Research On Microstructure And Properties Of Semi-solid Rheological Squeeze Casting TiB₂/Al-20Si Composite

Hypereutectic Al-Si alloys are widely used in automobile engine parts to achieve light weight due to their excellent abrasion resistance,thermal stability,and low coefficient of thermal expansion.However,due to the existence of coarse and brittle primary Si phases in the microstructure of the hypereutectic Al-Si alloy in the conventional casting condition,the continuity of the matrix is severely severed and the mechanical properties of the material are reduc ed.Therefore,in order to improve the microstructure of the hypereutectic Al-Si alloy and improve its mechanical properties,this paper uses the hypereutectic aluminum-silicon alloy Al-20Si alloy as the matrix and prepares the Ti B₂/Al-20Si composite by a mixed salt reaction method.The composite material is subjected to a semi-solid treatment,and finally the semi-solid slurry is directly extrusion cast.In this paper,the effect of cooling rate on the microstructure size,morphology and distribution in Ti B₂/Al-20Si composites is studied in order to improve the microstructure and mechanical properties of Al-20Si alloy.Based on this,the effects of Ti B ₂ particle content,extrusion rate and heat treatment on the microstructure and mechanical properties of Ti B ₂/Al-20Si composite rheological extrusion castings were studied.Wedge-shaped molds were used to study the size and morphology of the primary Si phase at different cooling rates and the distribution of Ti B ₂ particles in the matrix to evaluate the effect of the semi-solid over-flow cooling pulping technology on the microstructure of composites.The experimental results show that the primary Si phase size is uniformly distributed in the region with a large cooling rate,and the cooling rate is inversely proportional to the primary Si phase size.Ti B₂ particles are distributed in the matrix in the form of particles and agglomerations.Large clusters are moved to a lower cooling rate,and Ti B₂ particles is easily captured by the grains.As the cooling rate is higher,Ti B₂ captured by the primary Si phase.It was found that when the extrusion rate was 24mm/s,the primary Si phase in the Al-20Si alloy rheological extrusion casting had obvious primary Si segregation,and the eutectic Si phase was coarse and long needles.With the increase of Ti B₂particle content,the segregation phenomenon in the casting gradually weakened,the primary Si phase was evenly di stributed,the size became smaller,and the shape was more rounded.The eutectic Si phase changed from a coarse long needle to a short rod.The TEM and SAED results confirmed the existence of?1 2^—1^—0?_{Ti B2}//?111?_Alrelationship between Ti B₂ particles and Al grains,which can be used as an effective nucleation substrate for?-Al phase.It can be seen from HR-TEM that there is no obvious boundary at the interface between Ti B ₂ particles and the primary Si phase,and the bonding is good.When the content of Ti B₂ particles is 6%,the average size of the primary Si phase is 16.5?m and the shape factor is 0.77,which are uniformly distributed,and there is almost no agglomeration and soli d-liquid separation.At the same Ti B₂ particle content,as the extrusion rate increases,the smaller the size of the primary Si phase in the structure,the more uniform the distribution,the fewer defects in the structure,and the higher the density.The results of mechanical experiments show that as the content of Ti B ₂ particles increases,the tensile strength and elongation of the rheological extrusion specimens of Ti B₂/Al-20Si composites are significantly improved.When 6%Ti B₂ was added,the tensile strength of the composite reached 215.7MPa and the elongation reached4.2%,which were 95.4%and 82.6%higher than those of Al-20Si rheological extrusion castings,respectively.By analyzing the tensile fracture of Al-20Si alloy and composite materials,it is found that the fracture forms of the two are similar,and both are mainly brittle fractures.The size of the cleaving platform in the composite material is smaller and the crack propagation path is more complicated.Through heat treatment experiments on Al-20Si and Ti B₂/Al-20Si composites,the microstructure and mechanical properties of the samples before and after heat treatment were observed.After heat treatment,the primary Si phase in the structure is passivated,the needle-like eutectic Si phase is fu sed,and most of the spheroidized dispersion is distributed in the Al matrix,and the alloy matrix is significantly strengthened.When6 wt%Ti B₂ particles were added and subjected to heat treatment,the tensile strength of the material reached 288 MPa,wh ich was 162%higher than the non-heat treated Al-20Si alloy,and the elongation reached 5.8%and 152.1%.