Warm and hot punching of ultra high strength steel sheet

Warm and hot punching using resistance heating was developed to improve the quality of sheared edges of an ultra high strength steel sheet. As the heating temperature increased, the depth of the shiny burnished surface on the sheared edge increased and that of the rough fracture surface decreased. The rollover depth and burr height of the sheared edge became large above 800 °C. Although the roughness of the burnished surface was almost constant, the roughness of the fracture surface increased from 650 °C. The punching load was extremely reduced by the heating, i.e. 40% of the cold punching load at 650 °C and 15% at 1070 °C. The local resistance heating of the shearing region was efficient for the warm and hot shearing. It was found that the warm and hot shearing of ultra high strength steel sheets is effective for improving the quality of the sheared edge and in reducing the shearing load.     

5A90 Al-Li合金桁条电流自阻加热成形技术

Al-Li合金具有密度低、比强度高等优点,是实现飞行器结构减重的理想材料,在航天航空领域显示出了广阔的应用前景。先进的电加热成形技术是提高轻质合金成形性能的有效手段。通过高温拉伸实验研究了5A90 Al-Li合金的成形性能,确定了5A90 Al-Li合金的最佳成形温度为340℃。通过电流自阻加热升温实验,得到了加热5A90 Al-Li合金板材合适的电流密度为10 A·mm~(-2)。针对5A90 Al-Li合金室温成形性能差的问题,设计并制造了5A90 Al-Li合金桁条电流自阻加热成形装置,在此装置上进行了5A90 Al-Li合金桁条的电加热成形实验,得到了符合要求的桁条零件,并分析了温度、润滑条件等对成形质量的影响。这种新型的板材电流自阻加热成形工艺及装备具有较大的实际应用价值。     

Punching of ultra-high strength steel sheets using local resistance heating of shearing zone

A punching process using local resistance heating of a shearing zone was developed to shear ultra-high strength steel sheets. The shearing zone was heated by passing electric current between the sheet holder and the knockout in order to decrease the flow stress in the shearing, and the heating of the die and punch was prevented by no contact with the sheet during the heating. Electrode pins having an individual spring were employed to attain uniform heating of the shearing zone. The welding resistance of the heads of the electrode pins to the sheet by the heating was examined for Ag-W, Cu-W, Ag + WC and W. The Cu-W pins having the highest welding resistance were employed in a punching experiment of 980 MPa level ultra-high strength steel sheets. The punching load was considerably reduced by the heating, e.g., about 1/5 of the cold punching load at 800 °C. As the heating temperature increased, the depth of the shiny burnished surface on the sheared edge increased and that of the rough fracture surface decreased.     

Tailored die quenching of steel parts having strength distribution using bypass resistance heating in hot stamping

A tailored die quenching process of steel parts having a strength distribution using bypass resistance heating in hot stamping was developed. In the tailored die quenching process, zones requiring high strength in a quenchable steel sheet were heated, and then were quenched. In the bypass resistance heating, zones in contact with copper bypasses having a low resistance and large cross-sectional area were not heated due to the passage of the current though the copper bypasses. The bypass resistance heating was stable even for the heavy current in rapid heating of the steel sheets because of passage of current in one direction, and the electrical power loss was small. The hardenability for the bypass resistance heating was first examined by sandwiching a partially heated sheet between large steel blocks without deformation. Next, the tailored die quenching process using bypass resistance heating in the hat-shaped bending of the steel sheet was performed to form a part having high strength around the corners. A hat-shaped part having a tensile strength of approximately 1.5 GPa around the corners was formed, and the input energy and punching load in the bottom of the bent sheet were considerably smaller than those for whole heating.     

Springback behaviour in bending of ultra-high-strength steel sheets using CNC servo press

The springback behaviour of ultra-high-strength steel sheets in bending was investigated under controlled conditions using a CNC servo press. Although the ultra-high-strength steel sheets are attractive in reducing weight of cars, the amount of springback of the ultra-high-strength steel sheets in the forming is very large due to high strength. The CNC servo press has the function of accurately controlling the motion by two servo motors. The effects of the material, the finishing reduction in thickness, the forming speed and the holding time at the bottom dead centre on the amount of springback in V-shaped bending were examined. The scatter of the springback for the ultra-high-strength steel sheets was improved by using real thickness and not nominal thickness of each individual sheet in the control of the punch stroke. The amount of springback for the ultra-high-strength steel sheet in the V-shaped bending was much larger than that for the mild steel sheet, and the amount was decreased by the finishing reduction in thickness direction because of uniform stress distribution. The effects of the forming speed and the holding time at the bottom dead centre were small. The amount of springback for the steel sheets was almost proportional to the ratio of the tensile strength to the elastic modulus.     

