Finite element modeling and experimental results of brass elliptic cups using a new deep drawing process through conical dies

This paper introduces a new technique for deep drawing of elliptic cups through a conical die without blank holder or draw beads. In this technique an elliptic-cup is produced by pushing a circular blank using a flat-headed elliptic punch through a conical die with an elliptic aperture in a single stroke. A 3D parametric finite element (FE) model was built using the commercial FE-package ANSYS/APDL. Effects of die and punch geometry including, half-cone angle, die fillet radius, die aperture length and punch fillet radius on limiting drawing ratio (LDR), drawing load and thickness strain of the cup have been investigated numerically for optimal process design. A die with half cone angle of 18° has shown the best drawability for the new technique. An experimental set-up has been designed, manufactured, and used for experimental production of elliptical shaped sheet-metal cups. A total of seven punches having aspect ratios ranging from 2 to 2.25 and a die with an aspect ratio of 2 have been manufactured and used. Tensile tests were carried out to obtain the stress–strain behavior for the formed sheet metal. Experiments were conducted on blanks of brass (CuZn33) with initial thicknesses of 1.5, 1.9, 2.4 and 3 mm at different clearance ratios (c/t). Effects of blank thickness and clearance ratio on limiting drawing ratio, drawing load and thickness strain were numerically and experimentally investigated. Finite element model results showed good agreement with experimental results. An elliptic cup with a limiting drawing ratio (LDR) of 2.28 has been successfully achieved using the proposed technique and set-up.     

End formation of a round tube into a square section having small corner radii

Expansion and reduction are the two common end forming processes for tubes. In the tube end expansion process using a square punch, it is difficult to obtain a small corner radii due to the stretching of the tube around the punch corners. The wall thickness around the corners is small when compared to the side wall. Hence, a tube having a poor square look is formed. In this study, a 2-stage end expansion of a round tube end into a square section having an improved square look i.e. small corner radii and increase in wall thickness around corners is developed. In the 1st stage, the tube end is flared into a cone shape using a 30° conical die by axial compression. In the 2nd stage, the conical end of the tube is drawn through a taper square die using a conical bottom square punch, and a near square section is formed. A 15% ironing ratio is applied during the drawing process to flatten the side wall of the square. Experimental and FEM simulation were performed to evaluate and to verify the forming process. Although the height of the square section increases when the punch stroke at the 1st stage is increased. However, this increase is limited by the buckling of the pipe at the circular section of the thick blank tube. Since the conical end is drawn into a square section having different radial lengths, the bottom of the square section is uneven. The uneven bottom end is trimmed off in the later process. A square section having a maximum height of 32 mm after trimming is successfully obtained from the experiment for the punch stroke, S = 44 mm using an API 5 L tube.     

Enhancing formability in hydromechanical deep drawing process adding a shallow drawbead to the blank holder

In this paper, a new method was proposed in order to enhance the limiting drawing ratio (LDR) of AA5754-O in the hydromechanical deep drawing process (HDD). In the proposed method, a shallow drawbead was added to the blank holder to increase LDR so as to provide strain hardening of a large region on the flange of the sheet material in addition to pre-bulging process which affects particularly only the initial stage but not the later ongoing process. So the LDR of the AA5754-O was increased from 2.65 to 2.787 by enlarging the region of strain hardening in the flange and partially reducing wrinkling tendency due to occurred tensile stresses using the convenient pressure and blank holder force profiles. The importance levels and their convenient values for height of drawbead, pre-bulge height and pressure, surface roughness of the punch were determined with analysis of variance (ANOVA) is a statistical method. ANOVA analysis illustrated that adding a shallow drawbead to the blank holder is the most effective factor between the investigated factors for the HDD process. While the effects of the pre-bulging pressure and pre-bulging height were determined as quite small, the surface roughness of the punch was found unimportant compared to the effect of the shallow drawbead. The highest LDR value was obtained with 1 mm drawbead height, 5 mm pre-bulging height, 10 MPa pre-bulging pressure and 2.8 μm surface roughness of the punch.     

