Investigation On Microstructure And Properties Of Fe-based Amorphous Nanocrystalline Composite Coating By Laser Cladding

With the development of high-power lasers,Laser cladding technology is a new surface engineering technology that emerged in the 1970s.It is a new technology that combines laser heating and melting,material interaction in the molten pool and rapid solidification.The cooling rate is usually 10²-10⁶ K/s,which can achieve the necessary cooling rate for the formation of amorphous alloys.Therefore,the amorphous cladding layer can be prepared by laser cladding.In this paper,a Fe-based amorphous-nanocrystalline composite cladding layer was prepared by laser cladding on a Q235A low carbon steel surface using a 3 kW semiconductor laser,in an attempt to prepare an amorphous cladding layer with high hardness,high strength and good wear resistance,thereby exploring a new method of preparation of the bulk metallic glass?BMG?.The main research including the following contents.Element adjustment and preparation process adjustment of Fe-Co-Ni-B-Si-Nb amorphous alloy system.Observing the microstructure of the cladding layer by scanning electron microscope?SEM?.Analyzing the evolution and formation mechanism of the phase structure of the cladding layer using X-ray diffraction?XRD?.Characterizing the hardness distribution,wear resistance and strength of the cladding layer by using micro Vickers hardness tester,multi-functional friction,wear tester and universal testing machine.The main results are as follows:First,for the Fe-Co-Ni-B-Si-Nb bulk amorphous alloy system,in order to investigate the effect of Si element content on the amorphous forming ability,microstructure and mechanical properties of cladding layer,a[(Fe_0.6Co_0.2Ni_0.2)_0.75-0.03xB_0.2Si_0.05+0.03x]₉₆Nb₄?x=0,1,2,3,4,5,6?cladding layer was prepared by adding Si element.It is found that the addition of Si can increase the amorphous volume fraction of the cladding layer.When the Si content reaches 10.56 at.%,the amorphous volume fraction of the cladding layer is the highest.Moreover,the addition of Si element will cause the hard phase such as Fe₂B and Fe₃Si to be solidified in the cladding,which significantly enhances the microhardness of the cladding layer,up to 1000-1100 HV.The right amount of Si can improve the wear resistance of the cladding layer,which can reach 15times that of the traditional 45#steel.At room temperature,the addition of an appropriate amount of Si can also increase the compressive fracture strength of the cladding layer up to2880 MPa.Excessive Si element deteriorates the friction and wear properties and fracture strength properties of the cladding layer.Second,studies have shown that the laser power and scanning speed have a great influence on the microstructure and performance of the cladding layer,as the laser power increases,the melting width,melting height and dilution rate of the cladding layer increase gradually,the content of amorphous phase in the cladding layer decreases gradually,the grain size becomes larger,from 13nm to 22nm as the laser power increases from 1.5 kW to 3 kW,and the microhardness is gradually reduced from 1068 HV to 915 HV;As the scanning speed increases,the melting width,melting height and dilution rate of the cladding layer decrease gradually.When the scanning speed is 12 mm/s,the dilution rate of the cladding layer is 8.43%.The increase of the scanning speed can effectively control the grain growth in the cladding layer,thereby facilitating the formation of amorphous,and the size of the nanocrystal grain size in the cladding layer is also reduced,the grain size is 12 nm when the scanning speed is 12 mm/s,so that the microhardness is improved.Third,the laser remelting process can make the surface of the cladding layer more uniform,and the macroscopic forming surface is smoother.The increase in the remelting rate reduces the depth of the remelted layer.The remelting process has a significant effect on the hardness of the cladding layer,with an average increase of 100-200 HV.Finally,this paper successfully prepared Fe-based amorphous-nanocrystalline cladding layer with a certain amorphous volume content,which not only has good macroscopic shape since it can be metallurgically bonded to the substrate,but also its crystallinity is well controlled.The amorphous region in the middle area of the cladding layer is more than 40%,and the nanocrystal grain size is 12 nm.In summary,a relatively high amorphous FeCoNiBSiNb amorphous-nanocrystalline composite cladding layer is prepared by using a 3 kW semiconductor laser and the cladding layer has high hardness,good wear resistance and excellent fracture strength.