| Recently,the weight reduction in materials and structures is urgently needed with the rapid development of aerospace,transportation and other fields.Additive manufacturing(AM)is a competitive technique to meet the increasing demand.AM of Aluminum alloy has been received growing academic and industrial interest,due to the combination of structural optimization design and material light-weighting.However,high-strength aluminum alloys(such as Al-Cu-Mg,Al-Zn-Mg-Cu alloys),developed for traditional manufacturing processes,tend to cracking during AM due to the typical large solidification range,a columnar growth of dendrites and a directional grain structure.These defects deteriorate their mechanical properties severely,which limits the development and application of lightweight and high-strength aluminum alloy components made by additive manufacturing.It is a hot topic in the field of additive manufacturing of high-strength aluminum alloys to inhibit the hot cracking through composition design.In this work,the 2024 pre-alloyed powders were used as raw material.Firstly,the effect of laser powder bed fusion(L-PBF)process parameters on the processability of 2024 Al alloy was investigated,and the cracking mechanism was revealed.Secondly,2024-TiCxBy alloy(2024-TiCB aluminum alloy)was prepared by adding TiCxBy ceramic particles into the alloy by pre-alloying method,and the microstructure and properties of 2024-TiCB alloy fabricated by L-PBF were studied.In order to further improve the yield strength of 2024-TiCB aluminum alloy,1 wt.%Si was added to obtain 2024-TiCB+Si aluminum alloy.The microstructure and properties of the as-fabricated and heat-treated alloy were studied.The main conclusions of this work are as follows:(1)The extremely fast cooling rate and high temperature gradient in L-PBF processing,as well as layer-by-layer deposition characteristics,lead to the formation of coarse columnar grains with epitaxial growth.During solidification,the low melting point liquid phase(rich in Cu and Mg)is enriched at the grain boundaries in the form of liquid channels.At the terminal stage of solidification,intergranular cracking tends to occur under tensile stress due to the poor liquid feeding.The results show that it is difficult to eliminate the hot cracks in 2024 aluminum alloy only by optimizing the L-PBF process parameters.The formation of cracks greatly deteriorates the mechanical properties of the alloy,and the elongation of the L-PBF fabricated 2024 Al alloy is less than 1%.(2)The addition of TiCxBy ceramic particles promotes the grain nucleation during solidification,which not only refines the microstructure but also eliminates hot cracks.Crackfree 2024-TiCB aluminum alloy with grain size of～0.5 μm was obtained.Under the combined action of grain refinement and high-density dislocations,the mechanical properties of the alloy are greatly improved.The ultimate tensile strength,yield strength and elongation of the alloy are 441 ± 12 MPa,404± 12 MPa and 12.8± 1.3%,respectively.After solution treatment,the grain size of the alloy increases to～22 μm.In the subsequent aging process,only a few Al2Cu(Mg)strengthening phases were precipitated,and the effect of aging strengthening was not obvious.The yield strength of the sample after solid solution at 520℃ for 2h and aging at 190℃ for 10 h is decreased to 350±11 MPa.(3)The 2024-TiCB+Si aluminum alloy with Si content of～1 wt.%was obtained by mechanical mixing the AlSi10Mg and the 2024-TiCB powder.The alloy still exhibits good processability.The ultimate tensile strength,yield strength and elongation are 446±4 MPa,406±6 MPa and 13.1±1.4%,respectively,comparable to the as-fabricated 2024-TiCB aluminum alloy.The results show that the addition of a small amount of Si elements can largely affect the precipitation behavior of the alloy.After solution and aging treatment,a large number of θ’phases are well-distributed in the matrix,and thus the yield strength of the alloy increases from 406 MPa to～450 MPa. |
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