The main objective of this work was to experimentally and numerically evaluate the forming limit curve (FLC) of aluminum alloy 2B06. The FLC of 2B06 was measured by conducting the hemispherical dome test with specimens of different widths. The theoretical predictions of the FLC of 2B06 were based on the different instability theories and different yield functions. The comparison results show that the influence of the different yield functions can be ignored and the FLCs are basically same utilizing different yield functions based on the specific instability theory. While there is a significant difference among theoretical prediction curves based on three instability theories and the same yield function. The FLC based on SWIFT’s diffuse instability theory is higher than the measured curve. The right part of FLC based on HILL’s localized instability theory is invalid. The theoretical prediction curve based on M-K theory agrees well with the measured FLC. So, the theoretical curves based on M-K theory are effective for predicting the forming limit.

The forming limit diagram (FLD) and forming limit stress diagram (FLSD) of aluminium alloy 1060 under linear and nonlinear strain paths are investigated. The calculation of FLSD is based on experimental FLD using the method proposed by Stoughton. Different from the FLD that varies with the strain path, the FLSD is not sensitive to the strain path. Therefore, FLSD is convenient as a forming limit criterion for multi-stage sheet forming. The influences of the material’s yield criteria on FLSD are also discussed by comparison of the Hill’s 48, Hill’s 79 and Hosford non-quadratic criterion. The impacts of material hardening laws (Voce and Swift models) on translation of FLD and FLSD are analyzed. The Voce hardening law and the Hosford yield criterion are appropriate for the FLSD calculation of the aluminium alloy 1060. The stress calculation program and display interfaces of FLD and FLSD are developed on MATLAB, where the strain data can be input from experiment measurement or FEM calculations.

The plastic deformation process on hydroforming of aluminum alloy tube with rectangular sections was carried out by means of experiment and numerical simulation using solid elements. The stress and strain states of typical points were analyzed. The three-dimensional graphic representation of normal stress was done. The stress locus in the yielding-cylinder was obtained. It is shown that the plastic yielding firstly occurred in the outer layer of the corner. The inner layer of the corner at each end about 1/4 region is the last region which came into yielding. The strain state of the corner is shortened in the thickness direction and elongated in the hoop direction. The plastic deformation firstly occurred in the transition point and the equivalent strain of transition point was the largest all the time. The axial stress of the transition point is tensile stress all the time as the internal pressure increases.

To achieve the precision bending deformation, the springback behaviors under various forming conditions should be clarified preliminarily. Taking the thin-walled 6061-T4 Al-alloy tube of 50.8 mm×0.889 mm (outer diameter×wall thickness) as the objective, the single-factor experimental analysis and the 3D-FE based numerical orthogonal test are conducted to address the effects of forming parameters on the springback behaviors in 6061-T4 Al-alloy tube bending. The results show that: 1) The springback angle increases linearly with increasing of the bending angle. 2) The significant factors from high to low are the clearance between tube and mandrel, the bending radius, the friction between tube and pressure die, the clearance between tube and wiper die, the clearance between tube and pressure die, the coefficient of boost velocity, the friction between tube and mandrel, the number of mandrel balls. 3) The effect rules of significant parameters on springback of 6061-T4 Al-alloy tube are similar to those of stainless steel and Ti-alloy tubes. Springback becomes larger with increasing of the bending velocity, the tube-die clearance, the relative bending radius, the tube-pressure die friction and relative push assistant speed. While the springback decreases with increasing of the mandrel extension length, the number of mandrel balls and tube-mandrel friction.

Free bulging test was carried out at different temperatures ranging from 350 °C to 500 °C to evaluate the formability of AA6061 extruded tube, which can provide technology foundation for complex structures forming in hot metal gas forming (HMGF) process. Maximum expansion ratio (MER) and bursting pressure were obtained to evaluate directly the formability at heated conditions. Vickers hardness at different positions was measured. The fracture surface after bursting was observed with scanning electron microscope (SEM), and the microstructure change along axial and hoop directions was analyzed by electron backscattering diffraction (EBSD). The results show that the largest MER value is 86% at 425 °C. Bursting pressure decreases from 4.4 MPa to 1.5 MPa with temperature increasing. The Vickers hardness of fracture position is a little higher than other positions after gas bulging. The fracture mechanism is still the micro-pore aggregation fracture at elevated temperature, while overheated structure appears seriously at 500 °C. The initial fine equiaxial grain grows as temperature increases, which is elongated simultaneously in both axial and hoop directions.

