Low Formability Research Articles

Experimental investigation of distortion behavior of laser heat treated blanks

Aluminum alloys fulfill the growing demands for lightweight applications and are completely recyclable. The use of high-strength aluminum alloys offers the opportunity to replace safety-relevant steel components. Currently, the use of these alloys is limited due to their low formability and high springback during forming at room temperature. The use of tailored blanks with locally adapted material properties is one opportunity to overcome these limitations. By local heat treatment, zones are locally softened and thus crack critical areas during forming can be avoided. At present, blanks with tailored properties are not used extensively. One reason for this is component distortion due to thermal stress, which makes handling the heat-treated sheets more difficult. Aim of this study is the identification and reduction of thermally caused component distortion. Therefore, in this paper the influence of the initial material condition, the sheet thickness as well as the laser spot size on the distortion behavior is investigated.

Investigation of a radial toolpath in single point incremental forming

Single point incremental forming (SPIF) is a sheet material forming process recognized for its potential performance in low to medium volume production due to precluding the need for high-investment stamping dies. This process forms blanks instead through the use of local deformations, traditionally imposed utilizing a hemispherically-shaped tool. The dieless nature of this process makes it ideal at low volumes; however, several drawbacks of the process currently inhibit its widespread use in industry, including difficult-to-predict springback and poor surface finish.Another perceived drawback of SPIF is overall low formability. Conventional toolpaths form the blank through a series of successive passes in which the tool is generally constrained to the XY plane. Here a novel toolpath is proposed and tested experimentally and through simulation to determine its viability of improving formability. The toolpath investigated travels radially through the center of the workpiece, rather than along its periphery, in order to generate a more uniform thickness profile. A formability improvement of 21% was found utilizing this strategy.

The Present Situation of the Research and Development of the Electromagnetic Pulse Technology

Abstract The car industry is currently working to integrate the conflicting requirements of environmental legislation and customer demands for high-performance and safety by developing lightweight vehicles. To meet these very different goals, automotive components are being manufactured from alloys that include including lightweight materials such as aluminum and magnesium. However, existing welding and forming processes such as fusion welding, any that use heat in general, and low-velocity press forming can all cause defects. Therefore, Electromagnetic pulse technology has become the focus. Electromagnetic pulse technology is a high-velocity process that uses electromagnetic forces for welding and forming processes. In particular, because of its good flexibility, environmentally friendly process, and high-quality dissimilar weldment, the technology can potentially resolve the challenges in welding and forming processes for lightweight materials that have low weldability and formability. In particular, Korea’s research on electromagnetic pulse technology was started in 2008 by KITECH. These studies include the basic theory, crafting of applications, and simulation. Currently, electromagnetic pulse research has focused on the dissimilar sheets and tubular workpieces for applications in the automobile industry, which includes the propeller shaft. Although electromagnetic pulse technology research remains limited compared to other manufacturing processes, with the continuous increase in market demand in the automobile and electronics industries, interest in electromagnetic pulse technology research will also continue to increase

Development of Tool Paths for Multi-axis Single Stage Incremental Hole-flanging

In this paper, multi-axis single stage incremental hole-flanging is explored in place of three axis multi-stage tool paths in order to prevent flange conicity in hole-flanging by single point incremental forming (SPIF). The angle of inclination of the forming tool, the starting radius and step size of the tool paths are varied to investigate the feasibility of new tool paths. Using the proposed tool paths, an optimization is performed by Finite element (FE) analyses. The geometrical accuracy of the flange is maintained within tolerances relevant to industrial applications while minimizing the resultant forces acting on the tool and maintaining high formability (hole expansion ratio, HER). The FE analyses of the process reveal low formability and high forming forces for flanges formed at high angles of inclination of the forming tool. The tool paths with small diameters at the start point of the tool path result in high bulge heights and high resultant forces. Hence, the existing three axis tool paths are better than the proposed strategies. To prevent flange conicity, over bending of the flange edge to compensate for springback after forming is implemented on the three axis tool path strategy. This is achieved by adding a second tool path with an over bend angle at the end of the three axis tool path.

