Optimized Microstructure And Enhanced Mechanical Properties Of Heat Treatment-free Al Alloys

Heat treatment-free Al alloys are widely used because of their efficiency,energysaving and preventing deformation due to the elimination of long-time hightemperature heat treatment processes.The cast Al-Si-Cu and wrought Al-Mn alloys are two typical heat treatment-free Al alloys.However,the α-Al grains with welldeveloped dendritic spacing in the solidification microstructure of Al-Si-Cu alloys are usually coarse,which leads to many defects such as composition segregation,shrinkage and "snow spots".These defects can seriously endanger the uniformity and mechanical properties of the products.In addition,coarse grains in Al-Mn alloy ingots can also lead to thermal cracking phenomena and a variety of surface defects during subsequent deformation.Meanwhile,the lower mechanical properties of the two heat treatmentfree Al alloys under cast service conditions have failed to meet the demand for the preparation of high-performance Al alloys,which seriously restricts the further development and application of the two alloys.Therefore,it is of great practical importance for the refinement of grains,optimization of the solidification microstructure and improvement of mechanical properties.However,Al-Ti-B and AlTi-C master alloys commonly used to refine the grain of Al-Si alloys in industry can occur Si-induced refinement "poisoning" phenomenon,and there are problems of particle agglomeration and instability,resulting in poor refinement.Based on the above problems,a new Al-TCB master alloy(Al-based master alloy enriched with B-doped TiC particles,referred to as TCB)was applied to Al-9Si-3Cu alloy and Al-Mn alloy,and the effects of TCB particles on the refinement behavior,microstructure,extrusion behavior and mechanical properties of the two alloys were systematically investigated herein.On this basis,in order to further enhance the mechanical properties of the two alloys,the microstructure optimization and strengthening behavior of the micro-nano particles(submicron-TCB and nano-AlN)on Al-9Si-3Cu alloy and higher content TCB particles on Al-Mn alloy were researched.The main findings of this paper are as follows:(1)Microstructure refinement and nucleation mechanism of heat treatment-free Al-9Si-3Cu alloyThe new Al-TCB master alloy had excellent refinement effects on the Al-9Si3Cu alloy.The grain size of the Al-9Si-3Cu alloy could be refined to 112.1 μm from 3956.1 μm with the addition of only 0.5%Al-TCB master alloy.Then,the grain size of the Al-9Si-3Cu alloy further decreased as the content of Al-TCB master alloy continued to increase.The refinement limit was reached at 2%addition,where the grain size was about 79.9 μm.In addition,the refinement limit of the grain size in Al-9Si3Cu alloy could also be reached to 79.3 μm using 2%addition of Al-3B master alloy.On this basis,in order to break the refinement limit of a single alloy,Al-TCB master alloy and Al-3B master alloy were used together,both with 2%addition.Accordingly,the grain size of the Al-9Si-3Cu alloy was further refined to 48.1 um.Further study revealed that the efficient nucleation substrate for the synergistic refinement of the two alloys was a composite particle with three layers structure—TCB@TBC@AlB2.From the inside to the outside,they were TCB,TBC(short for Cdoped TiB2)and AlB2 in that order.This nucleation substrate was related to the evolution of TCB particles in the B-rich melt.First,after introducting the TCB particles into the Al-9Si-3Cu alloy melt,the surface of TCB particles preferentially evolved and reacted with B elements to form TiB2,and the newly generated(0001)TiB2 crystal surface tended to grow dependent on the(111)TCB crystal surface.Meanwhile,the C elements released from the TCB evolution were doped into the TiB2 crystal structure.Hence,the core-shell structure of TCB covered by TBC was formed.In addition,TBC had the same crystal structure as AlB2,and the mismatch between the densely arranged atomic planes of both was small.Therefore,TBC could also provide a heterogeneous nucleation substrate for the free B solute in the melt and promoted the precipitation of AlB2.Eventually,a composite particle with three layers structure was formed.It is noteworthy that no Si enrichment was found at the interface between the nucleation substrate provided by the external AlB2 shell and the α-Al matrix,indicating that the complex nucleation particles were immune to the Si refinement "poisoning" effect.(2)Microstructure optimization and properties enhancement of heat treatmentfree Al-9 Si-3 Cu alloy by micro-nano-particlesMicro-nano-particles(submicron-TCB and nano-AlN particles)could significantly optimize the solidification microstructure and achieve simultaneous enhancement of strength and elongation of the Al-9Si-3Cu alloy.The comprehensive analysis shows that the micro-nano-particles were distributed as clusters in the eutectic region of the Al-9Si-3Cu alloy,and the distribution of TCB and AlN particles within the clusters was relatively dispersed.The interfaces between submicron TCB particles and Al matrix were clean.It is worth noting that the nano-AlN particles resulted in a significant modification of eutectic Si by causing the "poisoning" of twin plane reentrant edge growth of Si and restricting the growth of Si crystals.Correspondingly,the morphology of Si transformed from flaky(with a 13.18 μm average length)to granular(with a 2.96 μm average length).Furthermore,the synergy of areas with and without particle clusters was emphasized to address their contributions to the simultaneous improvement of the strength and ductility.When inoculating the micro-nano-particle clusters into the Al-9Si-3Cu alloy,the yield strength,ultimate tensile strength,and elongation were 138 MPa,253 MPa,and 5.8%,respectively,corresponding to increases of 23.2%,23.4%,and 123.1%compared with those of the uninoculated Al-9Si-3Cu base alloy(112 MPa,205 MPa,and 2.6%,respectively).(3)Microstructure evolution and strengthening mechanism of heat treatment-free wrought Al-Mn alloyIn this work,TCB particles were introduced to an Al-Mn alloy with the Al-TCB master alloy.The effects of the TCB particles on the solidification microstructure,extrusion behavior,and mechanical properties of the Al-Mn alloy were systematically studied.The results showed that TCB particles could significantly refine α-Al grains by providing heterogeneous nucleation substrates and hindering the growth of α-grains.For example,when the amount of added TCB particles was 0.5 wt.%,average α-Al grain size could be refined to a minimum of 40.2 μm from 410.1 μm when no particles were added.After hot extrusion,TCB particles and fractured α-Al(Fe,Mn)Si formed streamlines along the extrusion direction.More importantly,TCB particles could inhibit the dynamic recrystallization process during hot extrusion and retain numerous geometrically necessary dislocations.Consequently,the mechanical properties of the Al-Mn alloy at room temperature(25℃)and elevated temperature(350℃)were significantly improved after adding TCB particles.When the amount of added TCB particles was 1.5 wt.%,the ultimate tensile strengths at 25℃ and 350℃ were increased by 25.6%and 32.4%.This work sheds light on the grain refinement and strengthening of heat treatment-free wrought Al-Mn alloys.