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.     

Effects of temperature and blank holding force on biaxial forming behavior of aluminum sheet alloys

Biaxial forming behavior is investigated for three aluminum sheet alloys (Al 5182 containing 1% Mn (5182+Mn), Al 5754, and 6111-T4) using a heated die and punch in the warm forming temperature range of 200–350 °C. It is found that, while all three alloys exhibit significant improvement in their formability compared with that at room temperature, the non-heat-treatable alloys 5182 + Mn and 5754 give higher part depths than that of heat-treatable 6111-T4. The formability generally increases with decreasing BHP (BHP), but increasing the forming temperature and/or BHP minimizes the wrinkling tendency and improves the forming performance. The stretchability of the sheet alloys increase with increasing temperature and increasing BHP. For the alloys and forming conditions involved in the current study, the formability, measured in terms of part depth, comes mainly from the drawing of metal into the die cavity, although stretching effects do influence the overall forming behavior. The optimum formability is achieved by setting the die temperature 50 °C higher than the punch temperature to enhance the drawing component. Setting the die temperature higher than the punch temperature also improves the strain distribution in a part in such a manner that postpones necking and fracture by altering the location of greatest thinning.     

Effect of aluminium sheet surface conditions on feasibility and quality of resistance spot welding

A study investigating the effect of sheet surface condition on resistance spot welding (RSW) of aluminium has been carried out. This concentrates on two automotive aluminium alloys; AA5754 and AA6111, used for structural and closure applications respectively. The results show the marked effect that surface condition can have on the RSW process. For AA5754 sheet incomplete removal of a ‘disrupted surface layer’ prior to surface pretreatment is shown to have a detrimental effect on the RSW process. The solid wax lubricant used to assist metal forming leads to unpredictable changes in contact resistance, and consequently affects the process stability. For AA6111 closures the final surface topography can influence the RSW process. Standard ‘mill’ and electro-discharge textured (EDT) finish sheet surfaces were examined and preliminary results suggest that both are suitable for welding. The successful application of RSW of aluminium sheet requires careful consideration of the sheet surface condition. This requires close collaboration between material suppliers and automotive manufacturers.     

Improvement of formability of 6xxx aluminum alloys using incremental forming technology

Aluminum sheet is becoming increasingly common as an automotive body panel material. The heat-treatable aluminum alloys of the 6xxx series are widely used as an outer panel material, due to their ability to precipitation harden during the paint-bake cycle, resulting in improved dent resistance. Increasing the formability of these alloys would allow for multiple parts of less complex geometry to be combined into a single more complex part, thereby avoiding the costs associated with any subsequent joining operations. Incremental forming is a process that can improve material formability through the use of short, recovery heat treatments applied between increments of deformation. The objective of this study was to investigate the incremental forming behavior of 6111-T4 an alloy, which is often used for exterior body panel applications. Interrupted tensile testing was used to simulate the incremental forming process. The effect of different heat-treatment parameters on mechanical properties was analyzed. The heat treat regimen developed for uniaxial testing was then applied to a series of plane strain tests using a hemispherical punch, to simulate the more complex states of stress found in forming operations.     

Using incremental forming to calibrate a void nucleation model for automotive aluminum sheet alloys

A novel method for the quantification of void nucleation rates in sheet material is presented. An incremental sheet forming process is employed to create large regions of homogeneous deformation, such that material density changes can then be used to quantify the evolution of void volume fraction with applied strain. This technique is employed to calibrate the void nucleation behaviour of three automotive aluminum sheet alloys (AA5182, AA5754 and AA6111) for incorporation into finite element method models which employ the Gurson–Tvergaard–Needleman (GTN) constitutive softening equations.     

