KEYWORDS: Interfaces, Aluminum, Titanium, Spatial light modulators, Temperature metrology, Chemical elements, Scanning electron microscopy, Chemical analysis, Thermal effects, Silicon
In this paper, Ti6Al4V/AlSi10Mg multi-material specimens were fabricated by selective laser melting (SLM). The influence of process parameters on the interfacial crack was discussed and the formation mechanism of interfacial crack under different process parameters was expounded through the simulation of temperature field. The microstructure, element distribution, phase composition and microhardness of the Ti/Al interface were investigated. The cooling rate and temperature gradient increased with the increase of laser power and scanning speed, which easily led to the interfacial crack. Using chess scanning strategy and increasing the preheating temperature of the substrate could effectively reduce the cooling rate, so as to reduce the stress and avoid the interfacial crack. There was a good metallurgical bonding between titanium alloy and aluminum alloy, the typical molten pool morphology could be seen at the interface. In the heat affected zone near the interface, the grain size of aluminum alloy became coarsen, because the lower thermal conductivity of titanium alloy and heat accumulation in the process of forming aluminum alloy. There were needle-like intermetallic compounds (IMCs) at the interface. According to the results of SEM and EDS, the thickness of IMCs was about 2-4 μm, and the composition of IMCs was mainly TiAl and TiAl3. The results of XRD showed that there were not only Ti3Al, TiAl, TiAl3 IMCs but also TiSi2 ceramic phase at the interface, which made the microhardness of the interface reached as high as 679 HV.
A 3D transient finite element model of Inconel 718 nickel-based superalloy processed by selective laser melting was developed to study the heat flux in the molten pool under the condition of different process parameters and its effect on the temperature field and the shape of molten pool, in which a solid heat transfer module and a laminar flow module in the finite element simulation software COMSOL Multiphysics was applied. In the simulation several special phenomenon characterized with SLM process was considered, such as the interaction between the laser and material with stochastic porosity distribution on powder bed, and the nonlinear change of thermophysical properties due to the state change of material and the influence of the Marangoni effect in the molten pool. The results show that the Marangoni effect caused by the surface tension of molten pool makes the convective heat flux play a leading role in the heat transfer process in the molten pool, and its depth/width ratio is changed by changing the magnitude and direction of the heat flow, what determines the shape of molten pool. The increase in laser power or reduced scanning speed can increase the heat input per unit time, and then lead to a Marangoni convection enhancement within the molten pool, thus enlarge the size of the molten pool. The experimental results are in good agreement with the simulation results.
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