Peter A. Friedman

Warm forming behavior of high strength aluminum alloy AA7075

Abstract The formability of aluminum alloy AA7075 at elevated temperature was investigated through experiment. Stress–strain relationship at different temperatures and forming speeds were investigated through tensile testing. Deep drawing and stretch formability were also tested through limiting drawing ratio (LDR) and limiting dome height (LDH) tests. Finally, post forming mechanical property testing was conducted to investigate the effects of exposure to warm forming temperatures on the mechanical properties. Results show that deep drawing and stretch formability of AA7075 can be significantly improved when the blank is heated to 140–220°C. At temperature over 260°C, formability and post forming mechanical properties begin to decrease due to the effect of the heating and forming processes on the materials temper.

Microstructural and mechanical investigation of aluminum tailor-welded blanks

The push to manufacture lighter-weight vehicles has forced the auto industry to look to alternative materials than steel for vehicle body structures. Aluminum is one such material that can greatly decrease the weight of vehicle body structures and is also consistent with existing manufacturing processes. As in steel structures, cost and weight can be saved in aluminum structures with the use of tailored blanks. These blanks consist of two or more sheets of dissimilar thicknesses and/or properties joined together through some type of welding process. This enables the design engineer to “tailor” the blank to meet the exact needs of a specific part. Cost savings can be gained by the elimination of reinforcement parts and the stamping dies used to manufacture them. Weight savings can be attained based on the fact that one thicker piece is more efficient than a welded structure and therefore can allow for down-gauging of parts.Although tailor-welded blanks (twbs) offer both potential weight and cost benefits, the continuous weldline and thickness differential in twbs can often result in difficulty in stamping. This problem is more severe in aluminum because of its limited formability as compared with typical drawing-quality steels. Additionally, welding of steel twbs tends to increase the strength of the weld material, which helps prevent failure in the weld during forming. Aluminum twbs do not experience this increase in strength and therefore may have a greater tendency to fail in the weld. In this study, several aspects of twbs manufactured from 6111-T4, 5754-O, and 5182-O aluminum alloys were analyzed and compared with those of a more conventional steel twb. The effect of gauge mismatch on the formability of these blanks is discussed as well as the overall potential of these blanks for automotive applications.

Superplastic response in Al-Mg sheet alloys

The ability to achieve large strains to failure coupled with extremely low flow stresses makes superplastic forming (SPF) an attractive option in the automotive industry for the manufacture of complex parts from aluminum (Al) sheet. However, a barrier to increased usage is the cost penalty associated with superplastic alloys, which are specially processed to have a small and stable grain size. In this article, high-temperature tensile tests are used to compare the superplastic performance of two different Al-Mg alloys that were specially processed for SPF with that of a conventionally processed Al-Mg alloy. The results of the tensile tests and optical microscopy are used to highlight the mechanisms that control deformation in each of these alloys under different test conditions. Failure in both types of materials was found to change from internal cavitation to external necking with increases in strain rate. The specially processed alloys experienced minimal grain growth or grain elongation during forming, and therefore it was assumed that deformation was controlled by grain boundary sliding. Contrary to this, the conventionally processed alloy experienced significant grain growth at the higher test temperatures, and hence it was concluded that deformation was at least partially controlled by some mechanism other than grain boundary sliding. The different deformation characteristics resulted in a different set of optimal forming conditions for the two types of materials. The SPF alloys displayed higher strains to failure at the slower strain rates and higher temperatures, while the conventionally processed alloy displayed higher strains to failure at the faster strain rates and lower temperatures.

Overview of superplastic forming research at ford motor company

In an effort to reduce vehicle weight, the automotive industry has switched to aluminum sheet for many closure panels. Although the application of aluminum is compatible with existing manufacturing processes and has attractive qualities such as low density, good mechanical properties, and high corrosion resistance, it has less room-temperature formability than steel. The expanded forming limits that are possible with superplastic forming can significantly improve the ability to manufacture complex shapes from materials with limited formability. Aluminum closure panels produced by superplastic forming have been used by Ford Motor Company for over a decade. However, applications have been limited to low-volume, specialty vehicles due to the relatively slow cycle time and the cost penalty associated with the specially processed sheet alloys. While there has been substantial research on the superplastic characteristics of aluminum alloys, the bulk of this work has focused on the development of aerospace alloys, which are often too costly and perhaps inappropriate for automotive applications. Additionally, there has been a limited amount of work done to develop the technologies required to support the higher production volumes of the automotive industry. This work presents an automotive perspective on superplastic forming and an overview of the research being performed at Ford Motor Company to increase the production volume so superplastic forming can be cost competitive with more traditional forming technologies.

