Study On Strengthening Mechanism And Corrosion Resistance Of Multi-Scale Microstructure TiB₂/Al-9Si-3Cu-0.8Zn Composites

Al-Si-Cu aluminum alloys have been widely used in various fields such as military equipment,aerospace,automotive parts and precision equipment due to its excellent mechanical properties,castability and machinability.However,with the progress of modern society,especially the rapid development of cutting-edge scientific and technological fields such as transportation and aerospace,the research only limited to Al-Si-Cu aluminum alloy itself has been unable to meet the dual requirements of the application environment for its performance and lightweight,which greatly limits the application value of Al-Si-Cu aluminum alloy.Therefore,to improve the strength and toughness of Al-Si-Cu aluminum alloy,the Al-9Si-3Cu-0.8Zn composites reinforced by TiB₂ particles with different mass fraction were successfully prepared from Al-K₂Ti F₆-KBF₄ system by mixed salt reaction method,and the thin-walled shell parts with complex structure were successfully prepared by high pressure die casting.The microstructure,mechanical properties and corrosion resistance of x TiB₂/Al-9Si-3Cu-0.8Zn composites were investigated by XRD,OM,SEM,TEM,tensile test at room temperature and electrochemical test.The microstructure of x TiB₂/Al-9Si-3Cu-0.8Zn composites was observed,and it was found that TiB₂ particles were irregular polyhedral structure and near sphere,with an average size of about 209.7nm.Most of the TiB₂ particles surrounded the grain boundary ofα-Al a clustered structure,whereas a few TiB₂ particles were distributed in the grain boundary.With the increase of TiB₂ particle content,the grain size of the composites decreases gradually,and the size of eutectic Si phase decreases obviously,which changes from long needle shape to short rod shape.After T6 heat treatment,the eutectic Si phase of the composites appeared obvious spheroidization,and the morphology gradually changed to near spherical shape.With the increase of TiB₂particle content,the average size and aspect ratio distribution of eutectic Si decreased from 19.91μm and 10.76 to 9.34μm and 1.21,and the spheroidization effect became more and more obvious.In addition,the Al₂Cu phase dissolved and precipitated,which changed from coarse skeleton to nanoscale lath and flake,and uniformly distributed in the Al matrix.TEM observation and analysis showed that the interface between TiB₂ particles andα-Al matrix was compact and clean without impurities.And the following crystal orientation relationship was found between the TiB₂ and the Al matrix:[-1 1 0 1]_TiB2//[0-1 1]_α-Al,（1 0-1 1）_TiB2//（1 1 1）_α-Al.The semi-coherent relationship between the TiB₂ particles andα-Al matrix was found by calculation of mismatch degree.Therefore,TiB₂ particles can be used as the core ofα-Al heterogeneous nucleation and promote grain refinement ofα-Al.Moreover,it was found that there exists a crystallographic orientation relationship between lath precipitation phase andα-Al matrix,which is[-2-1 2]_Al2Cu//[-1 0 0]_α-Al,（0 2 1）_Al2Cu//（0 2 0）_α-Al.At the same time,there was also a semi-coherent relationship between them,but there was no coherent relationship between flake precipitation phase andα-Al matrix.Therefore,nano-Al₂Cu precipitates exist in the matrix asθ’phase andθphase after T6 heat treatment,andθ’phase as the main strengthening phase.Tensile tests at room temperature showed that the tensile strength of the composites increases first and then decreases with the increase of TiB₂ particle content,while the elongation of the composites decreases continuously.When the content of TiB2 particles reaches 4wt%and after T6 heat treatment,the tensile strength of the composite reached 398MPa,which was 30.9%higher than that of the matrix alloy and63.8%higher than that before T6 heat treatment,but its elongation was only 1.4%.The strengthening mechanisms of composites mainly include load strengthening,CTE strengthening,fine grain strengthening and Orowan strengthening.For4wt%TiB₂/Al-9Si-3Cu-0.8Zn composites,the contribution of the four strengthening mechanisms to the yield strength is 2.7MPa,72.1MPa,3.8MPa and 35.6MPa,respectively.Therefore,the contribution of the strengthening mechanisms to the strength improvement of the composites was as follows:CTE strengthening>Orowan strengthening>fine grain strengthening>load strengthening,and the contribution of fine grain enhancement and load enhancement was very small.In addition,the spheroidization of micrometer eutectic Si phase and the precipitation of nanometer Al₂Cu phase after heat treatment have significant effects on the improvement of mechanical properties of the composites.Electrochemical tests showed that the corrosion resistance of the composites decreases slightly with the increase of TiB₂ content.The reason was that the existence of TiB₂ particles breaked the protective effect of oxide film on composites and TiB₂particles could act as the cathode of corrosion reaction to produce galvanic effect with Al matrix,which further promoted the corrosion of composites.In addition,casting defects such as micropores would become the starting point of corrosion reaction.while TiB₂ particles would increase the number and size of micropores in composites to a certain extent,which would increase the number of corrosion reaction sites and lead to the decrease of corrosion resistance of composites.The 4wt%TiB2/Al-9Si-3Cu-0.8Zn composite was fabricated by high pressure die casting.It was found that the grain size of the composite castings became smaller and smaller with the increase of filling distance.Howerer,the pore size in the castings increased from 3.48μm in the B part to 6.54μm in the D part,which resulted in a significant decrease in the mechanical properties of the D part with the smallest grain size.Therefore,the C part of the casting had the best mechanical properties.The tensile strength and elongation after heat treatment reached 421MPa and 3.0%,which increased by 5.8%and 114.3%respectively compared with that of gravity casting after heat treatment.