Numerical Investigations on the Influence of Superimposed Double-Sided Pressure on the Formability of Biaxially Stretched AA6111-T4 Sheet Metal

Lightweight materials have been widely used in aerospace, automobile industries to meet the requirement of structural weight reduction. Due to their limited plasticity at room temperature, however, lightweight materials always exhibit distinctly poor forming capability in comparison with conventional deep drawing steels. Based on the phenomenon that the superimposed hydrostatic pressure can improve the plasticity of metal, many kinds of double-sided pressure forming processes have been proposed. In the present study, the Gurson-Tvergaard-Needleman (GTN) damage model combined with finite element method is used to investigate the influence of double-sided pressure on the deformation behavior of biaxially stretched AA6111-T4 sheet metal, including nucleation and growth of microvoids, evaluation of stress triaxiality, and so forth. The Marciniak-Kuczynski (M-K) localized necking model is used to predict the right-hand side of the forming limit diagram (FLD) of sheet metal under superimposed double-sided pressure. It is found that the superimposed double-sided pressure has no obvious effect on the nucleation of microvoids. However, the superimposed double-sided pressure can suppress the growth and coalescence of microvoids. The forming limit curve (FLC) of the biaxially stretched AA6111-T4 sheet metal under the superimposed double-sided pressure is improved and the fracture locus shifts to the left. Furthermore, the formability increase value is sensitive to the strain path.     

Material Formability and Coil Design in Electromagnetic Forming

Pulsed electromagnetic forming is based on high-voltage discharge of capacitors through a coil. An intense transient magnetic field is generated in the coil and through interaction with the metal work-piece; pressure in the form of a magnetic pulse is built up to do the work. Data on formability of two aluminum alloys employed for exterior (6111-T4) and interior (5754) automotive body panels will be shown. Comparison of traditional Forming Limit Diagrams obtained by stretching of aluminum sheet with hemispherical punch to the results on formability, where hemispherical punch is replaced by a coil will be provided. It will be shown that material formability in high-rate forming conditions can significantly depend upon interaction with the forming die: electromagnetic forming into an open round window provides only slight improvement in formability, while forming in a V-shape die or into a conical die indicates a significant improvement. An important part of the electromagnetic forming technology is the design of the coil. The coil failure modes and measures preventing them are discussed. This article was presented at Materials Science & Technology 2006, Innovations in Metal Forming symposium held October 15-19, 2006 in Cincinnati, OH.     

Mechanism investigation of friction-related effects in single point incremental forming using a developed oblique roller-ball tool

Single point incremental forming (SPIF) is a highly versatile and flexible process for rapid manufacturing of complex sheet metal parts. In the SPIF process, a ball nose tool moves along a predefined tool path to form the sheet to desired shapes. Due to its unique ability in local deformation of sheet metal, the friction condition between the tool and sheet plays a significant role in material deformation. The effects of friction on surface finish, forming load, material deformation and formability are studied using a newly developed oblique roller ball (ORB) tool. Four grades of aluminum sheet including AA1100, AA2024, AA5052 and AA6111 are employed in the experiments. The material deformation under both the ORB tool and conventional rigid tool are studied by drilling a small hole in the sheet. The experimental results suggest that by reducing the friction resistance using the ORB tool, better surface quality, reduced forming load, smaller through-the-thickness-shear and higher formability can be achieved. To obtain a better understanding of the frictional effect, an analytical model is developed based on the analysis of the stress state in the SPIF deformation zone. Using the developed model, an explicit relationship between the stress state and forming parameters is established. The experimental observations are in good agreement with the developed model. The model can also be used to explain two contrary effects of friction and corresponding through-the-thickness-shear: increase of friction would potentially enhance the forming stability and suppress the necking; however, increase of friction would also increase the stress triaxiality and decrease the formability. The final role of the friction effect depends on the significance of each effect in SPIF process.     

变压边力条件下铝合金板的成形窗口

在由塑性成形理论和能量法分别建立的临界破裂和临界起皱压边力模型基础上,构建了变压边力情况下汽车用铝合金板的成形窗口,并分析了拉深比、材料参数和模具参数对成形窗口的影响规律.对比实验表明:在整个有效压边力范围内,数学模型构建的成形窗口能够与实验相吻合,且铝合金6111-T4和5052-O的成形窗口明显小于钢板st14的.而影响因素分析显示:随着拉深比降低、厚向异性系数(r)增加和凹模圆角半径(Rdp)增大,成形窗口明显扩大;n值增加,成形窗口虽然有所扩大,但是效果并不明显.     

