Research On Spray Forming Process Control And High Efficient Utilization Of Overspray Powder

High speed steel(HSS)is extensively used for making cutting tools and heat-and wear-resistant components.Spray forming is a novel rapid solidification and near-net shape forming technique,which makes it possiable for the fabrication of large-section HSS.However,it is difficult to control the spray forming process due to the numerous and interactional parameters.Besides,the overspray powder,the by-product of the spray forming process,significantly reduces the materials utilization.Therefore,multiple models were established for the simulation of spray forming process based on the 4 ton spray forming equipment in HEYE Special Steel Co.,Ltd.The powder metallurgy process for producing HSS from its overspray powder was established.The W6Mo5Cr4V2Co5Nb(825K)HSS was fabricated from its overspray powder.The research establishes the theoretical and technical foundation for the spray forming HSS.The results are as follows.The droplet dynamical model,thermodynamic model and particle size distribution model,the billet shape model and thermodynamic model were established.The billet top surface solid fraction,the billet shape and the billet temperature were calculated using Matlab,Ansys and Thermo-calc softwares.The prediction equation of the billet top surface solid fraction was established as F=Alvgi0.222 R0.19 Z0.40-A2?T,where vgi was the initial gas velocity,R was the gas to liquid mass flow ratio,Z was the deposition distance and ?T was the degree of superheat.When vgi=300m/s,R=0.55,Z=0.5m and ?T=100K,the W18Cr4V billet top surface solid fraction was 71.48%,the nozzles had good atomizing effect,the billet had good shape and microstructure,and the simulation matched well with the fact.The key to the shape control of the billet is the control of the scan trajectory and the substrate pull-down rate.The scanning trajectory is determined by the nozzle scanning frequency and the substrate rotation speed.The scanning trajectory needs to uniformly cover the billet surface,and the track period is preferably in the range of 10-15s.The eccentric distance,the inclination angle and the deposition distance determine the scanning position,the nozzle oscillating angle,the atomization coefficient and the deposition distance determine the scanning range.The substrate pull-down rate needs to be met v?K·SE·Acos?(t),otherwise would result in the overspray aggravation or even spray failure.The established models can be used to optimize the spray parameters of different HSSs and predict the billet state under certain spray parameters.The recommended spray parameters were:for the main nozzle,eccentric distance 0.12m,inclination angle 17°,atomization coefficient 0.0012m/s and 60nm-2,scanning frequency of 3Hz,for the auxiliary nozzle,eccentric distance 0.20m,2 inclination angle 40°,atomization coefficient of 0.003m/s and 150m-2,scanning frequency 3Hz,for the substrate,rotation speed 75rpm,pull-down rate 6.8 X 104m/s,under the parameters,the?550mm and 4ton spray formed 825K billet was produced.The utilization was 92.1%,the billet had relative density of ?98%and oxygen content of 39ppm.The spray formed 825K HSS can achieve hardness of 66.5HRC and bending strength of 3321 MPa after quenching at 1180°C and subsequent triple tempering at 540°C.On the whole,the research promoted the industrialization process of the spray formed high speed steel.Most of the 825K overspray powder is spherical,and the sphericity increases with the decreasing particle size.The microstructure is dendrites,and the carbides are M2C type and MC type carbides.The overspray powder has an oxygen content of 240 ppm and a stable non-metallic inclusion content of 100.93 ppm,wherein Si02 accounts for 91.03 wt%,Al2O3 is 4.66 wt%,and the rest are oxides of Ca,Mg,K,Ti,etc.,after 800? hydrogen reduction,magnetic separation,overspray powder oxygen content ?100ppm,non-metallic inclusion content ?20ppm.825K alloy with a density ?98%can be obtained using two-step sintering process(1200?/120mim?1260?/30min)from the overspray powder.The microstructure is cementite,ferrite,W-and Mo-rich M6C type carbide and V-rich MC type carbide,wherein M6C had particle size less than 5?m and small block shape inside the grain,and the maximum size at the grain boundary was up to 12?m with irregular block or strip shape,the diffusely distributed MC type carbide had particle size less than 3?m.The hardness of the sintered sample can reach 51.8HRC and the bending strength is 2042MPa,which is equivalent to the performance of high speed steel sintered from water atomized powder.After quenching at 1180? and tempering at 540?,the microstructure of the sample is tempered martensite,M6C type carbide,MC type carbide and a small amount of retained austenite.The hardness can reach 64.2HRC and the bending strength is 2858MPa,better than cast CW6Mo5Cr4V3 alloy(64~65.3HRC,2550MPa).