Microstructural Evolution And Control Of Ti-6Al-4V Alloy Produced By Selective Laser Melting

Selective laser melting(SLM),as a flexible powder-bed-based additive manufacturing technology,involves slicing a 3D-CAD model and layer by layer shaping and consolidation of primary powder materials.Also,nearly unlimited geometric freedom is offered through SLM,which realizes the integrated structure,function and manufacturing.Therefore,SLM is hailed as one of the most promising technologies for manufacturing Ti-6Al-4V alloy parts in aviation and aerospace fields with high geometric complexity and high dimensional accuracy.A characteristic and complex heat history involving multiple unsteady reheating and cooling cycles along horizontal and vertical directions is experienced by the solidified parts during SLM due to its process characteristics of point-,line-and layer-wise laser melting and solidification.Obviously,the resulted microstructural evolution and control of SLMed Ti-6A1-4V alloy during such heat history is under an unknown state.Thus,a thoroughly experimental investigation is conducted on the effect of horizontal and vertical thermal cycles as well as complex thermal history on the microstructural characteristics of the SLMed Ti-6A1-4V samples.The microstructural formation and evolution mechanisms of Ti-6A1-4V during SLM complex heat history are developed based on the experimental results and thermodynamic and kinetics calculations.Also,the cellular automaton solidification model is employed to simulate the microstructural evolution process in the as-deposited state for SLM process.Finally,based on experimental result,theoretical analysis and numerical simulation,the control of micro structure was realized by optimizing SLM process and adding post heat treatment in order to explore the method to improve its tensile properties and corrosion resistance.The major conclusions are listed as follows:SLM processing parameters act a significant role in determining the morphology,geometry,melting mode and microstructure of single-track molten pool.With increasing laser energy density,the morphology and melting mode of molten pool experiences a transition from V-shape of keyhole mode to U-shape of conduction mode.And the sizes are gradually reduced with increasing laser energy density.SLMed Ti-6Al-4V molten pools are composed of melting zone(?'),transition zone(?+?),and heat affected zone(?+?A modified threshold for melting mode is proposed by introducing to the SLM processing parameters of layer thickness.As the laser energy density(Ev)exceeds the threshold,i.e.EV?(?),the keyhole mode is obtained,otherwise the conductionmode is present.The peak temperature(TP)and times rather than the direction(horizontal or vertical)of thermal cycle are key influencing factor to microstructural evolution of SLMed Ti-6A1-4V alloy.During SLM horizontal thermal cycle of TP above liquidus temperature(TL),an overlap region with coarser martensite ?' forms in the interior of molten pool with finer martensite ?' The refinement of martensite ?' and improvement of microhardness occur during vertical thermal cycle of TP>TL.Such effects are enhanced by increasing the times of thermal cycles with TP>TL.No obvious effects on microstructure are exhibited by employing thermal cycle of TP<TL.More thermal cycles of TP>TL form along vertical direction,thus the microstructural evolution are more sensitive to vertical thermal cycles than horizontal thermal cycles.The times of horizontal thermal cycle with TP<TL can be calculated from width of molten pool and hatch spacing.The times of vertical thermal cycle with TP<TL can be determined by SLM processing parameters and the melting depth for every laser radiation(dp),i.e.dp =(?)When the sample is subjected to both horizontal and vertical thermal cycles of SLM,the microstructures of Ti-6A1-4V cuboid sample consists of a hierarchical structure of martensite including primary,secondary,tertiary and quartic a' martensites within columnar prior ? grains.The length and thickness as well as aspect ratio of primary and secondary martensites ?' are mainly in the range of 10-70 ?m,1.0-2.0 ?m and 5-40,respectively.Moreover,most martensites ?' show an angle of-30-60 or 30-60° between major axis and building direction.The martensitic size is linear positive correlation with hatch spacing,but shows a quadratic function relationship with scanning speed.Based on the magnitude of peak temperature,the thermal cycles of SLM heat history can be divided into five types including cycle 1 to cycle 5 with TP>TL,Ts<TP<TL,Tp<TP<Ts,Ms<TP<Tp and TP<Ms,respectively.The formation and evolution mechanisms of primary,secondary,tertiary and quartic a' martensites are proposed in SLMed Ti-6Al-4V sample during such five types of thermal cycles.Based on the mechanism,this particular martensitic structure ascribes to processing conditions of multiple thermal cycles during SLM,material characteristics of simultaneously possessing a and ? stability elements and substructure features of containing a high density of dislocations.Based on cellular automate method,a numerical model describing the microstructural evolution process of Ti-6A1-4V alloy during heat history of SLM is successfully developed.Simulation results of the microstructure in single-track,single-layer,thin wall and cuboid samples are in good accord with experimental observations.The three zones(melting zone,remelting zone and reheating zone)of Ti-6A1-4V alloy experiences four stage including powder melting stage,solid-liquid coexistence stage,multiple phase transformation stage and solid-state phase transformation stage during SLM process.Six kinds of evolution occur in the prior ?grain of SLMed Ti-6A1-4V alloy,including(I)disappearance,(?)morphology change,(IV)size increases and then became stable(V)size gradually increases(VI)unchanged.SLM process optimization can promote the formation of a small amount of a lamellae,while adding post heat treatment can control the microstructure including phase constituent,morphology,content,size and substructure.The tensile and yield strengths as well as elongation can reach 1050MPa,978MPa and 11%respectively in the fine duplex mixed microstructure of ?+? under heat treating of 800-900? and followed by furnace cooling.And,its corrosion resistance is superior to that of rolled sample.Therefore,the application potentials of SLMed Ti-6A1-4V alloy are demonstrated in the aspect of tensile properties and corrosion resistance by exploiting an appropriate microstructural control.