Coarsening of AA6013-T6 Precipitates During Sheet Warm Forming Applications

The use of warm forming for AA6xxx-T6 sheet is of interest to improve its formability; however, the effect warm forming may have on the coarsening of precipitates and the mechanical strength of these sheets has not been well studied. In this research, the coarsening behavior of AA6013-T6 precipitates has been explored, in the temperature range of 200-300 °C, and time of 30 s up to 50 h. Additionally, the effect of warm deformation on coarsening behavior was explored using: (1) simulated warm forming tests in a Gleeble thermo-mechanical simulator and (2) bi-axial warm deformation tests. Using a strong obstacle model to describe the yield strength (YS) evolution of the AA6013-T6 material, and a Lifshitz, Slyozov, and Wagner (LSW) particle coarsening law to describe the change in precipitate size with time, the coarsening kinetics were modeled for this alloy. The coarsening kinetics in the range of 220-300 °C followed a trend similar to that previously found for AA6111 for the 180-220 °C range. There was strong evidence that coarsening kinetics were not altered due to warm deformation above 220 °C. For warm forming between 200 and 220 °C, the YS of the AA6013-T6 material increased slightly, which could be attributed to strain hardening during warm deformation. Finally, a non-isothermal coarsening model was used to assess the potential reduction in the YS of AA6013-T6 for practical processing conditions related to auto-body manufacturing. The model calculations showed that 90% of the original AA6013-T6 YS could be maintained, for warm forming temperatures up to 280 °C, if the heating schedule used to get the part to the warm forming temperature was limited to 1 min.     

Quality of Trimming and its Effect on Stretch Flanging of Automotive Panels

Traditional trimming requires accurate alignment of the die shearing edges, typically 5–10% of the blank thickness. Increasing the clearance above the recommended value often leads to generation of burrs on the trimmed surface. These burrs may create difficulties for flanging and hemming operations. Details of trimming technology for panels made out of aluminum sheet AA6111-T4 with elastic offal support will be discussed, including such factors as die radii of the tooling, effect of tooling wear, and trimming angle on the quality of trimmed surface. Also, imperfections on the trimmed edge of the panel may result in reduced formability in stretched flanging and hemming operations. Experimental results quantifying the behavior of trimmed surface in stretching will be provided for both a conventional trimming process and a newly developed process. This article was presented at Materials Science & Technology 2007, Automotive and Ground Vehicles symposium held September 16-20, 2007, in Detroit, MI.     

A simple experimental tooling with internal pressure source used for evaluation of material formability in tube hydroforming

Material properties have powerful impact on the tube hydroforming (THF) process and the quality of the deformed tube, so it is important to select proper materials and evaluate the material formability prior to conducting the process. A simplified and applied tooling, which has no use for any external hydraulic pressure source but internal one, was designed for charactering the material formability in THF. A pressurized-fluid supplier is automatically established to provide the internal pressure and axial load synchronously required for THF, and the ratio of the two loads is achieved by proper design of the supplier. As a stand-alone hydraulic bulging fixture, the tooling can be worked on a conventional press, even on a single action press. Free bulge forming (FBF), bulge forming with axial loading (BFAL), free and restrained bulge forming (free and fixed ends) can be fulfilled by the tooling, and furthermore, bulge forming with proportional loading to some extend can be realized. Comparative bulge forming experiments under various forming conditions were carried out with the tooling to validate this project and the results suggest that restrained conditions on the tube ends highly affect the FBF, while the ratio of the two loads dominates the BFAL.     

