| The formation of undesirable coarse columnar grains and periodic hot tears（also known as hot cracks）has been a main obstacle in the additive manufacturing（AM）of high-strength aluminum alloys and their nanocomposites.Combined with alloy composition design and AM processing parameter optimization,it is the key to control solidification process to achieve grain refinement and hot tearing suppression via modification treatment（also known as inoculation treatment）.Some nano-particles and microalloying elements are effective inoculants for aluminum.TiC and Ti possess potential nucleation ability and high growth restriction factor,respectively.They are potential components of Al-Ti-C master alloys for grain refinement of cast aluminum alloys.However,TiC and Al3Ti tend to decompose in the Al melt,resulting in"poisoning"or fading of the grain refiners.Therefore,the application of TiC and Ti in grain refinement of AM high-strength aluminum alloys has not been fully studied.Based on the laser powder bed fusion（LPBF）technology,typical 2024（Al-Cu-Mg）and 7075（Al-Zn-Mg-Cu）high-strength aluminum alloys were taken as research objects.The aluminum alloy powders were functionalized by adding a trace of TiC or Ti H2 particles.During LPBF,the aluminum alloy was inoculated by the in-situ reaction of TiC/Ti H2 and Al in the micro molten pool to achieve grain refinement and dispersion strengthening simultaneously.The effects of TiC,Ti H2and their combined grain refiners on the LPBF-printability of high strength aluminum alloys and their composites were studied.The microstructures and mechanical properties were further adjusted by hot isostatic pressing（HIP）and T6 heat treatment.The grain refining mechanism,and strengthening and toughening mechanism were discussed.The 2024 aluminum alloy powders were functionalized with TiC nanoparticles and Ti H2submicron particles respectively.The effects of TiC and Ti H2 on the LPBF-printability of 2024aluminum alloy were studied.The grain refinement mechanisms of TiC and Ti H2 were discussed,respectively.The results show that the laser absorption of aluminum alloy powders is improved by adding TiC or/and Ti H2.The columnar to equiaxed transition during solidification can be triggered by adding TiC or/and Ti H2.Thus,grain refinement and in-situ multiphase hybrid strengthening are achieved simultaneously in the TiC/Ti H2 modified 2024aluminum alloy.In LPBF,TiC-triggered grain refinement can overcome the poisoning of TiC grain refiner during the traditional solidification process.The grain refinement mechanism includes promoting the heterogeneous nucleation ofα-Al by the ex-situ TiC and in-situ L12-Al3Ti nucleats,and restricting theα-Al grain growth by the residual TiC and in-situ lamellar Al4C3 nanoparticles.Ti H2-triggered grain refinement not only comes from the heterogeneous nucleation ofα-Al promoted by in-situ L12-Al3Ti nucleants,but also includes the growth restriction effect induced by Ti solute.Compared with TiC,Ti H2 with the same weight fraction shows more efficient grain refinement.With the content of TiC or Ti H2 grain refiners increasing from 0 to 2.5 wt%,the average grain size of 2024 aluminum alloy decreases by 91%and 94%,respectively.However,due to the introduction of a large number of pores,the strengthening effect of Ti H2 on the mechanical properties is lower than that of TiC.The maximum ultimate tensile strengths（UTS）of TiC/2024 and Ti H2/2024 samples are 409 MPa and 336 MPa,respectively.The yield strength（YS）are 352 MPa and 334 MPa,respectively.And the elongation（El）are approximately 7.9%and 3.0%,respectively.After T6 heat treatment,T phase,in-situ Al18Ti2Mg3 and D022-Al3Ti dispersed phase,G.P.zone andθ’’nano precipitates are precipitated in TiC/2024 and Ti H2/2024.The maximum UTSs of TiC/2024 and Ti H2/2024samples are approximately 507 MPa and 422 MPa,respectively,and the El are approximately6.6%and 4.3%,respectively.To exert the grain refining and strengthening effects of Ti H2 and TiC synergistically,a cooperative grain refining strategy was introduced.It includes heterogeneous nucleation,solute-driven growth restriction and nanoparticle-induced growth restriction effects.A novel grain refiner consisting of TiC and Ti H2 was designed to improve the microstructure and mechanical properties of 2024 aluminum alloy frabricated by LPBF.The results show that crack-free TiC-Ti H2 biphase modified 2024 aluminum alloy with significantly refined grains were successfully prepared by LPBF.To suppress hot tearing,the minimum addition amount of TiC-Ti H2（2 wt%）is lower than that of TiC alone（2.5 wt%）,and the relative density of the as-built sample（95.6%）is higher than that of Ti H2 alone（94.5%）.The in-situ L12-Al3Ti nucleant promotes the heterogeneous nucleation ofα-Al,and the addition of Ti solute restricts the grain growth.The introduction of TiC nanoparticles not only improves the density of the nucleants in the melt,but also pins the grain growth physically.The grains transform from the columnar grains with an average area of 729.0μm2 in 2024 aluminum alloy to equiaxed grains with an average area of 3.4μm2 in TiC（1 wt%）and Ti H2（1 wt%）biphase modified 2024 aluminum alloy（1TiC1Ti H2/2024）.Hot tears are eliminated essentially.Benefited from the elimination of columnar grains and hot tears,as well as the combined effects of refinement strengthening and Orowan strengthening,1TiC1Ti H2/2024 shows excellent mechanical properties.The UTS and El of the as-built alloys increase from 240±10 MPa and 0.3±0.2%of 2024 aluminum alloy to 390±15 MPa and 12.0±0.5%of 1TiC1Ti H2/2024 sample,respectively.The LPBF processing parameters of TiC-Ti H2 biphase modified 2024 aluminum alloy were optimized,and the effects of HIP densification process and T6 heat treatment process on the microstructure and mechanical properties were explored.The results show that1TiC1Ti H2/2024 possesses high resistance to hot tearing,but the relative density decreases from97.0%to 93.3%when the building rate increases from 0.36 mm3/s to 2.16 mm3/s.The resultant UTS and El of the as-built samples reduce from 395±20 MPa and 10.2±2.1%to 347±17MPa and 3.5±1.3%,respectively.By healing pores and controlling precipitates,HIP reduces the property difference between high-speed and low-speed printed alloys.After HIP,the UTS of high-speed（2.16 mm3/s）printing samples is 462±23 MPa,and the El is 12.6±2.4%.After T6 heat treatment,the UTS and El increase to 515±25 MPa and 16.8±1.9%,respectively,which are comparable to that of traditionally forged 2024.The TiC-Ti H2 biphasic modified 7075 aluminum alloy（AA7075）was fabricated by LPBF to verify the universality of TiC and Ti H2 coordinated grain refiners on improving the LPBF-printability of high-strength aluminum alloys.The results show that the columnar grains and hot tears in AA7075 can be eliminated by co-adding（1 wt%）TiC and（0.8 wt%）Ti H2.After T6heat treatment,the precipitation of Al18Mg3（Ti,Cr）2,G.P.zone andη’phases result in excellent UTS of 596±25 MPa,YS of 500±24 MPa and El of 10.0±2.5%.In addition,too high energy density will cause a large amount of burning loss of alloy elements and produce porosity.Nevertheless,too low energy density will produce a large number of non fusion defects due to insufficient energy,which will worsen the mechanical properties.The UTS and YS reach maxium vailues of 616 MPa and 550 MPa,respectively,when the laser power and scanning speed are 300 W and 1000 mm/s respectively,and the corresponding El is 7%.This study may provide a new way to improve the grain structure and mechanical properties of high-strength aluminum alloys and their composites prepared by LPBF. |
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