Preparation And Properties Of Laser Cladding Layer On Al-Si Alloy

In order to evade the defects of traditional surface treatment technology,such as high-cost,high-pollution,difficult automation and so on,and to solve the prominent problems of low microhardness,poor wear resistance and short service life of parts of cast aluminium alloy,the surface treatment of Al-Si alloy by Nd:YAG solid-state laser is studied in this paper.Preset method and synchronous powder feeding method are adopted.The structure and properties of composite reinforced cladding layer on aluminium alloy surface are systematically studied by means of SEM,EDS,XRD,microhardness tester and wear tester.The formation,dissolution,precipitation and strengthening mechanism of reinforcing phase during cladding are discussed.The main research contents and results are as follows:1.Single-pass cladding layer is prepared for AlSi7Mg by synchronous powder feeding method using Nickel-based Self-fluxing cladding material.It is found that fine NiAl phase,network structure Ni₃Al phase and M₇C₃ phase are formed in the layer,and the maximum microhardness of layer are 780HV.The experiment at room temperature under dry sliding condition showed that the lowest average friction coefficient is obtained when the load is 80N,and the average friction coefficient of the cladding layer range from 0.37 to 0.43.The growth rates and direction of grain in laser cladding pool are discussed.The regularity of grain growth and the mechanism of gradient distribution of grain morphology are revealed.2.Using Al-Ti-C powders as cladding material,multi-pass TiC reinforced layer is prepared on AlSi7Mg in situ by preset method.It is found that TiC with particle sizes of about 1μm and Al₃Ti with particle sizes of about 6-10μm are formed in the layer.The microhardness of the layer is gradient distribution,the highest value is 824HV,and the microhardness of the layer decreased rapidly at 0.25mm below the surface of layer.The distribution of thermal energy density in the layer is discussed and the dilution rate of the cladding layer is calculated.In this experiment,the energy density of the circular spot obeys the Gauss distribution,and the average dilution rate of the layer is 3.5%.3.WC particles are added into Nickel-based Self-fluxing material,and WC/Ni reinforced layer is prepared on AlSi7Mg by synchronous powder feeding method.Marangoni effect is used to reveal the distribution mechanism of WC particles in the layer.AlNi,Al₃Ni,M₇C₃ and M₂₃C₃ phases are formed in the reinforced layer,and the microhardness of the layer is gradient distributed with a maximum value of about1100HV.Dry sliding friction experiments are carried out at room temperature.The wear rate of the layer increased with the increase of load when the load is less than60N.When the load reached 80N,the material of the friction pair shifted and the wear rate decreased.Under the conditions of 20N and 40N,the average friction coefficient basically remains around 0.4;when the load is 60N,the average friction coefficient decreases to the minimum value of 0.137;when the load increases to 80N,the average friction coefficient increases to 0.67.The heat damage of WC in the layer is analyzed and discussed.The mechanism of the heat damage of WC particles in the layer is revealed by melting diffusion damage and collapsing damage respectively.4.The formation mechanism and control of cracks,pore,spheroidization and high dilution rate in laser cladding technology are analyzed and discussed.These studies get the follow conclusions:Cracks are mainly divided into hot cracking and cold cracking.Hot cracking is caused by high undercooling,brittle phase formation,and the difference of thermal expansion coefficient and volume between phases in layer.Cold cracking is mainly caused by the failure of effective release of residual stress.Scientific design of composition of cladding materials and control of morphology,size and distribution of reinforced particles are the ways to improve hot cracking.Heat treatment is the most effective way to improve cold cracking defects.Gas in layer comes from metallurgical reaction and external gas（protective gas,powder gas and water gas in powder）.Appropriate increase of laser power,decrease of scanning speed and proper composition of cladding material are the methods to improve the porosity defect of layer.Spheroidization defects can be divided into two types:metal particle spheroidization in layer and metal spheroidization on the surface of layer.The spheroidization of metal particles in layer is mainly due to the low specific energy,which causes the insufficient heat absorption of metal particles and spheroidization,and affects the performance of layer.Metal spheroidization on the surface of the cladding layer is due to the Leidenfrost phenomenon of metal droplets on the surface of the melt,which solidifies the metal droplets on the surface of the layer and affects the surface quality of the layer.Controlling the specific energy of laser and designing the size and composition of cladding materials can significantly improve the spheroidizing defects.The calculation of dilution rate can be divided into two methods:the calculation of measured components and the calculation of geometric dimensions.The factors affecting dilution rate include laser power,scanning speed,powder feeding rate and composition of cladding layer.In laser cladding technology,low dilution rate laser layer can be obtained by designing laser cladding materials scientifically,choosing low laser power,high scanning speed and high powder feeding rates.Among them,increasing powder feeding rates is the most effective way to reduce the dilution rate.