Failure Prediction for the Tearing of a Pin-Loaded Dual Phase Steel (DP980) Adjusting Guide

Owing to their outstanding strength, in recent years, there has been an increased use of advanced high-strength steel (AHSS) sheets in the automotive sector. Their low formability, however, poses a challenge to forming, and failure prediction requires accurate knowledge of its material behavior over a large strain range up to ultimate failure, in order to exploit their full capacity in forming, but also in crash events. For predicting the fracture of an adjusting guide loaded by a pin, first, the force–displacement data are extracted from tensile tests using DP980 specimens of diverse shapes, all of which represent a certain loading mode. Using digital image correlation (DIC), we determine the stress triaxialities corresponding to the diverse loading conditions and establish the triaxiality failure diagram (TFD), which serves as the basis for the generalized incremental stress state-dependent damage model (GISSMO). Then, the damage parameters (necking and failure strains) are determined for each loading mode by reverse engineering-based optimization. Finally, these damage parameters are applied to the adjusting guide, and the numerical results are compared with the experimental data. Comparisons of the external load–displacement curves and the local equivalent strain distributions show that using the damage model with the material parameters obtained in here allows for the accurate prediction of the guide’s failure behavior, and the applicability of GISSMO to complex loading cases.

A novel method to evaluate the high strain rate formability of sheet metals under impact hydroforming

In this study, a novel method was proposed to evaluate high strain rate (HSR) formability of Al-Cu-Mg 2B06-O sheets by impact hydroforming (IHF). IHF was suitable to manufacture hard-to-form sheets, since it combined the advantages of flexible liquid and impact impulse loading. Both 2D and 3D HSR formability curves were established to describe the relationship between impact energy, drawing height ratio (DHR) and deep drawing ratio (DDR). Moreover, the novel evaluation method was realized by finite element (FE) modeling using fluid-structure interaction (FSI) algorithm. FSI modeling was used to deal with the interaction of structure solid and flexible fluid. This evaluation means of FE modeling characterized with guiding production practice, saving evaluation costs, improving efficiency compared with corresponding experiments. The results obtained from FE modeling were in a remarkable agreement with those obtained from IHF experiments in the aspects of part shape, failure feature, thickness distribution. FE modeling showed that the limit deep drawing ratio (LDDR) and limit drawing height ratio (LDHR) were 1.99 and 1.04, respectively when the blank was completely deep drawn into the cavity of the die in one step only. FE modelling is proved to be reliable and efficient to estimate high strain rate formability of low formability metal sheets.

Comparison of the Mechanical Properties and Forming Behavior of Two Texture-Weakened Mg-Sheet Alloys Produced by Twin Roll Casting

The influence of rolling and annealing on the resulting mechanical properties and forming behavior of Mg-Zn-RE and Mg-Zn-Ca alloys produced via twin roll casting is investigated. After hot rolling followed by an annealing treatment, both alloys develop a fine-grained microstructure with average grain sizes less than 10 µm. A distinctive development of a texture with a pronounced split of the basal poles in the transverse direction, so called TD-split, is observed in both alloys during the annealing. Due to the fine microstructure and weak texture, which enhances the activation of basal slip, both alloys show large ductility with fracture strain higher than 30%. However, due to the TD-split texture, a high asymmetry of the yield stress is observed, where the yield stress in rolling direction is significantly higher than the yield stress along 45° and the transverse direction. Both alloys show high stretch formability at room temperature with Erichsen indices around 7. Despite a low planar anisotropy and high stretch formability, the Mg-Zn-RE alloy shows undesirable earing behavior, while the Mg-Zn-Ca alloy fractures during warm deep drawing. Moreover, the deep drawing operation using the as-rolled sheet of Mg-Zn-RE alloy can be successfully done. It is observed that the earing behavior can be effectively reduced by deviating from the TD-split texture. In this regard, cold rolling was explored to reduce the anisotropy of the ductile and formable alloy.

Influence of niobium solutes on the microstructural evolution of nickel during hot working

The effect of niobium additions on the microstructural evolution of nickel during hot working was investigated by optical and electron microscopy (SEM-based EBSD, electron backscatter diffraction). For this purpose, a series of wrought model alloys consisting of pure nickel and six Ni-xNb alloys (x = 0.01, 0.1, 1, 2, 5 and 10 wt% Nb) were prepared and tested in torsion to a true strain of 5 within the temperature range of 800 to 1000 °C and at three strain rates of 0.03, 0.1 and 0.3 s–1. The results revealed that the gradual Nb additions led to increasingly smaller grain sizes ranging from 800 to 40 μm and even up to 1 μm for the gripping area (undeformed head) and the gauge-length (deformed part) of the specimens, respectively. However, a saturation effect was observed at 5−wt% Nb in both cases. If the deformed Ni alloys containing 10-wt% Nb led to unsatisfactory results (in terms of volume fraction and recrystallized grain size) owing to low formability, however, under specific conditions of deformation, some of them gave good results comparable to those containing 5-wt% Nb. Overall, the gradual Nb additions affected, in complex ways, the kinetics and the nature of mechanisms of dynamic recrystallization (DRX) that turned out to be discontinuous dynamic recrystallization (DDRX), continuous dynamic recrystallization (CDRX) and geometric dynamic recrystallization (GDRX) operating separately and/or simultaneously, along with twinning as an additional mechanism.