Cold Deep Drawing of Commercial Magnesium Alloy Sheets

A cold deep drawing process for commercial AZ31 magnesium alloy sheets was developed. The commercial sheets were successfully formed into circular cups at room temperature by optimising the annealing temperature of the sheets, i.e. a limiting drawing ratio of 1.75 was attained for an annealing temperature of 500 °C. The increases in elongation, n-value and r-value, and the decrease in flow stress effective in the improvement of drawability were obtained for the annealing. The apparatus for cold deep drawing without heating becomes much simpler than that for the conventional warm deep drawing. The effects of the lubricant, the clearance between the die and the punch and the corner radius of the punch on the drawability were examined. The limiting drawing ratio was increased by applying force onto the edge of a blank through the die corner. In addition, cold deep drawing of magnesium alloy square cups was performed. It was found that comparatively shallow magnesium alloy cups are satisfactorily formed at room temperature without heating.     

Deep drawing of square cups with magnesium alloy AZ31 sheets

The square cup drawing of magnesium alloy AZ31 (aluminum 3%, zinc 1%) sheets was studied by both the experimental approach and the finite element analysis. The mechanical properties of AZ31 sheets at various forming temperatures were first obtained from the tensile tests and the forming limit tests. The test results indicate that AZ31 sheets exhibit poor formability at room temperature, but the formability could be improved significantly at elevated temperatures up to 200 °C. The test results were then employed in the finite element simulations to investigate the effects of process parameters, such as punch and die corner radii, and forming temperature, on the formability of square cup drawing with AZ31 sheets. In order to validate the finite element analysis, the deep drawing of square cups of AZ31 sheets at elevated temperatures was also performed. The experimental data show a good agreement with the simulation results, and the optimal forming temperature, punch radius and die corner radius were then determined for the square cup drawing of AZ31 sheets.     

Mechanical anisotropy and deep drawing behaviour of AZ31 and ZE10 magnesium alloy sheets

The influence of the initial microstructure on the deep drawability and the associated microstructural evolution in two different magnesium alloy sheets, AZ31 and ZE10, has been examined. Tensile testing at room temperature shows that the AZ31 sheet has high plastic strain ratios, r = 2–3, which are caused by strong basal-type texture. The ZE10 sheet shows lower r values, r ～ 1, as a result of its weak texture. Deep drawing experiments carried out over the temperature range 100–300 °C revealed that the ZE10 sheet can be successfully deep-drawn at lower temperatures than the AZ31 sheet. The ZE10 cups show earing despite the weak texture and low normal anisotropy, while earing of the AZ31 cups is negligible. In the ZE10 cups, deformation is accommodated mainly by 〈a〉 slips and by compression as well as secondary twinning. The occurrence of dynamic recrystallization is observed in successfully deep-drawn AZ31 cups.     

Two-stage cold stamping of magnesium alloy cups having small corner radius

A two-stage cold stamping process for forming magnesium alloy cups having a small corner radius from commercial magnesium alloy sheets was developed. In the 1st stage, a cup having large corner radius was formed by deep drawing using a punch having large corner radius, and the corner radius of the cup was decreased by compressing the side wall in the 2nd stage. In the deep drawing of the 1st stage, fracture was prevented by decreasing the concentration of deformation with the punch having large corner radius. The magnesium alloy sheets were annealed at 500 °C to increase the cold formability. Circular and square cups having small corner radii were formed by the two-stage cold stamping. For the circular cup, the height of the cup was increased by ironing the side wall in the 1st stage. The radii of the bottom and side corners of the square cup were reduced by a rubber punch for applying pressure at these corners in the 2nd stage. It was found that comparatively shallow magnesium alloy square cups used as cases of laptop computers and mobile phones can be satisfactorily formed at room temperature without heating by the two-stage stamping.     

Temperature conditions during ‘cold’ sheet metal stamping

This paper investigates the friction and deformation-induced heating that occurs during the stamping of high strength sheet steels, under room temperature conditions. A thermo-mechanical finite element model of a typical plane strain stamping process was developed to understand the temperature conditions experienced within the die and blank material; and this was validated against experimental measurements. A high level of correlation was achieved between the finite element model and experimental data for a range of operating conditions and parameters. The model showed that the heat generated during realistic production conditions can result in high temperatures of up to 108 °C and 181 °C in the blank and die materials, respectively, for what was traditionally expected to be ‘cold’ forming conditions. It was identified that frictional heating was primarily responsible for the peak temperatures at the die surface, whilst the peak blank temperatures were caused by a combination of frictional and deformation induced heating. The results provide new insights into the local conditions within the blank and die, and are of direct relevance to sheet formability and tool wear performance during industrial stamping processes.     

