Milling Surface and Subsurface Microstructure of Nickel-Based Single Crystal Superalloy DD5
Nickel-based superalloy have been widely used in the key high-temperature components of aeroengines,such as working blades,turbine disks,combustion chambers and so on,due to their excellent oxidation resistance and corrosion resistance,low ther-mal conductivity and good comprehensive mechanical properties in high-temperature environment.Because of the huge industrial val-ue,nickel-based superalloy has attracted people's extensive attention since it came out.After decades of development,nickel-based single crystal superalloy has emerged and become a key material for advanced aeroengine and gas turbine blades depend on its ultra-high strength/hardness and high temperature stability.After more than 40 years of development,people have successively developed five generations of nickel-based single crystal superalloys,and countries all over the world pay close attention to the development of relevant industries.However,nickel-based single crystal alloy is a typical difficult to machine material,which greatly limits its exten-sive industrial application.At present,the research on nickel-based single crystal superalloy is mainly focused on grinding and micro-machining,and some achievements have been made.But there is little research on milling of nickel-based single crystal superalloys.Compared with grinding and micro-machining,milling has the advantages of high efficiency and low cost.Based on this,in order to fill the blank in the research field of milling on nickel-based single crystal superalloys,the second generation nickel-based single crystal superalloy DD5 was selected as the research object in this work.Surface roughness(Ra),surface morphology,and subsurface micro-structure of nickel-based single crystal superalloy DD5 after milling were investigated by single factor experiments using machining center,3D profilometer,3D laser confocal microscope and field-emission scanning electron microscope.The results showed that sur-face roughness of DD5 workpiece after milling first decreased from 2.968 to 2.170 μm and then increased to 2.336 μm with the cutting linear speed(vs)increasing from 15.072 to 22.608 m·min-1.The main reason as that the increase of vs reducing the friction between the workpiece and the tool rake face,and then reducing the cutting force.In addition,with the increasing of vs,the chips were carried away from the machined surface quickly,and the tool concentrated the heat in the cutting area at high speed causing the thermal soft-ening effect.However,the excessive vs would increase the tool shock and then deteriorate milling surface quality.Surface roughness of DD5 work piece after milling increased from 1.893 to 2.981 μm with tool feed speed(vf)increasing from 500 to 1000 mm·min-1.This was due to the moving distance between the continuous cutter teeth increased with the increase of vf,leading to the cutting process prone to vibration.Surface roughness of DD5 workpiece after milling increased from 2.861 to 3.420 μm with cutting depth(ap)increas-ing from 0.05 to 0.15 mm,which was due to the volume of material removal increased and the chips were more difficult to discharge from the cutting area with the increasing of ap,inducing the accumulation of cutting heat.Compared with dry cutting,the use of cutting fluid significantly improved the surface quality of DD5 after milling,which was due to the cutting fluid penetrated between the tool and the machining surface,reducing the friction between the tool and the workpiece,and effectively slowing down the temperature rise in the processing area and decreasing the friction coefficient between the tool tooth and the chip.To sum up,high cutting speed,low feed speed and low cutting depth were beneficial to improve the surface quality after milling,and the use of cutting fluid could significantly alleviate the damage of milled surface.Compared with traditional polycrystalline superalloys GH4169,DD5 had relatively poor surface roughness after milling due to its ultra-high strength,hardness and single crystal characteristics,but would not adversely affect subse-quent grinding and polishing.In addition,the microstructure of DD5 subsurface after milling(vs=20.724 m·min-1,vf=700 mm·min-1,ap=0.1 mm)was composed of plastic deformation layer and work hardening layer.In the plastic deformation layer,γ'was twisted and stretched,y was compressed and narrowed.In the work hardening layer,obvious oxidation phenomenon occurred.The total thickness of work hardening layer and plastic deformation layer in the subsurface of DD5 after milling was about 11 μm,which would not affect the material macroscopic properties.Based on the above research results,it was believed that milling of nickel-based single crystal su-peralloy was effective,feasible and benefit to its extensive industrial application.This work provided a basis for analyzing the milling ability of nickel-based single crystal superalloy,and provided technical support for improving the processing quality.
nickel-based single crystal superalloy DD5millingsurface integritysubsurface
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