Joshua J. Jones

Electrically-Assisted Forming of Magnesium AZ31: Effect of Current Magnitude and Deformation Rate on Forgeability

Currently, the automotive and aircraft industries are considering increasing the use of magnesium within their products due to its favorable strength-to-weight characteristics. However, the implementation of this material is limited as a result of its formability. Partially addressing this issue, previous research has shown that electrically-assisted forming (EAF) improves the tensile formability of magnesium sheet metal. While these results are highly beneficial toward fabricating the skin of the vehicle, a technique for allowing the use of magnesium alloys in the production of the structural/mechanical components is also desirable. Given the influence that EAF has already exhibited on tensile deformation, the research herein focuses on incorporating this technique within compressive operations. The potential benefit of using EAF on compressive processes has been demonstrated in related research where other materials, such as titanium and aluminum, have shown improved compressive behavior. Therefore, this research endeavors to amalgamate these findings to Mg AZ31B-O, which is traditionally hard to forge. As such, to demonstrate the effects of EAF on this alloy, two series of tests were performed. First, the sensitivity of the alloy to the EAF process was determined by varying the current density and platen speed during an upsetting process (flat dies). Then, the ability to utilize impression (shaped) dies was examined. Through this study, it was shown for the first time that the EAF process increases the forgeability of this magnesium alloy through improvements such as decreased machine force requirements and increased achievable deformation. Additionally, the ability to form the desired final specimen geometry was achieved. Furthermore, this work also showed that this alloy is sensitive to any deformation rate changes when utilizing the EAF process. Last, a threshold current density was noted for this material where significant forgeability improvements could be realized once exceeded.

Investigation of the Machining of Titanium Components for Lightweight Vehicles

Due to titanium’s excellent strength-to-weight ratio and high corrosion resistance, titanium and its alloys have great potential to reduce energy usage in vehicles through a reduction in vehicle mass. The mass of a road vehicle is directly related to its energy consumption through inertial requirements and tire rolling resistance losses. However, when considering the manufacture of titanium automotive components, the machinability is poor, thus increasing processing cost through a trade-off between extended cycle time (labor cost) or increased tool wear (tooling cost). This fact has classified titanium as a “difficult-to-machine” material and consequently, titanium has been traditionally used for application areas having a comparatively higher end product cost such as in aerospace applications, the automotive racing segment, etc., as opposed to the consumer automotive segment. Herein, the problems associated with machining titanium are discussed, and a review of cutting tool technologies is presented that contributes to improving the machinability of titanium alloys. Additionally, nonconventional machining techniques such as High Speed Machining and Ultrasonic Machining are also reviewed. Also discussed are additional factors that need to be considered especially pertaining to the machining of titanium alloys, a crucial one being the non-conformity with standard tool wear models. Subsequently, the results of a controlled milling experiment on Ti-6Al-4V is presented, to evaluate the relationship between certain tool preparation/process parameters and tool wear for a comparison with traditional wear models. INTRODUCTION Titanium is the seventh most abundant metal and the fourth most abundant structural metal in earth’s crust behind aluminum, iron and magnesium. Titanium and its alloys are considered as alternatives in many engineering applications due to their superior properties such as retained strength at elevated temperatures, high chemical inertness and resistance to oxidation. Titanium has traditionally been utilized as a lightweight, very strong and exceedingly corrosion resistant material in the aerospace industry, electric power plants, seawater desalination plants, and heat exchanges. Also, it has been used in industrial applications such as petroleum refining, nuclear waste storage, food processing, pulp and paper plants, and marine applications [1]. Nevertheless, when considering the use of titanium as an automotive component material, there are several conflicting aspects that must be addressed. First of all, the cost of titanium is relatively high in comparison to other common engineering materials such as aluminum, magnesium, and steel. For this reason, it specifically calls for implementation and use only when extreme conditions are to be met, such as in the aerospace industry. The main reason for the increased cost is due to the limited demand from other market segments, thus making the extraction of the titanium ore expensive. Also, the processing costs for converting the ore into commercially usable titanium and its alloys is extensive and requires special processing procedures and involves vast batch production and careful process control, making them difficult to automate. Second, the difficulty in efficiently manufacturing titanium components has a significant adverse effect on processing cost which is mainly due to its low modulus of elasticity and high yield stress. Another manufacturing concern that arises during the machining of titanium is its susceptibility to work hardening during the cutting process and its tendency to react with many cutting tool materials causing substantial tool wear. Additionally, titanium has poor thermal conductivity properties, making heat dissipation a problem, again contributing to higher tool wear. Of primary concern however is the lack of material grade development outside the aerospace industry in which most of the alloys are developed for extreme conditions. This severely limits the currently available grades suited for automotive applications. Thus, a suite of lower strength alloys with properties specially catered for commercial automotive use needs to be developed. This paper examines most of the issues traditionally associated with the machinability of titanium and titanium alloys. As mentioned before, some methodologies and techniques are recommended for mitigating the non-desirable effects during titanium processing and analyzed in more detail, is its unique tool wear characteristics especially in light of manufacturing automotive components. Thus, this study is expected to primarily assist in the reduction of the processing cost of titanium and its alloys for automotive component manufacture. This will help reduce the operating cost of a road vehicle in terms of better fuel economy due to the reduced mass, which in turn translates to better energy efficiency. TITANIUM IN THE AUTOMOTIVE

A Process Comparison of Simple Stretch Forming Using Both Conventional and Electrically-Assisted Forming Techniques