Fabrication of noncircular multicore microtubes by superplastic dieless drawing process

A superplastic dieless drawing process that requires no dies or tools is applied to the drawing of a Zn–22Al superplastic alloy for noncircular microtubes such as square, rectangular and noncircular multi core tubes having square inner and rectangular outer cross-sections. In this study, the effects of heating condition, such as heating length and the use or nonuse of cooling device, on deformation behavior are investigated. As a result, a square microtube with 0.58 mm side and a rectangular microtube of 0.75 mm × 1.3 mm were fabricated after 3-pass superplastic dieless drawing. In addition, the fundamental deformation behavior of noncircular tubes combined with square and rectangular tubes during the dieless drawing process has been clarified experimentally. The cross-sectional shape of the noncircular tubes after the superplastic dieless drawing process tends to be maintained on the basis of the similarity law in case of a wide heating length compared with a narrow heating length. Furthermore, a noncircular microtube, which has inner square tubes with a 335 μm side, and an outer rectangular tube of 533 μm × 923 μm were fabricated successfully after a 4-pass superplastic dieless drawing process. Consequently, it was found that the superplastic dieless drawing is effective for the fabrication of noncircular multicore microtubes.     

A developed process for deep drawing of metal foil square cups

Deep drawing of non-axisymmetric cross-section cups from thin sheets or metal foils has become increasingly important, especially for miniaturization of mechanical components. However, with a thin sheet thickness, conventional deep drawing processes are not able to offer reasonable drawing ratios due to early formations of localized wrinkling and fractures at cup corners. In this paper, a friction aided deep drawing process has been developed to increase the deep drawability of thin sheets and metal foils. Productions of square cups have been chosen to verify the current proposed process since the shape provides recognizable non-homogeneous deformation, which can then be compared to conventional processes. In the proposed process, a circular blank holder of a square hole is divided into eight identical segments of 45°. During the deep drawing process, four of the eight segments will move radially inward while the other four segments will move radially outwards cyclically under a pre-determined blank holding pressure. A finite element model of the technique was used to simulate virtual experiments to evaluate and optimize the controlling parameters that influence the cup height and forming process. Taguchi and Pareto ANOVA statistical methods were subsequently used to determine the optimum conditions for best cup height. The results have shown that the new technique is capable of producing deep square cups from soft aluminum sheet (Al-O) of 0.5 mm thickness with a high drawing ratio of 3.3. In addition, it was also observed that the radial displacement was the most significant parameter in influencing the cup height.     

Evolution of springback and neutral layer of AZ31B magnesium alloy V-bending under warm forming conditions

The evolution of springback and neutral layer for AZ31B magnesium alloy sheet was investigated by V-bending tests at temperatures from 50 to 300 °C. Moreover, in order to perceive the influence of the punch radii on springback and offset of neutral layer, the tests with punch radii at 7.5, 8.1, 8.7 and 9.3 mm were conducted at 100 °C. The results show that the neutral layer shifts to the tension zone of the sheets. The coefficient of neutral layer (k-value) decreases with the increase of temperature and punch radii. This is mainly because of the asymmetry between the outer tension layer and inner compression layer during bending. The outer tension region is dominated by slip, while the inner compression region is dominated by twinning. With the increase of temperature, the asymmetry of tension–compression becomes weaker, and the offset of neutral layer decreases. The offset of neutral layer increases as punch radii decreases. The shift of neutral layer of AZ31B sheet results in the calculation of springback bigger than the reality.     

