A NEW FRICTION TEST PROCEDURE FOR THE IMPROVEMENT OF DRAWING AND SIMILAR PROCESSES

The development of drawing and similar room-temperature metal-forming processes is strongly related to efficient control of the contact and of the friction factors or coefficients. At this time, it is difficult to achieve accurate management of these factors and consequently there is major interest in new friction tests and analyses in most industrial plants. The ratios between the friction stress and the contact pressure in the contact zones between the tools and the workpieces are usually small in modern processes, say in the range of 0.05 to 0.18: these values increase when coatings and lubricants become ineffective and when wear appears on the tools. It is now important not only to secure the lowest friction stress but also to have accurate descriptions of the contact-zone behaviour throughout all the sequences of the forming process. For the above purpose, a new multi-step testing procedure has been developed, which consists of simulating the cumulative deformation of the workpiece contact-surfaces by the use of an inclined indenter which takes the place of the tools. A testing step is achieved with two indenter movements: in the first movement the indenter-the height of which is chosen so as to produce significant localized deformation-moves at the interface in the tangential direction, after which the distance between the initial surface and the deformed surface of the workpiece is typically in the range of 0.2 to 0.8 mm; the indenter then returns to its starting position without contacting the workpiece and moves again along the trajectory as previously defined. Successive steps, as described, are achieved to simulate at the workpiece surface what actually occurs during each forming sequence in the first movement and to measure updated friction-factors or coefficients in the second movement. Tangential forces, normal loads applied by the indenter to the workpiece surface and the temperature of the indenter are recorded during the movements. Thermo-mechanical models of the plastic flow occurring in the vicinity of the indenter are developed for the calculation of accurate friction factors/coefficients, to be implemented in friction constitutive equations. Experiments have been performed on 1017 steel workpieces with a quenched D4 indenter roughly polished to represent a tool damaged by abrasive wear. To date, phosphate coatings and lubricants have been used. Examples of analyses give a scheme for the determination of the mean isotropic friction factor or coefficient, defined for the former as the ration of the tangential stress to the shear yield stress, and for the latter as the ratio of the tangential stress to the contact pressure. It is shown that the proposed procedures and analyses can be used for most industrial workpieces which are drawn or formed in a similar manner. Friction factors/coefficients and new friction constitutive equations, when more accurate interface models are necessary, can thus be obtained. © 1990.