Preparation And Property Characterization Of Metallic Bioinspired Functional Surface Processed By Multi-element Coupling Laser Fabrication

Vermicular graphite cast iron(VGCI)and AZ91D magnesium alloy are two typical metallic materials for industrial applications.VGCI exhibits high hardness and good creep resistance,but along with relatively poor mechanical properties and wear resistance,which is the issue to be solved for better application.AZ91D possesses low density,high specific strength and thermal/electric conductivity,but with poor corrosion resistance,wear resistance,and mechanical properties.These poor properties result in obvious obstacle for industrial application.Consequently,inspired by typical structures of tree leaf and shell,this work investigated mechanicalproperties,wear resistance,and corrosion resistance of VGCI and AZ91D,by means of multi-element couling bionic laser surface treatment.The bionic functional surfaces were thereby fabricated on the surface of the two materials,for the purposes of improving the aforementioned properties.Furthermore,the result stability and process controllability of this technique were studied,to probe the promise of this technique in digital smart indudstrial manufacturing.In this work,the effects of this technique on unit morphology(network,stripe and spot),structure,distribution(width and depth of unit),and material characteristics(content gradient,grain size and precipitate)were investigated.The relationships of microstructure and hardness.mechanical properties,wear resistance and corrosion resistance were demonstrated.The strengthening mechanisms behind experimental results were proposed.Main conclusions are as follows:1.Two-element or three element network bionic coupling surfaces were fabricated on the surface of VGCI by means of multi-element coupling laser technique.Two-element or three-element stripe/spot bionic coupling surfaces were fabricated on the surface of AZ91D.Microstructure with great grain refinement was obtained.With decreasing output power and increasing scanning speed,physical size of unit,and content of size of precipitate all were reduced2.For the three-element network bionic coupling surface,the spacing of unit is 2mm.The microstructure was composed of Fe3C,martensite,Al2O3 and ZrO2 ceramic phase.Al2O3 and ZrO2 ceramic particulates do not influence the morphology and physical size of unit.In the range of 2.5wt%-22.5wt%,with the content increasing,the grain size was reduced3.Based on the two-element network bionic coupling surface,the hardness and tensile properties of the sample with the three-element network bionic coupling surface became even higher,by 2%-38%and 3%-16%,respectively.For the three-element network bionic coupling surface of VGCI,with the constant output power and scanning speed,with Al2O3 content increasing from 2.5 wt%to 22.5 wt%,hardness and tensile strength were increased by 71%-121%and 11%-24%,respectively.With ZrO2 content increasing from 2.5 wt%to 22.5 wt%,hardness and tensile strength were increased by 77%-132%and 23%-38%,respectively.4.The wear resistance of the three-element network bionic coupling surface was higher than that of the two-element network bionic coupling surface for VGCI.Grooves were shallower.Wear resistance was improved due to hard phase forming in microstructure Compared with the two-element network bionic coupling surface,the wear resistance of the three-element network bionic coupling surface was increased by 16%-59%.WithAl2O3 and ZrO2 content increasing from 2.5 wt%to 22.5 wt%,were resistance was successively increased by 56%-70%and 61%-81%,respectively5.For the three-element coupling bionic design for AZ91D,the microstructure was compsed of α-Mg and β-Mg17Al12 intermetallic compound.The induced Al did not influence of morphology and physical size of unit.With output power increasing,the grain size was larger,along with coarser β-Mg17Al12 precipitate.Meanwhile,the physical size of unit,such as depth and width,was increased.With scanning speed increasing,the grain size was smaller.Theβ-Mg17Al12 precipitate size was reduced.Meanwhile,the depth and width of unit was reduced too.6.For the two-element coupling laser fabrication of AZ91D,hardness and mechanical properties were both improved.When the output power increased from 1400W to 1800W,the physical size of unit was larger.The hardness and tensile strength were increased by 34%-57%and 18%-26%,respectively.When the scanning speed decreased from 900mm/min to 300/min,the physical size of unit was larger.The hardness and tensile strength were increased by 54%-78%and 21%-36%,respectively.The two-element fabrication and three-element fabrication results exhibited the same trend.It was suggested that with larger physical size of unit,the hardness and tensiles strength were higher.7.For the two-element coupling laser fabrication of AZ91D,wear resistance of AZ91D was greatly improved.When the output power increased from 1400W to 1800W,wear resistance was increased by 65%-71%.When the scanning speed decreased from 900mm/min to 300/min,the wear resistance was increased by 40%-74%.The two-element fabrication and three-element fabrication results exhibited the same trend8.A new concept was introduced:the specific length,for the purpose of characterizing the geometrical complexity of unit.Consequently,the geometrical complexity of unit was able to be calculated accurately.9.In the inner microstructure of unit of AZ91D,the surface potential distribution was very uniform.Galvanic cell was not formed,thus improving corrosion resistance of AZ91D.However,in the boundary microstructure of unit,severe galvanic effect was induced due to the non-uniform surface potential distribution,thus compromising the corrosion resistance of AZ91D.It was demonstrated that,morphology,distribution,area,and specific length of unit were key factors influencing corrosion behaviour of the bionic surface.With area of unit increasing,corrosion resistance was improved.With boundary length of unit increasing,corrosion resistance was compromised.10.Fabrication process parameters for VGCI and AZ91D were optimized.The trend and mechanism of relationship of microstructure,physical size of unit and hardness,mechanical properties,wear resistance,and corrosion resistance were proposed.The result stability and process controllability of this technique were demonstrated,to tailor optimal process for the fabrication of the bionic coupling surface.