Study On Microstructure And Mechanical Behavior Of Rheo-diecasting Hypereutectic Al-Si Alloys

As the most wildly used light metallic alloy,Al alloy is acknowledged has many advantages,such as low density,light weight,high specific strength,good thermal and electrical conductivity,easy to produce and recycle,etc,which make it comprehensively adopted in aviation,aerospace,automotive,computer,electronic communication and household appliance industries.Al-Si alloy is one kind of the most important Al alloy,its mechanical property,dimensional stability,wear resistance and heat resistance increases with increasing Si content.So,hypereutectic Al-Si alloy,especially high Si Al-Si alloy,are important directions in the development of high strength,wear resistance and high temperature resistance Al alloy.However,the coarse primary Si particles will be obtained as the Si content increased,which severely worsen the mechanical property and limit its widely application.Therefore,it is necessary to develop a new processing technique that can effectively refine the primary Si and increase the mechanical property of hypereutectic Al-Si alloy.High pressure die casting?HPDC?is one advanced liquid molding technique,by which grain refinement and near net shape forming could be achieved.Rheo-diecasting?RD?is considered to be one of the most promising metal molding techniques in 21century,which has taken both advantages of semisolid processing and HPDC,such as easy to obtain excellent microstructure,high productivity,low energy consumption,near net shape forming and low cost,etc.To date,researches about rheo-diecasting of Al-Si alloy mainly focus on eutectic and hypoeutectic Al-Si alloys,references on hypereutectic Al-Si alloy are rare.Researches on hypereutectic Al-Si alloy,especially high Si content Al-Si alloy,are of great prospects in refinement of primary Si,overcoming the defects of liquid casting,achieving excellent microstructure and mechanical property,and providing theoretical guides and experimental datum for the development of rheo-diecasting technique of hypereutectic Al-Si alloys.By means of optical microscope?OM?,scanning electron microscopy?SEM?,transmission electron microscopy?TEM?,X-ray analysis?XRD?and Differential Scanning Calorimetry?DSC?,the present work systematically investigated the effects of processing parameters of the Mechanical rotational barrel?MRB?and the combined treatment of RE modification and rheo-processing of MRB?CTRMR?on the microstructures of the semisolid slurry of Al-15Si-4Cu-0.5Mg and Al-20Si alloys.Then,adopted the optimized rheo-processing parameters to rheo-diecast the Al-15Si-4Cu-0.5Mg and Al-20Si alloys and investigated their microstructures and mechanical properties.Similarly,adopted the optimized the combined treatment of RE modification and rheo-processing of MRB parameters to rheo-diecast the Al-15Si-4Cu-0.5Mg and Al-20Si alloys and investigated their microstructures,mechanical properties and strengthening mechanism.The wear tests were applied to the Al-15Si-4Cu-0.5Mg alloy,in order to investigate the effects of rheo-processing on its wear resistant property and the worn out mechanism.The Flow-3D software was used to simulate the velocity,temperature and composition field of the hypereutectic Al-Si alloy during rheo-processing by MRB,and study the effects of pheo-processing on the nucleation and crystal growth of the primary Si.In order to investigated the rules that MRB processing parameters work on semisolid slurry,the slurry microstructural evolution of the Al-15Si-4Cu-0.5Mg and Al-20Si alloy under different rotational speeds and pouring temperature were studied.Meanwhile,the rules of the combined treatment of RE modification and rheo-processing of MRB that affected the semisolid slurry,the slurry microstructural evolution of the Al-15Si-4Cu-0.5Mg and Al-20Si alloy with 0.5 wt%RE?La,Ce?under different rotational speeds and pouring temperature were studied.The MRB can produce the semisolid slurry of the Al-15Si-4Cu-0.5Mg and Al-20Si alloy in the microstructures of fine primary Si particles,sheared-off Al matrix and the homogeneous minor phases.By the combined treatment of RE modification and rheo-processing of MRB,the average particle size and volume fraction of the primary Si were further lowered,but the roundness remained almost unchanged.The addition of RE?La,Ce?effectively modified the eutectic Si of the hypereutectic Al-Si alloys,the aspect ratio of the eutectic Si was greatl lowered y.The long needle like RE rich phase which is Al₄?FeMnSiCuRE?and Al₂Si₂RE appeared in the Al-15Si-4Cu-0.5Mg and Al-20Si alloy,respectively,but after rheo-processing they were refined and became homogeneous in the matrix.The optimized semisolid slurry preparation parameters of the Al-15Si-4Cu-0.5Mg alloy by MRB and combined treatment are 660 ^oC and 60 r/min.The optimized semisolid slurry preparation parameters of the Al-20Si alloy be MRB and combined treatment are 660 ^oC and 60 r/min.The microstructures of rheo-diecasting Al-15Si-4Cu-0.5Mg and Al-20Si alloy are superior to those cast by HPDC.The morphologies of primary Si changed from plate-like,blocky and unregular block shape to refined blocky shape,with smaller particle size,lower volume fraction and higher roundness.By rheo-diecasting,the dendritic Al matrix,skeletal?