Formability improvement in aluminum tailor-welded blanks via material combinations

Abstract The use of tailor-welded blanks (TWBs) in automotive applications is increasing due to the potential of weight and cost savings. These blanks are manufactured by seam welding two or more sheets of dissimilar gauge, properties, or both, to form a lighter and stiffer blank. This allows engineers to “tailor” the properties of the blank to meet the design requirements of a particular part. TWBs are used in such places as door inner panels, lift gates, and floor pans. Initial applications of TWBs were for steel alloys, but investigating the potential of using aluminum TWBs is also of interest. One of the problems encountered with stamping TWBs is the difference in load-bearing capacities of the dissimilar sheets that make up the TWB. This can result in a reduction in the formability of the TWB and possibly a movement of the weld from its design-intended location. This paper presents the results of investigating the use of different material combinations to manipulate this type of preferential straining in the TWB in an effort to minimize the movement of the weld line.

Texture development and hardening characteristics of steel sheets under plane-strain compression

Crystallographic texture development and hardening characteristics of a hot-rolled, low-carbon steel sheet due to cold rolling were investigated by idealizing the cold rolling process as plane-strain compression. The starting anisotropy of the test material was characterized by examination of the grain structure by optical microscopy and the preferred crystal orientation distribution by x-ray diffraction. Various heat treatments were used in an effort to remove the initial deformation texture resulting from hot rolling. The plastic anisotropy of the starting material was investigated with tensile tests on samples with the tensile axis parallel, 45°, and perpendicular to the rolling direction. The grain structure after plane-strain compression was studied by optical microscopy, and the new deformation texture was characterized by x-ray diffraction pole figures. These figures are compared with the theoretical pole figures produced from a Taylor-like polycrystal model based on a pencil-glide slip system. The uniaxial tensile stress-strain curve and the plane-strain, compressive stress-strain curve of the sheet were used to calibrate the material parameters in the model. The experimental pole figures were consistent with the findings in the theoretical study. The experimental and theoretical results suggest that the initial texture due to hot rolling was insignificant as compared with the texture induced by large strains under plane-strain compression.

Hot Draw Mechanical Preforming of an Automotive Door Inner Panel

A novel sheet metal forming technology based on aspects of both warm forming and superplastic forming has recently been developed. The new forming process, referred to as hot draw mechanical preforming (HDMP), uses two sequential steps to form a panel within a single tool at elevated temperature. In the first step, the cushion system acts on a binder and upper die to draw the blank over a punch which serves to draw in metal from the perimeter of the blank. In the second step gas pressure is applied to finish the panel details. This two step process of drawing in metal followed by gas forming can result in a significant expansion of the forming envelope for conventional AA5xxx series aluminum sheet alloys commonly used within the automotive industry. Similar to SPF, the HDMP process is performed within a single forming press equipped with heated platens and using gas pressure to shape the component during elevated temperature forming. However, the HDMP process utilizes a blankholder to control the flow of material into the forming cavity during the drawing stage and therefore requires the addition of an integrated cushion system in the bed of the press. HDMP dies are of interest in automotive applications because they maintain the low-investment attributes of SPF tooling while also significantly reducing the forming time as compared to conventional SPF. This work details the CAE based design of an HDMP die to form a one-piece aluminum door inner that can not be formed with conventionally forming processes. Critical aspects addressed in the development of the die include manufacturing targets, part design for manufacturing, and die design for operation at elevated temperature.Copyright

The Warm Forming Performance of Mg Sheet Materials

The warm forming performance of five different magnesium sheet materials was evaluated using a heated pan die. Forming maps were generated to determine the optimum temperature and die conditions for successful forming. All five materials exhibited excellent formability above 325°C, with a robust forming window. Below 300°C, significant differences in the materials were observed. Some materials could be formed into a pan at temperatures as low as 175°C while others could not be formed below 300°C. The differences in forming performance were investigated considering material properties, tribology, and micro structure. These trials included both continuously cast and direct chill cast materials, and demonstrated that the continuously cast materials can be successfully warm formed.

Manufacturing processes for light alloys

Abstract: This chapter highlights the manufacturing processes used to fabricate lightweight automotive parts. The chapter provides an overview of aluminum and magnesium alloys; describes design issues and manufacturing challenges for light alloys; highlights commonly used metal casting and metal forming processes; identifies enablers to significantly increase the use of light alloys in the production of automotive parts; and describes some of the promising metal forming technologies for light alloys.