Effects of hot rolled shear bands on formability and surface ridging of an ultra purified 21%Cr ferritic stainless steel

An ultra purified 21%Cr ferritic stainless steel (FSS) was hot rolled at a low finisher entry temperature (FET) of 750 °C to generate shear bands through large shear flow localizations in grain interiors. The effects of shear bands on the formability and surface ridging have been studied as compared to a conventional hot rolled band at the FET of 970 °C with few shear bands. The results showed that as compared to the final sheet produced with the FET of 970 °C, the average r-value for the final sheet with the FET of 750 °C was increased by 25%, and the surface roughness of the final sheet strained by 15% along the rolling direction was decreased by 40%. It has been observed that the existence of shear bands could enhance the nucleation for recrystallization during hot rolling and annealing for microstructure refinement, and modify the texture by intensifying the {1 1 1} textures in the final sheet. The influence of shear bands on sheet formability and resistance against surface ridging was discussed.     

Influence of initial texture on formability of AZ31B magnesium alloy sheets at different temperatures

Repeated unidirectional bending (RUB) process was carried out to improve the texture of AZ31B magnesium alloy sheets. Influence of initial texture on formability of AZ31B magnesium alloy sheets at different temperatures was investigated. Compared with the as-received sheets, the limiting drawing ratio of the RUB processed sheets increased to 1.3 at room temperature, 1.5 at 50 °C and 1.7 at 100 °C, respectively. The improvement of the press formability at lower temperatures can be attributed to the texture modification, which led to a smaller Lankford value and a larger strain hardening exponent. However, the press formability of the sheet with a weakened basal texture has no advantage at higher temperature. This is due to much smaller r-value that results in severe thinning in thickness direction during the stamping process which is unfavorable to forming. Anyhow it is likely that the texture control has more effect on the press formability at lower temperature.     

Self-piercing riveting of high tensile strength steel and aluminium alloy sheets using conventional rivet and die

High tensile strength steel sheets having different strengths were joined with an aluminium alloy sheet by a self-piercing rivet. In the joining, a conventional rivet and die used for aluminium alloy sheets were employed in order to have the versatility for various steel sheets. The effects of the flow stress of the high strength steel sheets and the combination of the sheets on the joinability of the sheets were investigated by finite element simulation and an experiment. As the tensile strength of the high strength steel sheet increases, the interlock for the upper high strength steel sheet increases due to the increase in flaring during the driving through the upper sheet, whereas that for the lower high strength steel sheet decreases. The joint strength for the lower high strength steel is comparatively smaller than that for the upper high strength steel sheet. It was found that the high tensile strength steel sheets below 590 MPa were fully joined with the aluminium alloy sheet even with the conventional self-piercing rivet and die.     

Experimental and numerical analysis of multilayered steel sheets upon bending

In this study, the bending formability of multilayered steel sheets is evaluated by tensile tests, V-bending tests, and hemming tests. Enhanced formability was observed in these experiments, namely, the constituent high-strength materials were elongated beyond the original fracture strain limit. As a result of this effect, multilayered steel sheets were successfully formed in V-bending tests and even in hemming tests. Observations using a scanning electron microscope verified that no delamination occurred at interfaces. To represent the geometrical features of a multilayered steel sheet, a solid-element model under an isostrain condition was utilized in finite element modeling, where the rule of mixtures was adopted to obtain the flow curve of the constituent high-strength material, and a good agreement with experimental results was observed. Analyses using this finite element model were conducted to investigate the effect of the geometry on the springback of multilayered steel sheets undergoing V-bending.     

织构与晶粒尺寸对AZ31镁合金薄板成形性能的影响

通过不同的轧制工艺,制备了4种具有不同晶粒尺寸和织构的镁合金板材;通过单向拉伸试验和室温埃克森试验,探讨了晶粒尺寸与织构对镁合金板材室温成形性能的影响。研究表明,晶粒细化虽然增强了板材的力学性能,但却不利于提高板材的胀形性能;基面织构减弱使板材沿厚向的变形能力提高,具有较好的胀形性能,但却造成板材屈服强度的降低。     

Effects of process parameters on warm and electromagnetic hybrid forming of magnesium alloy sheets

As the lightest structural metal, magnesium (Mg) is attracting increasing interest from both the industrial and academic fields. Magnesium alloy parts are mainly processed by die casting due to their poor sheet formability at room temperature. Warm forming is a popular method of forming; Mg alloy sheets produced in this manner show excellent formability around 200-400 °C. Electromagnetic forming (EMF) can improve the formability of metal sheets without the need for lubricants. In this paper, a new approach, called warm and electromagnetic hybrid forming (WEMF), is presented. The effects of voltage, capacity, and temperature on the bulging height of Mg alloy sheets are investigated. Results show that the bulging height of Mg sheets increases with moderate discharging energies. Enhancing the discharging voltage is also a more efficient method for increasing bulging height compared to simply increasing the capacity. When the discharging energy is kept constant, the bulging height first decreases (<150 °C) and then increases (>150 °C) from room temperature to 230 °C. The formability of Mg alloy sheets improves with increasing temperature, while the forming efficiency of WEMF decreases under similar conditions.     

