| Metal sheet forming has high material utilization,high precision and high efficiency,which plays an important role in modern manufacturing.In metal sheet forming process,friction coefficient directly affects the quality of stamping parts and is one of the important parameters of metal sheet forming performance.There are many factors affect friction,the current study on these factors are not sufficient,and friction models regarding these factors are relatively less.Aim at these problems,the paper established a friction model,and the effects of lubricant film thickness,relative sliding speed and coatings on metal sheet forming friction have been studied.The main contents are summarized as follows:Based on adhesion theory,by analyzing the contact state and stress situation of fluid lubrication,boundary lubrication and mixed lubrication and combining friction properties of coated metal sheet,a friction model considering both film thickness and relative sliding speed on the mixed lubrication state was proposed.It is concluded that the friction of actual coating metal sheet forming is the sum of the friction between sheet coating and tool and the friction between sheet coating and lubricants;the friction coefficient could be represented by combination film thickness and sliding speed.By flat sliding friction experiments of CR4 galvanized steel sheet,the paper studied the effects of film thickness and relative sliding speed on friction coefficient.Then friction model considering film thickness and sliding speed was applied to experiment results to obtain specific empirical model,and verified the accuracy of the model.It is concluded that frictioncoefficient decreases with the increase of film thickness and eventually tends to be stable;and in some speed range,it also decreases as the increase of relative sliding speed.The optimum lubrication status can be obtained by the equation between friction coefficient and film thickness.With the increase of film thickness and sliding speed,the percentage of the contact area between mold and sheet coating in the macroscopic contact area decreased,and theoretically the friction force decreases.The effects of CR4 steel sheet galvanized coating,mold chromed coating and different lubricants on the friction coefficient were studied by flat sliding experiments.It is concluded that both galvanized coating and chromed coating make the friction coefficient decrease while the CR4 steel galvanized coating is better.The effect of high viscosity of lubricating is better.Through the above research,the friction model considering lubricant film thickness and relative sliding speed has established,and the effects of lubricant film thickness,relative sliding speed and coatings on CR4 steel sheet forming friction have revealed,which provides basis and data support for selecting friction coefficient in actual production and simulation of metal sheet forming. |
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