Thermomechanical treatment is usually used to achieve good mechanical properties and microstructure of metal materials. The effect of pre-deformation on the microstructure and mechanical properties of 2219 aluminum alloy sheet by TMT process was investigated. The studies show that the yield strength and tensile strength increase firstly and decrease with the increase of pre-deformation under a certain heat treatment condition and total deformation. The tensile strength of 2219 aluminum alloy sheet reaches the maximum value when the pre-deformation is about 2%. It is also found that the very fine Al₂Cu appears inside the grain, which is the precipitated phase for the strengthening of the 2219 aluminium alloy.

The effects of internal pressure on forming defects, corner radius and thickness distribution of 5A02 aluminum alloy shear hydro-bending tubes were studied by experiment. Numerical simulation was conducted to analyze the effect of internal pressure on axial strain and invariable lines of thickness strain. The ultra-small bending tubes were successfully manufactured when the relative internal pressure, ratio of internal pressure and yield stress of the material, is higher than 0.2. The relative bending radius of the first outer corner decreases from 0.3 to 0.025 when the relative internal pressure increases from 0.2 to 1.2. The axial thickness distribution is different in intrados and extrados. The changing rate of thickness is larger with a higher internal pressure. The minimum thickness decreases from 1.45 mm to 0.87 mm when the relative internal pressure changes from 0.2 to 1.2. The tube is divided into feeding zone, the first corner, shearing zone, the second corner and holding zone. The strain of feeding zone and the first corner is compressive caused by the feeding. The strain of the second corner and holding zone is tensile for far away from feeding punches. The strain of shearing zone changes from compressive to tensile with rising of internal pressure. On one hand, the smaller corner radius formed by higher internal pressure blocks the feeding. On the other hand, the corner filling strengthens the extensive strain of shearing zone. In the feeding zone and holding zone, thickness strain is positive, and the tube thickens. In the corner and shear zones, thickness strain is negative, and the tube thins.

A shear hydro-bending process was proposed to solve the difficulties in forming aluminum alloy rectangular tubes with small bending radius, which cannot be integrally manufactured using conventional bending methods. Numerical simulation and experimental research were conducted on defects during shear bending process. Effects of the internal pressure, feeding ratio and die inner bend corner radius were comprehensively investigated. The results show that four defects including wrinkling, cracking, pitting and folding appear if the internal pressure and the axial feeding and transversal stroke are not appropriate. The shear hydro-bent tube is performed successfully as the internal pressure ranges from 0.2σ_s to 0.9σ_s and the feeding ratio ranges from 1.0 to 1.1. It can be concluded that the shear hydro-bending is suitable to form aluminum alloy rectangular tubes with small bending radius by controlling the internal pressure, the axial feeding and the transverse stroke. There is a reasonable process window of the internal pressure and the feeding ratio, in which the tube can be successfully formed without defects.

The aim of this work is to investigate the deformation behavior of commercial 5A06 aluminum alloy sheet for rapid gas forming at elevated temperature. So-called cup-shaped workpieces were formed under different conditions. The temperatures ranged from 325 °C to 500 °C. The gas pressure was 2.5 MPa and 4.0 MPa, with different duration time from 8 s to 120 s. The profiles, corner radius and thickness distribution of the formed specimens were measured and analyzed. The results show that the formability of the sheet is improved by increasing the testing temperature, duration time and gas pressure. Corner radius changes obviously before the temperature attains to 450 °C or within 30 s, and then trends to the minimum value (about 2.0 mm) gently. Higher forming gas pressure can be used to reduce the duration time at the appropriate temperature ranging from 400 °C to 500 °C. However, the sheet needs enough deformation time to conform final corner filling when the gas pressure is 4.0 MPa. The minimum wall thickness appears at the transition zone between corner and bottom.

