Compositional Design,microstructure And Mechanical Properties Of High Strength Aluminum Alloy Prepared By Selective Laser Melting

Because of its excellent physical,chemical and mechanical properties,aluminum alloys have been widely used in aerospace,manufacturing,transportation,electronics,defense and military fields.Nowadays,the workpieces of industries such as aerospace,transportation,and defense industry are developing towards the trend of light weight and integration.However,the traditional forming method has a long cycle and it is technically difficult to meet the development needs,so it is urgent to develop new forming methods.Addictive Manufacturing technology can achieve the accurate forming of complex three-dimensional structures,and obtain the structure and performance that are not available in traditional forming methods.At present,most commonly addictive manufacturing technology used in aluminum alloy is selective laser melting(SLM).The alloy system which can adapt this technique is very limited,and most of them are AlSi10Mg series and derivative alloys.Due to the unique high cooling speed and complex forming process of SLM technology,many traditional high-strength aluminum alloy(such as Al7075)will form defects such as thermal cracks during SLM process,making it difficult to form complex structural parts.In view of the above problems,this paper has carried out research on the composition design,microstructure and mechanical properties of new high-strength aluminum alloys for SLM.In this paper,binary and ternary alloys such as Al-Tm(Fe,Ni)-(Zr)are used as the research system.Alloy powder is prepared by gas atomization technology,and materials are formed by SLM technology.X-ray diffractometer(XRD),scanning electron microscope(SEM&EBSD),transmission electron microscope(TEM)are used to characterize their structure,and the microhardness,tensile properties at room temperature,tensile properties at high temperature,fracture toughness of different alloys are tested systematically.The effects of the components in the SLM formed aluminum alloy on the structure and mechanical properties are studied.The study finds that the Al-Fe binary alloy near the eutectic point component has good fluidity,and a compact,crack-free aluminum alloy sample can be prepared by SLM technology.Low-solubility elements in traditional aluminum alloys,such as Fe and Ni,which are difficult to strengthen,can form nanoscale(60-200nm)cell structures during SLM forming.The formation of this cellular structure is due to the non-equilibrium solidification of the molten pool,and the solidification front produces dendrite-like or cell-like growth under large temperature gradients and extremely fast cooling rates.As the Fe content increases(1-5.9at%),the formability of SLM Al-Fe binary alloy decreases and the strength increases.Especially,the microhardness of AlFe_2.5 alloy in this study reaches Hv172,the yield strength at room temperature reaches 400MPa,and the tensile strength reaches434MPa,which is significantly higher than the hardness(Hv130),yield strength(268MPa)and tensile strength(378MPa)of SLM AlSi₁₀Mg alloy.In addition,the room temperature tensile plasticity of AlFe_2.5 is 1.72%,and the notched fracture toughness(Kq)is 19.5MPam^1/2,which shows good toughness and plasticity.The strengthening mechanism of the new SLM Al-Fe alloy is mainly derived from the hindrance of dislocation slippage by the cell structure,which is consistent with the Hall-Petch relationship.In order to further refine the grains and improve the toughness,based on the above-mentioned Al-Fe binary system,Zr is introduced as a nucleation element,and the microstructure and mechanical properties of the SLM Al-Fe-Zr ternary alloy are systematically studied.The study finds that Zr element has two main effects:one is to precipitate the primary Al₃Zr particles as a nucleating agent to refine the grains;the other is to precipitate the Al₃Zr precipitation phase to produce a precipitation strengthening effect.EBSD test results show that the grain size of the SLM Al-Fe-Zr ternary alloy is about 20?m,which is 20%of the SLM Al-Fe binary alloy.By optimizing the composition of the Zr content,the designed SLM AlFe_2.5Zr_0.3 alloy has a room temperature yield strength of 405MPa,a tensile strength of 450 MPa,and a plasticity of 2.28%;while the SLM AlFe_2.5Zr₁alloy has a room temperature yield strength of 426 MPa and a tensile strength of 523MPa.In addition,the ternary system also has good high temperature strength.For example,the yield strength of SLM AlFe_2.5Zr_0.3 alloy at 315? is 215MPa,which is much higher than that of SLM AlSi₁₀Mg alloy(?70MPa).The in-situ notched fracture toughness test show that the fracture toughness Kq value of SLM AlFe_2.5Zr_0.3 alloy exceeds 35.7 MPam^1/2,which is much higher than that of SLM AlSi₁₀Mg alloy(Kq=27.6MPam^1/2).According to the analysis,the fracture mechanism is that the cracks propagate along the weak zone of the weld pool boundary,while the equiaxed grain zone of the SLM Al-Fe-Zr ternary alloy bath pool makes the cracks tortuously expand,thereby improving the fracture toughness.Using similar composition design concepts,we have also developed a new SLM Al-Ni-Zr ternary alloy that exhibits high strength(247MPa yield strength)and good plasticity(?4%).Through reasonable heat treatment,the mechanical properties of SLM Al-Ni-Zr can be further improved.So this study can provide theoretical guidance for the development of new high-strength and tough aluminum alloys for additive manufacturing.