Application of Combined W-Temper and Cold Forming Technology to High-Strength Aluminum Alloy Automotive Parts

In this study, a room temperature forming process for high-strength aluminum alloys was investigated. To overcome the low formability of the peak-aged 7075 aluminum alloy (7075-T6) at room temperature, the sheet metal was subjected to solution heat treatment followed by rapid quenching. Mechanical tests, including uniaxial and balanced biaxial tension tests, were performed, and the Nakajima test was conducted to evaluate the formability of the sheet metal. On the basis of the plastic deformation behavior, finite element models including the plastic yielding and strain hardening laws were constructed. The proposed forming process incorporating the W-temper heat treatment was successfully applied to a realistic automotive part, which could not be manufactured by conventional room temperature forming without prior heat treatment. The numerical accuracies among various plasticity models were evaluated from a comparison of the thickness profiles. In addition, the paint-bake process was applied to the automotive component to evaluate the strength recovery.

Investigations on hole-flanging by paddle forming and a comparison with single point incremental forming

The use of dedicated dies to carry out hole-flanging operations is costly for small batch production. As a result, hole-flanging by single point incremental forming (SPIF) was put forward because it offers advantages such as low equipment cost, high formability, and flexibility compared to conventional hole-flanging. However, drawbacks of hole-flanging by SPIF such as poor geometrical accuracy and long process times are obstacles to its industrial take-up. This paper presents a novel incremental hole-flanging method which uses paddle-shaped tools rotating at high speeds while being fed axially into the sheets to form flanges, with the aim of preserving the advantages of incremental forming while reducing the drawbacks related to process time. The results of the experiments show higher formability for the paddle forming process compared to hole-flanging by multi-stage SPIF and conventional hole-flanging. The high formability in paddle forming is attributed to the presence of shear which delays fracture. Furthermore, heat generated due to friction between the paddle and metal blanks leads to a rise in the blank temperature which enhances formability. Optical strain measurements of the formed flanges reveal that the paddle speed and feed have major influences on the forming mechanism of the process and the process borders. A maximum hole expansion ratio (HER) of 3.3 was obtained from the experiments in a process time of ∼2 s, showing that the process combines high formability and low process times.

Effects of YSZ powder properties on its corrosion behaviour for solid oxide membrane (SOM) electrolysis process

The solid oxide membrane (SOM) process is an environmentally friendly and innovative technology that can produce valuable metals directly from their oxides. In the SOM process, a yttria-stabilized zirconia (YSZ) tube is normally immersed the molten salt containing dissolved metal oxides and oxygen anions that move to the SOM tube where they are reduced without any emission. However, the YSZ phase transition and yttrium dissolution by the reaction with the flux reduce the SOM stability and lifetime. Here, we investigated the effect of powder characteristics on YSZ stability in the SOM electrolysis environment. Thus, powder with good formability such as high flowability and tap/untap density showed a 2% decrease in electrical conductivity, but a 17% decrease in low formability powders. The cause of the degradation is the penetration of salt ions into the lattice and the elution of yttrium ions, which results in a phase transition.

Improvement of Die Corner Filling of Stepped Tubes Using Warm Hybrid Forming

Aluminum and magnesium alloys are of materials for decreasing vehicle weight and consequently reducing fuel consumption. However, forming limitations regarding their low formability at room temperature are found when being manufactured by conventional forming processes. For this reason, development of new forming techniques, such as warm tube hydroforming, is needed to overcome such limitations. In addition, production of parts with sharp corners is nearly impossible using conventional forming processes. This paper investigates the possibility of forming stepped tubes with high expansion ratio, sharp corner radii and precise geometric shape using a developed hybrid hydroforming and bending method. To assess tube formability, the bulge test was adopted with different forming temperatures and axial feeds. It is shown that using the feed of 35 mm and feed rate of 15 mm/min, a stepped tube with 47.6 % expansion ratio and corner filling ratio of about 100 % (part with sharp corners) could be achieved when adopting the developed hybrid hydroforming and bending method at 150 ᵒC.