The effects of interfacial adhesion strength between skin sheet and core polymer on the formability of AA5052/ polyethylene/AA5052 sandwich sheets was investigated. A numerical simulation model considering the interface conditions between skin sheet and core materials was developed for simulating the forming process of sandwich sheet. Comparisons between the experimental results and calculations verify the proposed model. Then, the rigid punch tests and the NAKAZIMA forming tests were carried out for sandwich sheets with three kinds of interface conditions (separation, adhesion and stick). The influences of interfacial adhesion strength on the damage behavior of skin sheet and the forming limit diagrams (FLD) of sandwich sheets were investigated. The results show that the interface stress can suppress the increasing of void volume fraction and then postpone the fracture of skin sheet. The FLD of sandwich sheet with stick interfacial condition is higher than those of sandwich sheets with adhesion and separation interfacial conditions. It can be concluded that the FLD of sandwich sheet shifts to higher value with the increasing of interfacial adhesion strength.

Equal channel angular pressing and torsion (ECAPT) was applied to fabricate SiC_p-Al composites from the mixture of 35% SiC (volume fraction) particles and 65% Al powder. Based on microstructure observation and the quadrat method, the distributions of SiC particles in different positions of the samples and in different forming stages were studied. And the particle distribution was contrasted with that in the samples fabricated by multi-pass equal channel angular pressing (ECAP). The results show that the shear strain influences the distribution of SiC particles significantly. The distribution homogeneity of SiC particles increases greatly from the compaction stage to the angular pressing stage during ECAPT. The improvement of the particle distribution homogeneity through single pass of ECAPT process is close to that through two passes of ECAP. The relative homogeneous distribution of SiC particles is obtained through ECAPT.

According to warm hydroforming with poor thickness uniformity and low expansion ratio, a new approach named warm hydroforming with non-uniform temperature field was presented. Warm hydroforming die with nonuniform temperature field was designed and temperature field along tube axis was established with differential temperature gradient. Then, the effect of the temperature difference between the forming zone and the feeding zone on thickness uniformity along the part axis was studied in a certain loading path. Thickening of the feeding zone decreases and the thickness uniformity of tubular part is improved by selecting appropriate temperature difference, for this experiment, suitable temperature difference is 150 °C. Further, the effect of the preform shape formed by warm hydroforming on the limit expansion ratio of magnesium alloy tube was studied, using preform with wrinkles and preform with expansion ratio of 35% hydroformed by using differential temperature. The limit expansion ratio reaches 66.2% by using the preform with expansion ratio of 35%.

The warm tension-rotation bending process of AZ31 magnesium alloy as-extruded profile with thin-walled and multi-rib was researched by numerical and experimental methods. The effects of process parameters on springback characteristics of AZ31 bent profile were investigated. The results indicate that when the forming temperature increases from 100 °C to 200 °C, the numerical and experimental springback angles of AZ31 bent profile all decrease, the experimental springback angle decreases from 11.6° to 10.7°, and the springback ratio reduces from 11.26% to 10.39%. The relationship between the forming temperature and the springback angle seems to be linear. When the bending angle increases from 100° to 110°, the numerical and experimental springback angles of AZ31 bent profile all increase, the experimental springback angle increases from 10.8° to 11.5°, and the springback ratio increases from 10.48% to 11.16%. When the pre-tension amount increases from 0.2% to 1.1%, the numerical and experimental springback angles of AZ31 bent profile all decrease, the experimental springback angle decreases from 12.5° to 9.8°, and the springback ratio decreases from 12.14% to 9.51%.

AZ80 alloy semisolid billets were fabricated by a new strain induced melt activated method (SIMA), which involved the predeformation of as-cast AZ81 alloy via equal channel angular extrusion (ECAE) and the following semisolid isothermal treatment of ECAE-processed AZ80 alloys. The results show that highly strain-induced effect is successfully achieved by ECAE due to refined microstructure and the mechanical properties are enhanced. High-quality AZ80 semisolid billets with fine and spherical grains are fabricated by new SIMA method. The results of thixoforged experiment confirm that enhanced mechanical properties including yield strength of 216.9 MPa, ultimate tensile strength of 312.4 MPa and elongation of 26% are successfully achieved. It also confirms that new SIMA method is a very desirable method for fabricating AZ80 alloy semisolid billets.

