Evaluation of Thermal Mechanisms to Predict the Transient Electroplastic Effect in Aluminum and an Investigation of Electrically Assisted Drilling

摘要

The objective of this research is twofold: first, to evaluate if the microscale Joule heating theory can predict the transient electroplastic effect in 7075-T6 aluminum. Second, to determine if electrical application can have a significant impact on drilling of 1500MPa steel, and if the operation is predictable using a modified Merchant's machining model. Both 7075-T6 and 1500 MPa steel are of interest to the automotive industry due to their high strength-to-weight ratios. These metals are important to aid in lightweighting to meet increasingly strict governmental fuel economy standards. However, the strength of the steel makes it difficult to machine in post-forming operations. The ductility of the aluminum makes it impossible to form using conventional methods, especially for deep parts such as a body side outer. A potential fix to these problems is electrical augmentation to locally or globally soften the metal. It has been shown that electricity can increase ductility/formability in metals while also decreasing the forming loads and stresses required (this group of phenomena is termed the electroplastic effect). While the effects of electricity are well known, the underlying mechanisms are not, resulting in four key theories, two of which have already been disproven.;This research examines one of the remaining two theories to predict the transient electroplastic effect. The microscale Joule heating theory suggests that microscale hot spots develop inside of the metal in areas of high electrical resistivity, such as grain boundaries where dislocations pile up during deformation. A coupled mechanical-thermal-electrical model was partitioned with grains, grain boundaries, and precipitates. Temperature and dislocation density-dependent electrical resistivity was used in order to evaluate the microscale Joule heating theory. It was found that this theory cannot fully explain the resultant stress drop caused during the transient phase of electrically-assisted pulsed tension. During model testing it was discovered that electricity changes the strain hardening behavior of aluminum. To further investigate, the effect of electricity on precipitates was explored through measurement of precipitate size and distribution in specimens treated with different electrical treatments.;An electrically-assisted drilling experiment was designed, fabricated, and tested to determine the effect of electricity on a drilling process. A design of experiments study was conducted on 1008 steel to determine if electric current had a significant effect on process temperature, axial force, and tool wear compared to inputs of feedrate and spindle RPM. It was found that current was dominant and that tool wear and cutting forces could be decreased with electric current. The first electrically-assisted drilling model was created by modifying Merchant's machining model. This model was found to have shortcomings due to knowledge limitations on friction and equipment limitations on temperature measurement. The knowledge generated from the 1008 experiments was used to further the constraining limits of the drilling process, leading to 1000% tool life improvement on drilling of 1500 MPa steel while increasing the achievable feedrate for cutting by 200%.