Fractional Cooling Strategy of the Hot-Stamping Process and Its Influence on Formability and Mechanical Properties of Ultra-High-Strength Steel Parts

The effects of forming temperature on the formability and product properties of hot-stamping boron steel B1500HS were investigated. Based on the fractional cooling strategy, boron steel sheets were heated to achieve full austenitization before they were removed from the furnace and cooled to the forming temperature using different cooling methods. Subsequently,they were simultaneously press-formed and quenched inside the tool until the martensitic transformation was finished. A series of thermal tensile tests were conducted to study the effects of forming temperatures on the stamping performance indices, including elongation, yield ratio, and hardening exponent. Then, the mechanical properties and microstructures of the hot-stamped products were characterized. Finally, an irregular part was formed using different fractional cooling strategies, while its formability and springback phenomena were discussed. The results show that using a fast-cooling method to reach 650 °C as the forming temperature optimizes the formability of the tested B1500HS boron steel. The best mechanical properties and smallest springback values were achieved using this optimal strategy.     

Springback characteristics of steel sheets for warm U-draw bending

The purpose of this study is to investigate the characteristics of springback for various process conditions of the U-draw bending operation. The process variables are the forming temperature and the tool geometry, including punch profile radius (Rp) and die profile radius (Rd). In order to control springback, the use of a warm forming method is applied. For the warm draw-bending, five steps of temperature ranges, from room temperature to 200°C, were adopted. Two kinds of steel sheets, cold rolled carbon steel (SCP1, for general purposes in the automobile industries) and TRIP (transformation-induced plasticity) steel, were adopted. TRIP steel was the newly developed high-strength steel sheet. The results indicated that elevated temperature and the geometry of tools in the two kinds of steel sheet affected the springback.     

Identification of heat treatments for better formability in an aluminum-lithium alloy sheet

Research in the weight of an automobile is a continuous process among auto manufacturers. The “body in white” (BIW, i.e., the body of the car) deserves attention, being a major contributor to the weight of the vehicle. By virtue of a high strength to weight ratio (density smaller than aluminum) and a higher Young’s modulus than aluminum, aluminum-lithium alloy sheet appears to hold promise as an autobody material. Because auto components are required in large numbers and are formed at room temperature, formability under these conditions becomes significant. Aluminum-lithium alloys acquire, because of aging over a short period of time, a good amount of strength and hence dent resistance. In principle, they can be given, through suitable heat treatments, a high formability as well as dent resistance, i.e., an ideal combination of properties. To this end, tensile properties have been determined for a number of heat treatments comprising three different solutionizing temperatures and for three aging times at each of the three aging temperatures. Considerable influence of heat treatment was observed on the mechanical properties (which in turn characterize both formability and dent resistance), such as the strain hardening exponent, average normal anisotropy, yield stress, ultimate tensile stress, and percentage elongation to failure. For each property, the best three heat treatments leading to a high formability were identified. Consequently, heat treatments that imparted the greatest formability for processes such as deep drawing and stretch forming have been identified. The investigations show that the best heat treatment for one property may not be the best for another property, calling for a compromise to obtain the most practicable heat treatment schedule. Results shed light on not only the biaxial formability but also springback behavior that is important in the BIW components. Further, the properties obtained from the heat treatment giving good formability in deep drawing were used to simulate car body fender and the S-rail using sheet metal forming simulation software PAMSTAMP2G. A comparison of simulation of aluminum-lithium alloy fender and S-rail with those made from steel demonstrates advantages using aluminum-lithium alloys in terms of weight reduction. Finally, based on the current oil prices and the projected demand for oil in the next decade, aluminum-lithium alloys seem to have an edge despite the difficulties in manufacturing, assembly, and joining of the aluminum-lithium components.     