In order to improve the mechanical properties of ZK60 magnesium alloy components, the multi-axial forging (MAF) plus partial remelting route was used in this study. The effect of remelting temperature and holding time on microstructure of semi-solid ZK60 magnesium alloy was also studied. Furthermore, the tensile mechanical properties of ZK60 , magnesium alloy components produced by multi-axial forging plus partial remelting route were compared at different thixoforging temperatures. The results showed that the multi-axial forging followed by partial remelting is an effective route to produce semi-solid ZK60 magnesium alloy for thixoforming. During the partial remelting, with prolonging the holding time and increasing the remelting temperatures, the solid grain size increased. With the increase of remelting temperature, the degree of spheroidization tended to be improved. With increasing the thixoforming temperature from 560 °C to 574 °C, the tensile mechanical properties of ZK60 magnesium alloy were improved.

The microstructure development in the semi-solid state of AM60B magnesium alloy prepared by strain induced melt activation (SIMA) was described. In the SIMA route, cyclic extrusion compression (CEC) and conventional compression were used to predeform as-cast AM60B, respectively. Partial remelting and thixoforming were also carried out within the semi-solid zone. The results showed that coarse-grained structure disappeared and a fine-grained structure resulted in the CEC formed alloy. However, coarse grains and recrystallized grains co-existed in the compression formed alloy. During partial remelting, the alloy treated by CEC exhibited ideal, fine microstructure, in which completely spheroidized grains contain little entrapped liquid. Polygonal grains were spheroidized to some extent but the previous irregular shape was still obvious in the compression formed alloy. The mechanical properties of the thixoformed AM60B produced by CEC plus partial remelting were better than those of the thixoformed alloy produced by compression plus partial remelting.

Pure magnesium bars were prepared by two-pass cumulative high ratio extrusion with as-cast Mg as the original material and then the final as-extruded bars were annealed. The effect of extrusion deformation and annealing treatment on the microstructure, mechanical properties and fracture behaviour of the Mg were investigated by optical microscopy (OM), mechanical properties test and scanning electron microscopy (SEM), respectively. The results show that the grain size is obviously refined by the effect of dynamic recrystallization during the extrusion deformation. Thus the room-temperature mechanical properties and fracture behaviour of the material were significantly improved. After the first extrusion, the coarse as-cast grain size was reduced to 35 μm, and the yield strength (YS), ultimate tensile strength (UTS) and elongation of the bar achieved 84 MPa, 189 MPa, and 12%, respectively. After the further extrusion, the YS of as-extruded bar was over 120 MPa; however, the elongation decreased due to work hardening. Finally the grain size of the as-extruded bar was 9-10 μm after annealing treatment, and its YS, UTS and elongation of the bar achieved 124 MPa, 199 MPa, and 10.7%, respectively. The microstructures and mechanical performance of the material were enhanced obviously.

Microstructure evolution and mechanical properties of AZ80 alloy reheated from the as-cast and deformed states were investigated. A new method, cyclic closed-die forging (CCDF), was employed to deformation of AZ80 as a recrystallization and partial melting (RAP) process. During partial remelting, finer, rounder and more homogeneous grains can be obtained from CCDF-formed alloys than from as-cast alloys. Prolonging isothermal holding time from 0 to 40 min, the mean grain size of solid particles in as-cast state decreased initially and then increased, however, that of CCDF-formed alloys increased continuously. The degree of spheroidization was improved in as-cast alloys with prolonging holding time. In contrast, in CCDF-formed alloys, the value of shape factor increased initially and then decreased. Microstructure evolution during remelting is dominated by many factors, for example distortion energy providing recrystallization driving force, Ostwald ripening mechanism, grain coalescence. Compared with the as-cast alloys, the CCDF-formed AZ80 alloy got a significant improvement in tensile properties. YS, UTS and elongation increased by 89%, 45% and 242% respectively. This can be mainly attributed to the grain refinement and elimination of defects.