A common manufacturing process typically used to create large surface contours in sheet metal is stretch forming. With this process, the ability to create geometrically accurate parts and smooth surfaces is achievable, yet there are certain limits when considering the achievable elongation of the material and the inability to produce sharp contours in the sheet metal. Present research using Electrically-Assisted Manufacturing (EAM) has shown that applying direct electrical current to the workpiece during the forming process can increase the formability and reduce springback of the material, while also lowering the required forming forces. Seeing the advantageous qualities of EAM, this study examines the use of EAM for a simple stretch forming process. Specifically, this research examines this stretch forming process with regards to how the location where the electrical current is applied to the material affects the process, the achievable forming depth without fracture, and the application direction of the current. Overall results displayed that the directional flow of electrical current and the application location did not affect the obtained forming forces or forming depths using EAM.Copyright

Comparison of Electrically-Assisted and Conventional Friction Stir Welding Processes by Feed Force and Torque

The process of friction stir welding involves high tool forces and requires robust machinery; the forces involved make tool wear a predominant problem. As a result, many alternatives have been proposed in decreasing tool forces such as laser assisted friction stir welding and ultra-sound assisted friction stir welding. However, these alternatives are not commercially successful on a large scale due to scalability and capital/maintenance costs.In an attempt to reduce forces in a cost-feasible manner, electrically-assisted friction stir welding (EAFSW) is studied in this work. EAFSW is a result of applying the concept of electrically-assisted manufacturing (i.e., passing high direct electrical current through a workpiece during processing) to the conventional friction stir welding process. The concept of EAFSW is a relatively new adaptation of conventional frictional stir welding, which is well established. The expected benefits are reduction in the feed force and torque, which allow for improved processing productivity as well as the possibility for deeper penetration of the weld.Copyright

Introduction to Electrically Assisted Forming

Electrically assisted forming (EAF) is a recently introduced metal-forming technique capable of enhancing a metal’s formability during deformation and reducing springback after deformation.

Empirical Modeling of Direct Electric Current Effect on Machining Cutting Force

Metallic materials can be made more ductile and be formed at lower forces through the application of electrical current during deformation, termed Electrically-Assisted Forming (EAF). The current provides a degree of resistive heating, but also facilitates deformation by direct electrical mechanisms (termed the electroplastic effect). It is envisioned that this approach, currently applied to bulk/sheet deformation, could also be used to reduce the flow stress in the deformation zone of the machining shear plane. The objective of this work is to study and model the effect of electric current on forces in machining in order to relate the force reduction to the current level and machining process parameters. To perform this, skiving tests and orthogonal machining tests are performed with varying electrical conditions. It is shown that application of electric current does reduce machining force by up to 60% under certain conditions.Copyright

Constant Current Density Compression Behavior of 304 Stainless Steel and Ti-6Al-4V During Electrically-Assisted Forming

A metal forming technique which has more recently come of interest as an alternative to processes that use elevated temperatures at some stage during manufacturing is Electrically-Assisted Forming (EAF). EAF is a processing technique which applies electrical current through the workpiece concurrently while the material is being formed. At present, this method has only been studied on an experimental level in laboratory settings, and the heuristic results show increased fracture strain, reduced flow stress, and reduced springback; the enhanced process capability is beyond the range that would be expected from pure resistive heating alone. Thus far, when applying the electrical current through the workpiece during deformation, the current magnitude flowing through the workpiece has remained constant. Hence, for a compression loading, the current flux or density decreases as a result of an increasing specimen area. This work examines the effect of a non-constant current density (NCCD) and a constant current density (CCD) on the deformation behavior of 304 Stainless Steel and Ti-6Al-4V during uniaxial compression testing. Additionally, the application of a CCD is used to modify existing empirically-based EAF flow stress models for these materials. From this testing, it is shown that a CCD during forming can significantly reduce the flow stress of the material as compared to the NCCD tests. The reductions in the flow stress were increased at higher strains by approximately 30% and 15% for the 304 Stainless Steel and Ti-6Al-4V, respectively. More importantly, these flow stress curves are better representative of how the material responds to an applied electrical current as the specimen shape change is removed from the results. Also, the NCCD tests were approximated using an existing empirically-based EAF flow stress model and the CCD tests concluded that a new flow stress predictor model be introduced.Copyright

Applications of Electrically Assisted Manufacturing

Within this book, a modeling strategy for the EAF technique is explained for both compression and tension. Both strategies separate the thermal softening effects from the direct electrical effects and thus produce temperature and force profiles for their respective processes. However, in the real world, manufacturing processes are rarely exclusively compression or tension. Therefore, within this chapter, manufacturing processes that can be applicable to EAF will be explained. These include bending, stretch forming, machining, friction stir welding, and miscellaneous other EAF-industrialization research by researchers other than the authors. In addition, this chapter will include experimental EAF findings for compression, tension, channel formation, springback, and various types of forming.

Microstructure and Phase Effects on EAF

This chapter evaluates the microstructure and its impact on EAF. This chapter is divided into three sections where Sect. 8.1 discusses the impact that different starting grain sizes have on the thermal and mechanical profiles of EAF in compression. Section 8.2 evaluates differences in starting dislocation density (from specimen pre-working) and how they impact mechanical EAF profiles. Section 8.3 provides analysis for post-formed microstructure of tensile sheet specimens using statistical methods. To illustrate these concepts, the chapter contains experimental testing and analysis.

The Effect of Electric Current on Metals

This chapter describes the fundamentals behind electroplasticity in metals. Specifically, it focuses on electrical current flow, previous electroplastic theories, and an overall explanation of the electroplastic effect on metals. This overall theory will be supported with experimental results, and electroplastic conclusions will be drawn at the end of the chapter.