Evaluation of the Formability and Dimensional Accuracy Improvement of Ti6Al4V in Warm SPIF Processes

Single Point Incremental Forming (SPIF) has great potential as it can be easily implemented and the forming process does not require the use of dies. However, its application to high performance alloys such as Ti6Al4V has not been resolved due to its characteristic low formability. In the literature, studies on the warm SPIF process applied to this alloy report improved formability. However, in some of these studies the procedures used are complex and in others the surface finishes obtained are unsatisfactory. The present study proposes a methodology consisting of a simple heating device which permits working at moderate temperatures and quantifies the benefit of the temperature on the alloy formability, the forces acting during the process, and finally the dimensional precision of the parts produced. Working temperatures in the range of 300–400 °C significantly reduce forces, increase formability of the alloy, and substantially reduce springback. However, the springback values suggest the need for dimensional compensation at the design stage of products.

A Springback Prediction of 1.5 GPa Grade Steel in Roll Forming Process for Automotive Sill-Side Inner Component

Roll forming has been widely used to produce steel sheet with low formability such as Ultra High Strength Steel (UHSS). It allows the steel sheet to be formed through successive bending process into a desired shape which even cannot be formed by press brake forming. Although the process effectively improves the formability of UHSS, there still the remains accuracy issue such as springback, flair, bow and so on. Especially, springback of UHSS is one of the major challenges in roll forming process as much as press forming process. In this paper, the springback of 1.5 GPa grade steel in roll forming process was numerically investigated for automotive sill-side inner component. The material behavior was described by using the selected hardening models: isotropic hardening (Piecewise linear model), linear kinematic hardening (Prager model [6]), nonlinear kinematic hardening model (Yoshida-Uemori model [7]). A commercial software LS-DYNA was utilized for the analysis. Eighteen successive roll stages were modelled for the simulation. From the results, it was found that the springback prediction during roll forming process could be successfully achieved when the complicated material behaviors including Bauschinger effect, nonlinear transient hardening, and changeable unloading modulus are taken into account for the Finite Element (FE) simulation.

Experimental Investigation of Formability and Surface Finish into Resistance Single-Point Incremental Forming of Ti–6Al–4V Titanium Alloy Using Taguchi Design

Considering its low-cost equipment, single-point incremental forming (SPIF) can be appropriate for rapid prototyping of sheet metal parts, since a simple hemispherical head tool is usually implemented without any dedicated dies. Incremental forming at elevated temperatures assisted by electric current, which is hereafter called resistance SPIF, is an innovative process for forming sheet metals with a high-strength-to-weight ratio and a low formability at room temperature. In the present research, the effect of utilizing various lubricants on both the formability and surface finish of the Ti–6Al–4V titanium alloy sheet is investigated using the resistance SPIF of the truncated cone at different feed rates, vertical pitches, and currents. To this end, the Taguchi design of experiment and the analysis of variance (ANOVA) are employed. A cooling water system is designed and applied to the forming tool to improve both its life and the duty cycle of the process. The results show that the formability of Ti–6Al–4V sheet can be enhanced using the resistance SPIF, which strongly depends on the lubrication condition. In order of significance, the lubricant and current have the most influence on the maximum achievable forming depth and the lubricant, the feed rate, and the vertical pitch have the highest effects on the surface roughness. Based on the ANOVA results, the graphite-based anti-seize compound and the graphite powder are suggested for an appropriate surface finish and the highest formability, respectively, in the resistance SPIF within the range of the considered parameters.

Effect of Artificial Aging Temperature on Mechanical Properties of 6061 Aluminum Alloy

Aluminum alloys have been attracted by several engineering sectors due to their excellent strengthweight ratio and corrosion resistant properties. These are categorized into 1, 2, 3, 4, 5, 6, 7and 8xxx on the basis of alloying elements. Among these 6xxx series contains aluminum–magnesium–silicon as alloying elements and are widely used in extruded products and automotive body panels. The major advantages of these alloys are good corrosion resistance, medium strength, low cost, age hardening response no yield point phenomenon and Ludering. 6xxx series alloys generally have lower formability than other aluminum alloys which restrict their utilization for wide applications. Keeping in view of the shortcomings in the set of mechanical properties of 6xxx series the efforts were made to improve the tensile strength and toughness properties through age hardening. In present study heat treatment cycles were studied for 6061 aluminum alloy. Three different age hardening temperatures 160, 200 and 240oC were selected. The obtained results showed that 17.26, 7.69, and 10.51% improvement in tensile strength, toughness and hardness respectively was achieved with solution treatment at 380oC followed by an aging 240oC. Microstructural study revealed that substantial improvements in the mechanical properties of 6061 aluminum alloy under heat treatment were achieved due to precipitation of Mg2Si secondary phase.