Accessing collision welding process window for titanium/copper welds with vaporizing foil actuators and grooved targets

A method for accurate, low-cost, lab-scale determination of the optimal collision angles and velocities for collision welding of a given combination of materials has been introduced. 0.508 mm thick grade 2 CP Ti sheets were launched at various velocities toward a Cu 110 target with grooves of angles ranging from 8° to 28°, machined on the collision side. Capacitor bank-driven aluminum vaporizing foil actuators operated at input energy levels up to 12 kJ and currents up to 140 kA were used to launch the flyer sheets. Velocity was measured with high temporal resolution using a photonic Doppler velocimetry (PDV) system. Collision velocities ranged from 440 m/s to 860 m/s. The welded assemblies were sectioned and the weld interfaces were observed via scanning electron microscopy. For each collision angle there were certain collision velocities which yielded a wavy interface. Welding velocity for transition from smooth to wavy interfaces for each collision angle was used to determine the corresponding transition Reynolds number and was compared to existing results in literature. The uniqueness of this process lies in its small scale and ease of implementation.     

Effect of crystalline anisotropy and forming conditions on thinning and rupturing in deep drawing of copper single crystal

Copper single crystal has excellent electrical properties and ductility, and it has a peculiar application in micro-manufacturing. In this paper, a specific mold was designed and made in order to conduct deep drawing of copper single crystal and evaluate the crystalline orientation effect to the local thinning and rupturing in the process. As a contrast, a finite element subroutine (VUMAT in ABAQUS) based on the crystal plasticity theory was developed to simulate the deep drawing process according to the experimental configurations. The results show that the (1 1 0) blank has better deep drawing performance than (0 0 1) blank; The crack of (0 0 1) blank originates at 〈1 0 0〉 orientation in the plane because 〈1 0 0〉 orientation has poor plasticity; friction is a crucial factor to the forming quality in a small scale deep drawing process; the simulations are in good agreement with the experiments, which also indicates the crystal plasticity model is very necessary to study the plastic forming of metallic material, especially the crystalline characteristics should be considered in the research.     

Effect of oxide scale on temperature-dependent interfacial heat transfer in hot stamping process

An optimization-based numerical procedure was developed to determine the temperature-dependent interfacial heat transfer coefficient (IHTC). The effects of temperature, pressure and oxide scale thickness were analyzed, for oxide thickness between 9 μm and 156 μm and pressure from 8 MPa to 42 MPa. Oxide scales and contact pressure both show distinctive effects on IHTC in the cooling process. The average IHTC decreases about 2461 W/(m² °C) with the increase of oxide scale thickness and increases 2620 W/(m² °C) with the increase of pressure. Based on the two-way ANOVA, the effect of contact pressure influences the IHTC most. Their mutual interaction is negligible. The IHTC decreases when the average temperature between the blank and die surface is above 250 °C and increases when the latent heat release.     

Improvement of formability in deep drawing of ultra-high strength steel sheets by coating of die

The formability of deep drawn ultra-high strength steel sheets in dies coated with either titanium nitride (TiN) or Vanadium carbide (VC) at different drawing speeds and ironing ratios was investigated. TiN was deposited via chemical vapour deposition and physical vapour deposition (PVD) while thermal diffusion was used for VC deposition. In non-coated dies, seizure occurred on both surfaces of the die and the side wall of the drawn cup irrespective of the deep drawing conditions. The deep drawability is improved with coating of die. Whereas in coated dies, seizure became significant only during deep drawing extreme conditions of 120 mm/s for TiN-coated dies; and this was prevented in VC-coated dies across all drawing conditions. The VC-coated die was suitable for deep drawing of ultra-high strength steel sheets. The delayed fractured observed in the ultra-high strength steel cups occurred for a large amount of ironing ratio and drawing speeds; and this can be prevented by appropriate heat treatment.     

Influence of anisotropy of the magnesium alloy AZ31 sheets on warm negative incremental forming

Single point incremental forming of the magnesium alloy AZ31 sheets, which were fabricated by hot extrusion, slab + hot/cold rolling, strip-casting rolling and cross-rolling, respectively, was investigated at elevated temperatures. The results show that the anisotropy of the sheets fabricated by casting slab + hot/cold rolling and cross-rolling is not remarkable, and the formability is improved significantly. The circular, square and rotary cone parts were performed with satisfactory surface quality and without any microcracks successfully, and which is superior to those of the extruded sheet and the one-way rolled sheet. Therefore, anisotropy of the sheets has remarkable effects on the surface quality of the formed parts, and the effect becomes weakened with increasing temperature. It is proposed that cross-rolling sheet is much more suitable for warm SPIF process.     