In order to study the workability of Ti-6Al-4V alloy in the hot forming process for sheets and profiles, the stress-strain experimental data from isothermal hot tensile tests of flat specimens, in the temperature range of 923-1023 K and strain rate range of 0.0005-0.05 s^-1 were used to develop the constitutive equation. Arrhenius and Norton-Hoff constitutive models were proposed to characterize the tensile behavior. The fitting results suggest that both Arrhenius constitutive equation (material constants consider the compensation of strain) and modified Norton-Hoff one can predict flow stress of Ti-6Al-4V alloy under most experimental conditions. Further, the modified Norton-Hoff model is more accurate and precise than Arrhenius model.

TiB whiskers reinforced Ti6Al4V (TC4) alloy matrix composite (TiB_w/Ti64) was fabricated by in-situ reactive hot pressing. The high temperature deformation behavior of the composite was studied by hot compression tests carried out in the temperature range of 900-1100 °C and strain rate range of 0.001-10 s^-1. The results show that the flow stress of the composite decreases with the increase of temperature and the decrease of strain rate. At strain rate of 10 s^-1, discontinuous yielding followed by flow oscillations can be observed, especially in the β phase region. Constitutive equations were constructed based on the peak flow stresses of the stress-strain curves in α+β phase region and β phase region, respectively. Activation energies for the plastic deformation were also calculated in different temperature ranges according to the constitutive equations, namely, 822.3 kJ/mol in α+β phase region and 209.4 kJ/mol in β phase region. The deformed morphologies of the reinforcement network microstructure and the matrix microstructure greatly depend on the deformation regions and parameters.

Different shapes of pre-forming die were designed using MSC. MARC. The influence of surface friction of pre-forming die and final forming die on final thickness distribution was analyzed. The results show that the thinning of the blank near the periphery and the bottom of deep cylinder benefits to the uniform thickness. And higher friction coefficient of pre-forming die (0.57) can efficiently reduce the thickness of this regions and result in a more uniform final thickness distribution. Lower friction coefficient of forming die can make the sheet tend to integral formation, also results in uniform thickness distribution. The friction of pre-forming die is increased by machining and the friction of forming die is decreased by spraying BN ceramic powder. The aerospace Ti-6Al-4V deep cylinder with uniform thickness (1.50-1.78 mm) is fabricated successfully by using friction changing and direct-reverse superplastic forming method.

The mechanical properties of TA2 extruded tube were tested by uniaxial tensile test at high temperatures ranging from 700 °C to 850 °C and different strain rates changing from 4×10^-4 s^-1 to 4×10^-1 s^-1. The tube bulging test was carried out at elevated temperatures up to 950 °C to evaluate the formability of TA2 extruded tube for hot metal gas forming (HMGF). The total elongation and the maximum expansion ratio of the tube were obtained. The bursting pressure and fracturing manner were analyzed. The results show that the tensile strength of TA2 tube decreases practically with temperature increasing and strain rate decreasing. Meanwhile, the total elongation is between 142% and 331%. In tube bulging test, special experimental setup was established by the way of induction heating. The bursting pressure decreases from 6.5 MPa to 1.2 MPa as temperature increases. The maximum expansion ratio increases firstly and reaches the maximum value of about 70% at 890 °C, and then decreases. Fracture occurs along hoop direction, axial direction and random direction at different temperatures. The ideal temperature range for TA2 tube forming is from 860 °C to 920 °C in HMGF process.

In order to study the role of sintering neck in porous titanium with helical pores, the effect of size and position of sintering neck on compressive mechanical properties of porous titanium single cell was studied by using numerical simulation method. The results show that the compressive mechanical properties of the porous titanium unit cell are determined by the helical pore structure and sintering neck. Contribution coefficient of sintering neck is approximately 3.5 times larger than that of helical pore structure. With the increase of the relative diameter of sintering neck, compressive yield stress and elastic modulus of the cell are constantly increased. The sintering point of C1 is the most important sintering position. Under the same condition, increasing the size of sintering neck at C1 is much effective to the increasing of compressive properties.