Evolution of Texture during Hot Rolling of a New Magnesium Alloy

High-strength wrought magnesium alloys are one of the sought-after materials in the automotive sector owing to the demands for weight reduction in the automobiles due to fuel economy and CO2 emission. However, because of low room temperature strength and formability of Mg alloys, only a few applications in wrought form have been explored with these materials. In the present investigation, a high strength, good ductility and low cost wrought magnesium alloy with Mg-Sn-Zn composition have been developed and subjected to conventional wrought processing. Hot rolling was carried out at 350°C without homogenization and after homogenization at 300°C and 330°C. The phase stability, microstructure and texture of the alloy has been investigated for as-cast, homogenized and hot rolled conditions. The compositional and microstructural characterization was carried out by Electron Probe Micro-analysis (EPMA) and optical microscopy respectively. Texture evolution was investigated by X-ray diffraction method. A strong (0002) basal texture develops after hot rolling without homogenization. The (0002) basal texture has been weekend by splitting of poles and double peak distribution when hot rolling was carried out after homogenization.

Microstructure and Texture of MX20 after Conventional Rolling and Rolling from Twin Rolled Cast Strip

Two main impediments are currenty discussed with respect to the industrial application of magnesium sheets. First, the low formability of magnesium sheets requires many rolling passes to roll cast slabs to final gauge, which leads, second, to high costs for the production of magnesium sheets. An alternative cost-saving process chain for magnesium sheets with enhanced properties is the feedstock production by twin roll casting (TRC). In the TRC process, liquid metal proceeds from a furnace over a pipe into a crucible and then flows between a pair of counter rotating, internally cooled rolls. The metal solidifies upon touch with the cooled rolls and gets rolled to a strip. This paper refers to the comparison of the two processing routes on the example of the aluminum-free magnesium alloy MX20 (2 wt% Mn and 0.5 wt% Ca). Both kinds of production processes like casting and twin roll casting have an influence on the microstructure and texture of the feedstock material for the subsequent rolling process. The paper reports on the results of casting and twin roll casting experiments of this alloy. Furthermore, rolling trials are conducted and the deformation behavior of the sheets are presented and discussed with respect to the developed microstructures and textures. The different morphology of precipitates in the cast and twin roll cast feedstock material is used to improve the ductility of the magnesium alloy MX20.

Effects of CaO addition and strain rate on the texture evolution of AZ31 alloy

The controlling alloying elements and deformation parameters are significant factors of the plastic deformation of Mg alloys due to their low formability and difficult-to-control texture. In this study, we investigated the effect of both CaO addition and the strain rate on the texture evolution and flow behavior of AZ31 alloys by carrying out hot torsion. The dislocation components of the deformed AZ31 alloys were evaluated using X-ray line profile analysis (XLPA). The XLPA results showed that the strain rate variation significantly affected the dislocation components and dislocation density of the alloys while CaO addition has only dislocation density effect. According to the results, weaken texture evolution by CaO addition is based on not variation of slip modes but grain boundary mobility. Also, it was determined that high strain rate deformation affect texture evolution by fortifying <a>-type slip mode.

Investigation on Forming Behaviour of Sheet Metal by Test Using Cylindrical Cup with Hole

Nowadays, there are several grades of sheet metal used in the automotive industry. Highstrength steel sheets, particularly, have been widely used in order to reduce the weight of vehicles,which is strongly related to their fuel consumption rate. However, it is generally known that thestrength of the sheets, which is relatively higher than that of the conventional carbon steel sheets,results in their low formability. In this work, the limiting drawing ratio and forming behavior of sheetmetal that is conventionally used for automobile parts were evaluated by test using cylindrical cupwith hole. The feasibility to use limiting cup height for comparing formability of sheet metal was alsoincluded in the investigation. The sheet materials used in the experiments are aluminium, cold rolledsteel, high strength steel and advanced high strength steel. The process parameters for this study weredie corner radius and blank holder force. Workpiece materials were prepared with a circular shapeand with a diameter of 80 millimetres. In the center of the circular workpiece, a 12-millimetrediameter hole was drilled to observe the formability of each of the materials. The advantage of usingan initial blank with a hole in the center by the cylindrical cup drawing test is that the cup does notfail from changes of the thickness of material near the punch radius at the bottom of the cup. Thelimiting cup height of the investigated materials were evaluated by test using the cylindrical cup withhole. The results show that the limiting cup height values have a relationship to the limiting drawingratio values of the investigated materials. Testing using cylindrical cup with hole by evaluating thelimiting cup height value is feasible for comparing the formability of sheet metals.