-AlFeMnSi phase are sheared off,and the Al₂Cu clusters are homogenized.By the combined treatment of RE modification and rheo-processing of MRB,the average particle size and volume fraction of the primary Si were further lowered and the roundness was increased,meanwhile,the segregated long needle-like Al₄?FeMnSiCuRE?and Al₂Si₂RE were refined and homogenized.The effects of rheo-diecasting and combined treatment on the mechanical properties and solidification characteristics of Al-15Si-4Cu-0.5Mg and Al-20Si alloy were illuminated.By RDC and CTRMR,the Al-15Si-4Cu-0.5Mg poured at 660 ^oC and the Al-20Si alloy poured at 750 ^oC obtained the best mechanical properties.The wear-resistance of liquid cast and rheo-cast Al-15Si-4Cu-0.5Mg alloy were investigated,and their worn out mechanisms were illuminated:In low wear velocity and wear load conditions,abrasive wear of the Al matrix is the dominant mechanism.The improvement of the microstructure of the rheo-casting Al-15Si-4Cu-0.5Mg,such as refined and rounded primary Si,refinement of Al₂Cu cluster and?-AlFeMnSi phase,improved the local resistance to the outer stress which alleviated the damages to the oxidation layer on the sample surface,and finally improve the wear resistance.In high wear velocity and wear load conditions,adhesive wear is the dominant wear mechanism.The coarse primary Si particles and skeletal?-AlFeMnSi phase in the liquid cast Al-15Si-4Cu-0.5Mg alloy are easy to fracture and peel off from the matrix,which improve the oxidation and worn out speed of the matrix.In rheo-cast Al-15Si-4Cu-0.5Mg alloy,the refinement of the primary Si and?-AlFeMnSi phase lower the possibility of breaking off and came out of the matrix,which improve the wear resistance of the alloy.The Flow-3D software was used to simulate the velocity,temperature and composition field of the hypereutectic Al-Si alloy during rheo-processing by MRB.The simulation results showed that the shearing stress from the MRB and the gravity lead to the circular spiral motion of the melt,and the intensive melt convection happened in the melt during the motion.The chilling effect happed to the melt and the composition became homogeneous during rheo-processing by MRB.The effects of the MRB affect the nucleation and crystal growth of the melt during rheo-processing were illuminated:The chilling released the melt's sensible heat instantly,which greatly increased the supercooling and improved the nucleation rate.The released of the latent heat make most of the crystal embryo remelt,and those who stuck to the surface of the barrel obtained continuous supercooling were survived and grew as the nucleus.The highly rotated barrel make the composition and temperature fields of the melt became homogeneous,and the nucleus may be washed off the surface into the melt became the free nucleus and grew up in a globular way.The present work illuminated the effects of rheo-processing on the nucleation and crystal growth of the primary Si.The chilling effects generated by MRB enlarged the supre-cooling of the melt,increased the probability of preexisted Si atoms crash into each other and transform into decahedron and five-fold polyhedron and phase transformation driving force,which activated the nucleation of star-like primary Si and finally result in the large amounts of star-like primary Si particles.The supre-cooling of the melt was increased by rheo-processing,which increased the nucleation rate of octahedral Si.The alloying elements near the edges and corners of the octahedral Si diffuse more quickly by rheo-processing,which promote the corners and edges grow faster,and finally the octahedral grow as the imperfect octahedron Si with hollows inside.The strong shearing stress supplied by MRB may generate dislocation cluster at the areas that locate near the edges and corners which are near to the solid-liquid front.The severe collisions between Si particles and shearing stress generated by MRB may crack the octahedral Si at the{100}planes near the corners and edges,and then the cracks propagate rapidly along the dislocation cluster{110}planes and break the Si particle.The present work illuminated the effects of CTRMR on the growth of primary Si.The chilling effects effectively increased the nucleation rate of the primary Si,and the strong melt convection generated by MRB made more La and Ce atoms absorbed onto the Si growth front along the<112>directions,which greatly improved the impurity induced twinning effect.The La and Ce elements became more homogeneous by rheo-processing,which during solidifying period surround or stick to the primary Si and hinder its further coarsening.The optimized slurry preparation parameter was successfully used to rheo-diecast the compressive cylinder for automobile air conditioner of Al-15Si-4Cu-0.5Mg alloy.The products were excellent in quality and could be heat treated,with much higher compressive ability than conventional high pressure die casting products,which show that the rheo-processing technology of hypereutectic Al-Si alloy is of great values to be applied to the industry production.