Powders with nominal composition of Ti-45Al-10Nb (mole fraction, %) were prepared by mechanical ball milling and were then consolidated by hot-pressed sintering in vacuum atmosphere. The microstructures of 12 h-milling composite powders and sintered bulk materials were characterized by OM, XRD, SEM and EDX. The results showed that the composite powders were completely homogeneous, and fine-grained Ti/Al/Nb composite powders were made. The microstructure of the consolidated alloy was composed of fine equiaxed grains with large pure Nb particles. And the results also indicated that the compressive strength and ductility of the sintered TiAl-based bulk material at RT could be improved effectively by adding 10% Nb element. The yield strength (σ_0.2) and fracture strength (σ_b) of the alloy were 842 MPa and 1314 MPa, respectively, and the compressive ductility (δp) was 12.4%.

Smart hot stamping processes of ultra-high strength steel parts have been developed to overcome difficulties in the present hot stamping process. Using rapid resistance heating, the stamping equipment became considerably simpler than that using furnace heating. The formability was improved by high speed hot stamping using a mechanical servo press in comparison with the conventional process using a hydraulic press. Parts having a different distribution of strength were produced by tailored die quenching using local bypass resistance heating. Die-quenched steel parts and ultra-high strength steel sheets were sheared by reducing the flow stress in the shearing zone using local resistance heating. An ultra-high strength steel gear drum was produced by hot spline forming using resistance heating of side wall of a cup. V-shaped hot forming using sealed air was developed to produce ultra-high strength hollow axle beams used for suspensions of automobiles. A one shot hot stamping process consisting of resistance heating, forming, shearing and die quenching was applicable to comparatively small high strength steel parts.

In order to establish a new processing technique of producing steels with fine grain, asymmetric hot rolling and symmetric rolling were performed. The asymmetric condition was introduced by applying mismatched roll diameters. The diameter ratios between big and small rolls were 1.00, 1.05 and 1.11. The rolling temperatures were between 900 °C and 1100 °C. The thickness reduction of workpiece was set at 15%, 30% and 60%. The results showed that asymmetric rolling produced higher volume fraction of recrystallization grain and smaller average grain size at the center layer of rolled sample than symmetric rolling. The diameter ratio of 1.05 tended to generate the highest recrystallization level and the smallest average grain size. With high temperature and low thickness reduction value, the grain growth was obvious. The conventional dynamic recrystallization mechanism was prevailing, while accompanied by twinning. At low temperature of 900 °C, even at thickness reduction of 60%, symmetric rolling cannot initiate dynamic recrystallization as asymmetric rolling did. The asymmetric rolling force was smaller than that of symmetric rolling, except at 900 °C and thickness reduction of 60%. The rolling force also increased with descending temperature and climbing thickness reduction.

High performance components, e.g., fasteners, nowadays are usually made out of cold forged and heat treated steels like steel 1.5525 (20MnB4). To overcome the problems of heat treatment, e.g., low surface quality, new workpiece materials for cold forging should be found to achieve the needlessness of heat treatment after cold forging. One possible material is given by high nitrogen steels like steel 1.3815 (X8CrMnN19-19). Due to the high strain hardening of these materials the process and tool design for an industrial batch process are challenging and should be conducted by FE-simulation. The numerical results show that, high strength tool materials, like PM-steels or cemented carbides, in most cases, are inevitable. Additionally to the selection of suitable tool materials, the tool layout should be developed further to achieve a high loadability of the tools. The FE‑models, used for process and tool design, are validated with respect to the materials’ flow and occurring forming force to assure a proper design process. Also the comparison of strength of components made out of steel 1.5525 in quenched and tempered conditions and steel 1.3815 in strain hardened condition is done. The results show that the component made of steel 1.3815 has a significantly higher strength than the component made of steel 1.5525. This shows that by the use of high nitrogen steels a high performance component can be manufactured by cold forging.