Numerical and Experimental Study of the Efficient Parameters on Hydromechanical Deep Drawing of Square Parts

In hydromechanical deep drawing (HDD), a chamber of fluid replaces the matrix and the final form of part is determined based on the form of rigid punch. Allowable working zone in this process indicates the applicable range of chamber pressure versus drawing ratio to achieve a rupture-free drawing. In this article, the HDD of the square parts was studied using the finite element method (FEM) and the effect of different parameters of the process such as pre-bulging pressure, chamber pressure, and friction coefficient on the working zone was investigated. The results showed that increasing of the friction between blank and die or blank and blank-holder confines the working zone, while by increasing of the friction between blank and punch, the working zone becomes larger. A study was also carried out using experimental setup for verifying the FEM results. Finally, the numerical results were compared with experimental ones.     

Evaluation of stamping lubricants in forming advanced high strength steels (AHSS) using deep drawing and ironing tests

In forming AHSS, the lubricant must reduce the friction between die and sheet as well as the effect of heat generated from deformation and friction, especially in forming at high stroking rates. In this study, the effectiveness of stamping lubricants was evaluated by using the deep drawing and ironing tests. Various stamping lubricants were tested in forming of DP590 GA round cup samples. In these tests, the performance of lubricants was ranked via evaluation criteria that include punch force and the geometry of tested specimens. Deep drawing tests were conducted at two different blank holder forces, BHF (30 and 70 ton) at a constant ram speed (70 mm/s). The ironing tests were conducted to evaluate the performance of lubricants at higher tool–workpiece interface pressure than that is present in deep drawing. Polymer-based thin film lubricants with pressure additives (e.g. Lubricants A and B) were more effective than other lubricants as shown by the force (e.g. maximum punch force and applicable BHF without cup fracture) and geometry indicators (e.g. draw-in length, flange perimeter and sidewall thinning).The pressure and temperature distributions at the die–sheet interface were predicted by FE simulation of deep drawing and ironing tests. As expected, the value of interface pressure and temperature were maximum at the die corner radius.     

Effect of hot rolled grain size on the precipitation kinetics of nitrides in low carbon Al-killed steel

The precipitation of nitrides plays a general role in the industrial processing of deep drawing quality Al-killed low carbon steels. In this paper, the effect of hot rolled grain size on the precipitation of nitrides has been analysed. To evaluate the effect of grain size on the nitride precipitation kinetics, thermoelectric power based investigations have been performed on hot and cold rolled specimens.In the hot rolled state, the precipitation of nitrides occurs more intensively in the fine grain size microstructure (average grain size = 9 μm) than in the large grain size microstructure (average grain size = 23 μm) until the precipitated fraction of nitrides reaches about 70%. In the cold rolled state the effect of grain size is much less significant; probably the precipitation process occurs simultaneously at the grain boundaries and along dislocations. According to the simulation results, significant differences can be found between the precipitated fraction of nitrides in fine and large grain size sheets coiled in the temperature range 550–650 °C. In this interval, the precipitated nitride fraction is about two times larger in a fine grain microstructure (9 μm) than in sheets with 23 μm average grain size. The local position in the coil also affects significantly the precipitated fraction of nitrides. In the outer ring of the coil, less than 20% precipitated fraction is predicted in coiling temperature range 550–700 °C. However, in the middle ring of a hot rolled coil, the precipitated fraction changes from 5% to 85% with increasing coiling temperature from 550 to 700 °C.     

Development of incremental deep drawing process

This paper deals with development of an incremental deep drawing process. On a newly developed incremental deep drawing set-up, the aluminium sheets are formed; the forming is carried out by deep-drawing the blank as in the conventional method but incrementally. Fractures at the punch or die corner in the blank may or may not occur depending on the conditions; the process parameters involved are punch size, punch corner radius, increment in punch displacement, blank holding force or pressure, etc. It is thus shown that different shapes are formed by one set of common tools. It is thereby confirmed that incremental deep drawing is possible without using a particular tool set for a particular shape.