Pulse Current Auxiliary Thermal Deep Drawing of SiCp/2024Al Composite Sheet with Poor Formability

The pulse current auxiliary thermal deep drawing (PCATDD) of SiCp/2024Al composite sheets with poor formability was investigated and an integrated high-efficient heating and deep drawing process design was developed to improve the formability of SiCp/2024Al composites. The average pulse current density was achieved at 21.7?A/mm² with temperature of SiCp/2024Al composites reaching around 673?K in the 50?s. The temperature uniformity of the sheet electrified by the high-intensity pulse current and temperature gradients between the sliding core and top flat of female die were investigated to achieve precise temperature control of sheet and die. During electrification, the stainless steel inserts between the sheet and the copper electrodes successfully prevented heat dissipation and promoted temperature uniformity. Meanwhile, the temperature gradients can be efficiently controlled by blowing high-speed air. The workpiece showed good shape retention, surface quality, and high geometry accuracy. The present study has verified successfully the feasibility of process procedure for PCATDD of SiCp/2024Al composites sheet.     

Enhancement of bending formability of brittle sheet metal in multilayer metallic sheets

The formability of multilayer metallic sheets is evaluated by tensile, V-bending, hat bending and hemming tests. A monolithic type-420J2 stainless steel sheet cannot be formed because of poor elongation as small as 1.7%. Marked enhancement of the bending formability was observed in the bending of type-420J2 stainless steel sheets when they are layered by type-304 stainless steel sheets and composed into a multilayer metallic sheet. The mechanism of the enhancement of the formability of type-420J2 stainless steel in a multilayer metallic sheet is investigated analytically by focusing on the delay of the initiation of necking, and by performing stress analysis by finite element method (FEM).     

The Effect of Adhesion and Tensile Properties on the Formability of Laminated Steels

Laminated steel has been implemented in vehicle structures by several automotive manufacturers to reduce in-cabin noise. This study provides an understanding of how the adhesion between the steel skin and the viscoelastic polymer core affects laminate formability. Material properties, including peel strength, shear strength, and tensile strength were determined. The presence of the viscoelastic core was found to slightly reduce tensile properties of the laminate compared to the skin sheet. Forming limit diagrams were also determined. These indicated that the viscoelastic core properties can significantly affect formability of laminated steel compared to that of solid steel sheet. In general, the formability of laminated steel was found to be similar to or less than that of the much thinner skin sheet material, which indicates that its formability should be less than that of solid steel of the same gauge. This article was presented at Materials Science & Technology 2007, Automotive and Ground Vehicles symposium held September 16-20, 2007, in Detroit, MI.     

Effect of interfacial oxidation occurring during the duplex process combining surface nanocrystallisation and co-rolling

This paper presents an investigation of the interface quality of nanocristallised 316 L stainless steel multilayer structures. They were produced by a duplex process, combining the Surface Mechanical Attrition Treatment (SMAT) and the co-rolling process at two different annealing temperatures (550 °C and 650 °C). Oxide layers were observed at the interfaces between the sheets and their morphology was characterised by optical microscopy. Their chemical composition was determined by Energy Dispersive X-ray spectrometry. The microstructure near the interfaces was analysed by Transmission Electron Microscopy (TEM). In the laminate co-rolled at 550 °C, the presence of ultrafine grains was demonstrated. Additional tensile tests have shown an influence of the annealing temperature on the yield strength, as well as on the resistance of the interfaces of the co-rolled multilayer structures.     

Effect of heat treatment on formability in 0.15C−1.5Si−1.5Mn multiphase cold-rolled steel sheet

The effects of volume fraction and the stability of retained austenite on the formability of a 0.15C−1.5Si−1.5Mn (hereafter all in wt.%) TRIP-aided multiphase cold-rolled steel sheet were investigated after various heat treatments. The steel sheets were intercritically annealed at 800°C, and isothermally treated at 400°C and 430°C. Microstructural observation, tensile tests and limiting dome height (LDH) tests were conducted on the heat-treated sheet specimens, and the changes in retained austenite volume fraction as a function of tensile strain were measured using an X-ray diffractometer. The results showed a plausible relationship between formability and retained austenite stability. Although the same amount of retained austenite was obtained after isothermal holding at different temperatures, better formability was obtained in the specimens with the higher stability of retained austenite. If the stability of the retained austenite is high, the strain-induced transformation of retained austenite to martensite can be stably progressed, resulting in a delay of necking to the high strain region and improvement in formability.     

Plastic Joining of Ultra High Strength Steel and Aluminium Alloy Sheets by Self Piercing Rivet

Ultra high strength steel and aluminium alloy sheets were plastically joined by a self piercing rivet driven through the upper sheet and spread in the lower sheet with a die. The self piercing rivet directly pierces into the sheets without drilling the sheets beforehand unlike the conventional rivets. Insufficient driving though the upper sheet and fracture of the lower sheet occur due to the high hardness and low ductility of the ultra sheet, respectively. An ultra high strength steel sheet having a tensile strength of 980MPa and an aluminium alloy sheet were successfully joined by optimising shapes of the die.