The dynamic recrystallization (DRX) rules are the significant foundation for the effective control of DRX behavior, consequently achieving fine grains and qualified microstructure in the needle piercing extrusion for AISI304 stainless steel pipe. A reliable multi-scale FE model was developed for the extrusion process of seamless AISI304 stainless steel pipe (d29 mm×4.5 mm) under the DEFORM-2D software environment, and then the influence rules of the key extrusion parameters were numerically unfolded, namely the initial billet temperature and extrusion speed, on the DRX volume fraction, average grain size and their distributions by comprehensive simulations. The outcome establishes the basis and guidelines for the optimal design and steady control of the extrusion process in terms of the microstructure of the extruded pipe.

The flow behaviour and flash formation of ring component for 45^# steel in continuous drive friction welding (CDFW) process were investigated. A 3D thermo-mechanical coupled finite element method was used to conduct this research. Seven welding schemes with different friction pressures, friction times and rotational velocities were carried out. The influences of friction pressure, friction time and rotational velocity on the material flow behaviour at the friction surface and flash formation were analyzed. Research results show that higher peak temperature, larger region with high temperature and larger axial pressure are all good for the increase of material flow velocity. During the CDFW process, the material near the edge of friction surface flows towards the outside of joint, which makes the appearance of flashes. With the increase of rotational velocity, friction time and friction pressure, the dimensions and the bending degree of flashes increase. The reasonable welding parameters of ring structure for 45^# steel, whose inner diameter and outer diameter are 50 mm and 80 mm, respectively, are the friction pressure of 100 MPa, the friction time of 4 s and the rotational velocity of 1600 r/min.

The hot stamping process of ultra high strength (UHS) sheet metal is an innovative way which is used in automobile increasingly for the manufacturing of components with a ultra high ultimate tensile strength (UTS). Due to the high nonlinear elastoplastic and thermo-mechanical responses of material during the hot forming process, practically it is difficult to investigate such hot forming presses only via experiments. Therefore, it is necessary to develop a 3D elastoplastic coupled thermo-mechanical finite element model (FEM) of UHS sheet metal hot forming. Meanwhile, the mechanical properties of UHS steel at elevated temperatures were characterized and the corresponding material constitutive, which is strain, strain rate and temperature dependent, is developed on the basis of unaxial tensile test at elevated temperatures. In addition, hot stamping experiments were conducted and combined with the simulation to investigate the effect of tool temperature and punch speed on the hot stamping of UHS sheet metal for the square-box-shaped (SBS) component. Considering the UTS of hot formed component, the optimum tool temperature and the punch speed are achieved.

Experiments were designed to manufacture square-box-shaped part, and the effects of hot stamping process parameters including blank holder force (BHF), forming temperature and tool temperature on the hot formation quality were investigated. Since the hot formation quality is highly sensitive to BHF, a BHF controlling system was developed using six hydraulic cylinders to improve the accuracy of applied BHF to ±10 N. The experimental results showed that a mixture microstructure of martensite and bainite with large fraction of martensite at forming temperature of 850 °C was obtained in the hot stamped part, while the microstructure was dominated by the softened phase of pearlite as the forming temperature decreased to 550 °C. The tensile strength was raised from 1550 MPa to 1750 MPa as the tool temperature decreased from 200 °C to ambient temperature. The optimum BHF of 1.62 MPa was determined which can avoid the formation of drawbacks of wrinkle and crack.

The magnetic pulse joining (MPJ) between 3A21 aluminum alloy and steel 20 tubes was experimentally investigated with a solenoid coil assisted by a field shaper. The mechanical properties and microstructure of MPJ joints were tested and observed. The results show that the metallurgical joints can be obtained at a voltage of 15 kV, a radial gap of 1.2-1.4 mm, and a slope angle of 3°-7° in the lapping area. The joint comprises a transition zone with different widths, two matrix metals and two interfaces between the zone and the two metals. The interface presents a typical wavy pattern and mutual diffusion of Fe and Al elements happens in the zone. The transition zone is composed of Fe-Al intermetallics, micro cracks and micro pores. The microhardness of the transition zone is much higher than that of the matrix metals.

