Research On Strengthening Mechanism Of High Strength Aluminum Alloy Processed By Laser Selective Melting

Selective laser melting(SLM)is a mainstream technology in metal additive manufacturing(AM),but most traditional Al-Mg alloys are prone to cracking during SLM process,which is difficult to meet the application requirements.The current solution is to add Sc element with excess of eutectic composition(greater than 0.6wt.%)in the alloy,namely microalloying.In this case,Sc and Zr form primary and secondary Al3(Sc,Zr)precipitates,which can inhibit cracking and improve strength.However,the excessive use of Sc elements greatly increases the cost of raw materials.In this work,multi-element microalloying was adopted to reduce the amount of Sc elements.Different aging treatments were performed after SLM forming to regulate the precipitation and microstructure of various secondary phases.Finally,the Sc content is reduced to 0.4wt.%and the tensile strength of SLM-processed Al-Mg alloy is not less than 540 MPa.Besides,its strengthening mechanism of multi-phase reinforcement was elucidated by studying on the relationship between the microstructure and mechanical properties,which provides a beneficial exploration for the development of the additive manufacturing of high-strength Al alloy with low Sc.The main research findings are as follows:For the cracking phenomenon during SLM process,simulation experiments of laser remelting as-cast alloy were used to evaluate the cracking tendency of the alloy.The alloy component suitable for SLM process was selected by analyzing the microstructure and hardness of as-cast and remelted alloys before and after aging.The results show that cracks in the remelted zone mainly propagate along the grain boundaries of columnar grains.By adjusting the content of microalloying elements Sc,Zr,and Er,the molten pool shows an alternating distribution of equiaxed fine grain regions(FG-regions)and columnar coarse grain regions(CG-regions),which effectively suppresses the cracking phenomenon.After laser remelting,coarse primary phases precipitated by supersaturation of as-cast alloys dissolve back into the matrix and the age-hardening effect is improved.The final selected alloy composition is Al-5Mg-0.5Mn-0.4Sc-0.4Zr-0.4Er(wt.%).The influence of laser power,scanning speed and hatch spacing on the surface quality of single-track samples,density,porosity,morphology and hardness of bulk samples were studied,which were used for parameter optimization.The results indicate that the better processing quality of single tracks is obtained when laser power ranged from 340 W to 400 W and scanning speed ranged from 700 mm/s to 1100 mm/s.When the volume energy density is between 115 J/mm3 and 150 J/mm3,the density of bulk samples is not less than 98%.The optimized processing parameters are laser power of 380 W,scanning speed of 900 mm/s and scanning speed of 110 μm.The grain characteristics,composition and distribution of primary phases as well as the heterogeneous nucleation of FG regions were investigated and the formation mechanism of microstructure for the SLM-processed alloy was elucidated.On this basis,the mechanism of fine grain strengthening was revealed by the test of nano-indentation hardness,microhardness and tensile properties.The results show that the microstructure of the SLM-processed alloy exhibits alternating distribution of submicron equiaxed fine grains(～0.66 μm)and micrometer columnar coarse grains(～3.86 μm).FG regions are formed through heterogeneous nucleation of Al2MgO4 and Al3(Sc,Er,Zr)primary phases and the columnar grains are formed by epitaxial growth of fine grains.During plastic deformation,the fine grain results in a higher fraction of grain boundary dislocation sink.At the same time,the heterogeneous microstructure of bimodal grain size distribution promotes the generation of a large number of geometrically necessary dislocations(GND).The pile-ups of GND increases the total dislocation density,so the strength of the alloy is greatly improved.According to Hall-Petch relationship,the contribution of fine grain strengthening is～148 MPa,which is more than three times that of the traditional rapid cooling ingot.The influence of the one-stage aging treatment on phases,microstructure and mechanical properties of the SLM-processed alloy were investigated and the mechanism of precipitation strengthening was elucidated.The two-stage aging treatment was adopted to regulate the coarsening of non coherent precipitates in the one-stage aged alloys,thereby suppressing the intergranular fracture.The results show that the tensile strength of the alloy aged at 375℃ for 4 h can reach 543.5 MPa,which is about 30%higher than the SLM-processed alloy.The coherent Al3(Sc,Er,Zr)phase(size of～4.64 nm)is precipitated dispersedly within grains and its contribution to strengthening can come from coherency strengthening,order strengthening and modulus strengthening when the dispersoids get sheared by dislocations.At the same time,incoherent Al6Mn and ErMn2 phases(size of～101.67 nm)are precipitated both within grains and at grain boundaries.Their contribution to strength originate from Orowan strengthening when these dispersoids get looped by dislocations.However,some incoherent phases of the one-stage aged alloy are prone to coarsening,which results in the intergranular fracture.By using the two-stage aging treatment of 325℃/4 h+375℃/2 h,the coarsening can be suppressed.The elongation of the alloy increases to 14.6%without losing strength(tensile strength of 548.7 MPa).