Experimental Observations of Quasi-Static-Dynamic Formability in Biaxially Strained AA5052-O

To establish the efficacy of electromagnetically assisted sheet metal stamping (EMAS), a series of combined hydraulic bulging and electromagnetic forming (EMF) experiments are presented to evaluate the biaxial quasi-static-dynamic formability of an aluminum alloy (AA5052-O) sheet material. Data on formability are plotted in principal strain space and show an enhanced biaxial formability beyond the corresponding experimental results from conventional forming limit diagram. The plastic strains produced by the combined process are a little larger than or at least similar with those obtained in the fully dynamic EMF process. In addition, the biaxial forming limits of aluminum sheets undergoing both very low and high quasi-static prestraining are almost similar in quasi-static-dynamic bulging process. Limit formability seems to depend largely on the high-velocity loading condition as dictated by EMF. It appears that in quasi-static-dynamic forming, quasi-static loading is not of primary importance to the material’s formability. Based on these observations, one may be able to develop forming operations that take advantage of this formability improvement of quasi-static-dynamic deformation. Also, this could enable the use of a quasi-static preform fairly close to the quasi-static material limits for the design of an EMAS process.     

Parametric study on crashworthiness of automotive sheet alloy

Crashworthiness (or collision performance) is a critical design factor in optimizing automotive part products. In this study, a numerical sensitivity analysis was performed to investigate the effects of material properties on crashworthiness for automotive sheet materials. As standard material parameters, the measured mechanical properties of AA 6111-T4 sheet were considered, based on the anisotropic non-quadratic yield function, YId2004-18p, and a combination type non-linear isotropic-kinematic hardening law. The constitutive law was implemented into codes of the commercial finite element program ABAQUS, using user material subroutines. As for process simulations, spring-back after forming as well as sheet forming and collision were included in this evaluation.     

不同材料型材拉弯成形性的比较

介绍了型材拉弯成形性的比较方法,通过数值模拟,建立不同型材的拉弯成形性图,用于评估截面畸变等缺陷产生的难易;以某轿车门框复杂截面型材为例,进行了AA6060—T4和TRIP800两种型材拉弯成形性的对比,表明铝合金型材比钢型材具有更好的拉弯成形性,可得到较高的成形精度,有利于轿车车身轻量化设计。     

Friction Stir Processed AA5182-O and AA6111-T4 Aluminum Alloys. Part 2: Tensile Properties and Strain Field Evolution

In this second part, a state-of-the-art digital image correlation (DIC) technique was used to compute true stress-true strain curves beyond diffuse necking for friction stir processed AA5182-O and AA6111-T4 aluminum alloys. Of particular interest were differences in key tensile properties, such as initial yield point, and ultimate tensile strength, between the base and friction stir processed materials. Tensile coupons cut from the same material used to investigate crystallographic texture via the electron backscatter diffraction technique in Part 1, were strained to failure in a miniature tensile stage. The evolution of two-dimensional strain fields in both the base and friction stir processed materials was explored with incremental and cumulative strain maps computed from digital grids superimposed on each image after testing was completed. The impact of friction stir processing on strain localization just prior to fracture was revealed through changes in incremental strain map contour profiles. It is suggested that grain size refinement due to friction stir processing has a prominent effect on strength, while texture plays a secondary role. This article was presented at Materials Science & Technology 2006, Innovations in Metal Forming symposium held in Cincinnati, OH, October 15-19, 2006.     

Friction Stir Processed AA5182-O and AA6111-T4 Aluminum Alloys. Part 1: Electron Backscattered Diffraction Analysis

The present article is the first part in a two-part series in which crystallographic texture developed during friction stir processing of AA5182-O and AA6111-T4 is characterized and its impact on tensile properties explored. For the texture measurements, coupons were cut from the friction stir processed zone at selected orientations relative to the direction of tool translation. Texture was characterized with electron backscatter diffraction (EBSD) in a scanning electron microscope. Measurements were made at key positions along the coupon surfaces and texture differences between the two friction stir processed Al alloys are discussed in detail. Grain size variations were also measured in both the base and friction stir processed materials and subsequently compared. In part 2, a state-of-art digital image correlation technique is used to investigate tensile properties of both friction stir processed Al alloys. The impact of crystallographic texture on mechanical properties is also explored in this latter part. This article was presented at Materials Science & Technology 2006, Innovations in Metal Forming symposium held in Cincinnati, OH, October 15-19, 2006.     