In order to overcome the deficiencies of the traditional connection of dissimilar sheet metals, a new method “cold-pressing deformation joining” was proposed. By using finite element simulation software DEFORM^TM-3D, the joining process of the pure aluminium and stainless steel was simulated while the sample size is 100 mm×30 mm and the thicknesses are 1 mm and 0.5 mm, respectively. The results indicate that the punching load presents an increasing trend with the increase of the punch downward displacements. The thickness of stainless steel sheet decreases gradually from the centre to the two sides in the process. In contrast, the thickness of aluminum sheet changes a lot. Experimental scheme and simulation are in good agreement. The unidirectional tensile test demonstrates that with the increase of punch downward displacements, the average thickness of connection area decreases while the connection strength increases before decreasing. The thicknesses in gauge length of connection are not uniformly distributed and the position 15 mm away from the aluminum end shows the largest thickness.

The bi-layered tubing components provide an alternative solution to make the best use of corrosion-resistant alloys and low-alloy steels. The numerical simulation and hydro bending experiments were carried out to analyze the wrinkling behavior of carbon steel/Al-alloy bi-layered tubes with different thickness ratios and internal pressures. Two types of instabilities, namely the bifurcation instability of inner tube and the limit load instability of outer tube are noticed and examined. It is indicated that the onsets of the wrinkling of inner and outer tube are delayed with increasing the thickness ratio. The bending capacity and the stability of bi-layered tube are remarkably improved as the thickness ratio increases. The optimized range of the thickness ratio is determined by numerical simulation. It is shown that separation between two layers occurs with a lower level of internal pressure, which causes the bifurcation instability of inner tube. However, the stability of inner tube is evidently enhanced with increasing the internal pressure, resulting in larger improvements of bending limit and moment capacity. The numerical predictions are verified by the hydro bending experiments with different internal pressures. Through the analysis, the selection of the internal pressure and outer tube thickness, and the mechanisms of increasing stabilities of the inner and outer tubes are clarified. The knowledge can be transferred to other bi-layered pipes with different materials and dimensions.

Reticulated open-cell Ni-Cr-Fe foams were manufactured by gas-phase codeposition of Cr and Fe onto the struts of pure Ni foam at 1050 °C, and then the samples were homogenization treated at 1200 °C in a vacuum atmosphere. The quasi-static compressive behavior and energy absorption characteristics of the Ni-Cr-Fe alloy foams with different Cr and Fe contents were discussed. The mechanical properties of these open-cell Ni-Cr-Fe alloy foams were also comparable with the pure Ni foams or the hypothetical Ni-Cr-Fe foam model. The results show that although the alloy foam struts show the similar hardness, the compressive strengths and energy absorption properties of the open-cell Ni-Cr-Fe alloy foams increase with increasing the Cr and Fe contents. The stress-strain behaviours of the Ni-Cr-Fe alloy foams are as smooth as those of nickel foams, indicating that the Ni-Cr-Fe alloy foams are the characteristic of typical ductile metallic foam. The energy absorption capabilities of the Ni-Cr-Fe alloy foams exhibit 22 times higher than that of the pure Ni foams. Simultaneously, the compressive strength of the Ni-Cr-Fe alloy foams at ambient temperature is in agreement with theoretical prediction by Gibson-Ashby model.

The research was carried out on the sheet metal samples of oxygen-free high purity copper to investigate the size effect on the indentation depth induced by laser shock. Samples were annealed at different temperatures and holding time to obtain microstructures with different grain sizes at the cross-section of thickness. Two spot sizes were used for experiments to obtain different loading sizes with different numbers of grains covered. The depth of indentation was measured to analyze size effects on the deformation at high strain rates. The results show that although the same pulse energy is adopted, the laser shock with the larger laser spot induces deep indentation more easily. The grain size has a notable effect on the indentation depth. It is difficult to generate plastic deformation in material with fine grains. The interactive effect of the thickness, laser spot, and grain sizes are found not to be important for the indentation depth when samples with coarse grains are used for experiments.