Micro wire drawing: effect of lubricant and analysis of the mechanical properties of steel and copper wires

Recently, fine wires on the order of 300 to 100 μm in diameter have become popular for mechanical and electrical -applications such as micro springs, micro pins, printer mesh,cutting wire for Electrical Discharge Machining (EDM) and wire for cutting silicon, quartz and other semiconductor materials. The wires are produced by cold micro wire drawing. Recent studies1 have demonstrated that at micro scale the so called “size effect” determines differences in the production processes when passing from the macro to the micro scale. In particular, it has been found that the main differences are related to the material behavior and the friction. As concerns friction, it has been found2 that friction can increase by a factor above 20 and that the conventionally used friction laws cannot incorporate the size effect. Instead, the general Wanheim/Bay friction law can be used to describe friction at micro scale. The material behavior changes with miniaturization, caused by size effects that occur when a process is scaled down from conventional size to micro scale. Scaling down the dimensions of the part as in microforming, the ratio of the grains on the surface layer with the internal grains increases, and according to metal physics theory, stress hardening is reduced. For this reason, when microspecimens are tested in compression or in tension, the increased contribution of external grains to the integral flow stress causes a reduction of the strength of the material as explained by the surface layer model. When the external surface of microparts is not free during deformation, but interacts with the tooling system (as in mirco wire drawing), some experimentations show that the material presents an increased flow stress localized in few tens of microns at the external surface of the part3, which can not be detected by the tensile or compression test. This investigation, after an introduction on the experimental apparatus and the procedure, focuses on the cold drawing of low-carbon steel and commercially pure copper, where the wire diameter is reduced from 300 to 269 μm. Different lubrication conditions have been tested and relevant drawing forces have been monitored. The tensile strength of the drawn wire and the hardness distribution in the cross section has been measured. The drawn wires have been electro-chemically polished to remove their external layer and the resulting wire (the reduced one) has also been subjected to the tensile test. From the comparison between the strength of the drawn wire and that of the reduced wire, the strength of the external layer has been estimated. Finally, a grain size analysis in the cross section of the undeformed and drawn wire has been performed. Differences in strengthening of these materials have been evidenced as well as in the final grain distribution observed in the drawn wires.