The Effect of Strain-Rate Sensitivity on Formability of AA 5754-O at Cold and Warm Temperatures

Aluminum-magnesium (Al-Mg) alloys have been widely used in diverse applications ranging from automotive bodies to food processing industries because of their excellent high-strength-to-weight ratio, corrosion resistance, and recyclability potential. The formability of these alloys is decreased at room temperature (RT) and is related with the strain-rate sensitivity. This study presents the effect of strain-rate sensitivity on formability of AA 5754-O alloy sheet at a test temperature range of −60 to 250 °C by duplicate tensile test at different strain rates. The test results indicated that the formability change with positive or negative strain-rate sensitivity values. It was observed that the strain-rate sensitivity values increased at negative temperatures with respect to RT. The best formability condition for this alloy in the test ranges was observed at 250 °C and 0.0016 s⁻¹.     

An investigation of enhanced formability in AA5182-O Al during high-rate free-forming at room-temperature: Quantification of deformation history

The goal of this work is to improve our understanding of formability enhancement in aluminum (Al) sheet alloys that has generally been observed during high-strain-rate forming. In the work presented here, experiments and numerical modeling were used to investigate the room-temperature formability of AA5182-O Al alloy sheet (1 mm thick) at high strain-rates using the electro-hydraulic forming (EHF) technique. A finite element model, using Johnson–Cook constitutive equation, was developed to simulate the high-rate forming behavior of Al under EHF and test samples were designed to obtain different strain paths at the apex of the EHF domes. The deformation history of Al sheets, under free-forming conditions and inside a conical die, was experimentally determined and compared to the model predictions. Experimental data shows that the high-rate formability of AA5182-O Al at minor strains of ∼−0.1 and ∼0.05, relative to its corresponding quasi-static formability, was enhanced locally by ∼2.5× and ∼6.5× under free-forming and when forming inside the conical die, respectively. The in-plane peak engineering strain-rate associated with the enhanced formability during free-forming was measured to be ∼3900/s while the pre-impact strain-rate during conical-die forming was estimated to be ∼4230/s. The strain-path associated with enhanced formability was experimentally determined under a free-forming case and was found to be in good agreement with that predicted by the numerical model. To the authors’ knowledge, these results are the first to experimentally quantify the deformation history associated with enhanced formability that has often been reported in the literature.     

Influence of short-term heat treatment on the microstructure and mechanical properties of EN AW-6060 T4 extrusion profiles—Part B

In the automotive industry aluminium and its corresponding semi-finished products contribute an essential part to the aim of weight reduction in car body structures. Aluminium alloys of the 6000 series with Mg and Si contents are preferred because of the possibility to increase strength by ageing processes. However, the cold formability in comparison to other materials like mild steels is quite low and due to this, complex parts are only producible at higher temperatures. Therefore, the so called Tailor Heat Treatment was developed to improve the cold formability of aluminium alloys. In this approach, a short-term heat treatment is conducted to achieve a local softening of the material due to dissolution of Mg and Si clusters (retrogression). This effect is used to improve the material flow, relief critical forming zones and enhance the overall formability of the material. Afterwards, strength can be increased again by ageing processes. However, up till now a holistic process understanding, taking into account all process parameters as well as a microstructural explanation is missing. Therefore, the focus of the fundamental investigations lies on connections between the mechanical properties and short-term heat treatment with industry-relevant heating rates as well as natural and artificial ageing process. Conclusively, the evolution of the mechanical properties with regard to the natural ageing process is compared with findings of DSC analysis, which were discussed in Part A. Based on these results, a process window is derived for the subsequent forming process and the final mechanical properties of the final part in dependency of the forming history as well as the artificial ageing process, are identified.