parts. However, lack of δ phase will reduce the high temperature strength of grain boundaries and then influence the high temperature mechanical properties. As a result, the suitable heat treatment is used to regulate the precipitation of δ phase and the moderate amount of δ phases precipitate along grain boundaries, further improving high temperature mechanical properties of IN718 alloy.

Thermofluid field of molten pool and its effects during selective laser melting (SLM) of Inconel 718 alloy

Abstract: A physical model coupled with heat transfer and fluid flow was developed to investigate the thermofluid field of molten pool and its effects on SLM process of Inconel 718 alloy, in which a heat source considering the porous properties of powder bed and its reflection to laser beam is used. The simulation results showed that surface tension caused by temperature gradient on the surface of molten pool drives to Marangoni convection, which makes fluid flow state mainly an outward convection during SLM process. Marangoni convection includes convective and conductive heat flux, both of them have effects of on molten pool shape, but the effect of convective heat flux is dominant because its magnitude is one order larger than that of conductive heat flux. The convective heat flux accelerates the flow rate of the molten metal, benefits to heat dissipation. The convective heat flux makes the molten pool wider, while the conductive heat flux makes comparably the molten pool deeper and wider. Furthermore, heat accumulation caused by multiple scanning increases convection and conduction heat flux resulting in the increase of the width and depth of the molten pool, but no change of dominant role of convective heat flux to the shape of the molten pool.

Mechanical properties tailoring of topology optimized and selective laser melting fabricated Ti6Al4V lattice structure.

Abstract: This paper investigates the effect of porosity and unit cell size variation in topology optimized (TOP) and selective laser melting (SLM) fabricated Ti6Al4V lattice structure on the mechanical properties including compressive strength, failure mechanism and dynamic elastic modulus. Meanwhile, mathematical relations between mechanical properties and geometric parameters are obtained based on Gibson-Ashby model. The results show that both ultimate compressive strength (σ = 23∼498 MPa) and dynamic elastic modulus (E = 3.5∼55.47 GPa) of TOP lattice structures gradually decrease with the increase in porosity and unit cell size. The analysis combining experimental and numerical results indicates that TOP lattice structures are elastic-brittle porous material and have two failure mechanisms. The numerical predicted stress-strain curves are compared with the experimental ones. The numerical models incorporating the Johnson-Cook damage model could predict the slip direction of 45° failure band and ultimate compressive strength. Classical Gibson-Ashby model was used to predict the relation between relative density and mechanical properties of lattice structures. The exponential factors (n) of fitted models are obviously affected by unit cell size, which are determined by the number of unit cells in compressive test and SLM manufacturability in dynamic elastic modulus test. A 3D Modulus-Density-Unit Cell Size model is innovatively proposed, which can provide theoretical basis of tailoring orthopedic implant filled with functional gradient TOP lattice structures.

Influence of the particle size distribution on surface quality and mechanical properties in AM steel parts

Abstract: Purpose – A recent study confirmed that the particle size distribution of a metallic powder material has a major influence on the density of a part produced by selective laser melting (SLM). Although it is possible to get high density values with different powder types, the processing parameters have to be adjusted accordingly, affecting the process productivity. However, the particle size distribution does not only affect the density but also the surface quality and the mechanical properties of the parts. The purpose of this paper is to investigate the effect of three different powder granulations on the resulting part density, surface quality and mechanical properties of the materials produced.Design/methodology/approach – The scan surface quality and mechanical properties of three different particle size distributions and two layer thicknesses of 30 and 45 μm were compared. The scan velocities for the different powder types have been adjusted in order to guarantee a part density≥99.5 per cent.Findings ...

Microstructure and hardness studies of Inconel 718 manufactured by selective laser melting before and after solution heat treatment

Abstract: The microstructure of Additive Manufactured (AM) Inconel 718 in general and Selective Laser Melting (SLM), in particular is different from the material produced by conventional methods due to the rapid solidification process associated with the former. As a result, the widely adapted standard solution heat treatment temperature (