A new fast method called “Pseudo Inverse Approach” (PIA) for the multi-stage axi-symmetrical cold forging modelling is presented. The approach is based on the knowledge of the final part shape. Some intermediate configurations are introduced and corrected by using a free surface method to consider the deformation paths without contact treatment. A new direct algorithm of plasticity is developed using the notion of equivalent stress and the tensile curve, leading to a very efficient and robust plastic integration procedure. Numerical tests show that the Pseudo Inverse Approach is very fast compared with the incremental approach. The PIA is used in an optimization procedure for the preliminary preform tool design in multi-stage cold forging processes. This optimization problem aims to minimize the equivalent plastic strain and the punch force during the forging process. The preform tool shapes are represented by B-spline curves. The vertical positions of the control points of B-spline are taken as design variables. The evolution of the objective functions shows the importance of the tool preform shape optimization for the forging quality and energy saving. The forging results obtained by using the PIA are compared with those obtained by the classical incremental approaches to show the efficiency and accuracy of the PIA.

This paper reports the work associated with the development of a desktop hot-embossing machine system and tools which would enable volume production of polymeric tubular micro-components for various applications. The development was undertaken by considering factors such as machine dynamic performance, precision guides to tools, tool heating and cooling, raw material feeding and end-part collection, control strategy. It was assisted with FE analysis and a series of forming experiment. An integrated machine system has now been constructed and tested with the forming of several demonstration-components. A good result was obtained from these tests.

An explicit model controlled by a linear equations set was developed. This model was directly solved by the complete pivot GAUSSIAN elimination method without any iteration. In addition, crystallographic-system based solving procedure was proposed to reduce the additional calculation caused by grain rotation. By establishing crystal plasticity finite element model (CPFEM), the model was verified by comparing the predicted texture to the experimental results. Then, the model was applied to predict textures under different deformation states achieved by adjusting the ratio (k) of the loading velocities in Z and Y directions. The results show that the model is reliable in texture prediction (good agreement with the experiments in compression, tension, simple shear and plane-strain compression) and much more efficient (more than 100 times) than the implicit model; with the increasing of k, the strong texture progresses from ±35º to normal direction to fiber texture in the {111} plane and enhances in intensity; the texture intensity drops dramatically when the strain rate increases from 0.1 s^-1 to 100 s^-1, while drops slowly when the strain rate increases from 100 s^-1 to 7×10⁴ s^-1, which indicates the computational stability of the model for simulation of ultra-high strain rate deformation.

The friction effect is very important in micro-forming process and differs from the usual forming process. A new method of evaluating the friction factor in tests by combining micro-compression was presented. The friction behavior and the flow stress were investigated in micro-expression by theory analysis and experiment. To investigate the effects of specimen size on the flow stress, a series of compression tests were carried out for specimens with different diameters. The results show that with the decrease of specimen size, the flow stress shows a gradually downward trend. The bulging value increases with increasing the friction factor. Combining the bulging method and theory analysis, the experimental friction factor was obtained and the flow stress was calibrated without friction. Thus, the real stress-strain curve was obtained, which provides the accurate material model for the subsequent finite element simulation on micro-forming.

An analytical approach for predicting the critical blank holding force (BHF) was presented. Using energy method, the analysis provides the circumferential stress and the equivalent strain as functions of radius under the plane strain and the equivalent strain is inversely proportional to the radius respectively. The maximum relative errors of the circumferential stress and the equivalent strain are 22.3% and 35.9% respectively under the two conditions for some dimensions of sheet and die. In addition, the relationship between BHF and wrinkle number was obtained under the assumption that wrinkle shape is expressed by power function. The critical BHF under plane strain was analyzed for the wrinkle shapes when the power is less than, equal to or greater than 1. The effects of wrinkle shapes on the critical BHF are also presented.

To better understand the radial spread behavior and propose the effective control method, the FE analysis for radial spread in three-roll cross rolling (TRCR) was carried out using ABAQUS\Explicit procedure. The evolution and distribution laws of radial spread were studied, and the effects of four key forming parameters on the radial spread were investigated. The results obtained show that radial spread of the central region firstly increases, then decreases, and that of the upper or lower region always increases in TRCR process. The radial spread distribution is symmetrical about the axial centre after TRCR, it decreases from the upper and lower regions to the central region. A larger feed velocity or smaller rotational speed contributes to the larger average radial spread and more unevenness of inner surface, a larger friction coefficient or smaller initial blank temperature contributes to the smaller average radial spread and more unevenness of inner surface. The results can provide scientific basis for radial spread control and reasonable parameter design.

The thermal deep drawing process with vacuum environment was put forward to solve the problems in the forming of the high strength and difficult-to-deformation sheet metals such as difficult to form under normal temperature and easily to be oxidized at high temperatures. The vacuum hot drawing system was discussed and developed based on the integration of vacuum heating technology and variable blank holding force (BHF) system, which consisted of mainframe system, hydraulic system, vacuum system, heating system, water-cooling system, air-cooling system and computer control system, and the key parameters such as temperature, BHF and punch velocity were regulated in real time using PID closed-loop control technique. The deep drawing test of 2.0 mm-thick molybdenum sheets with temperature of 870 °C and vacuum degree of 10^-2 Pa was conducted on the system, and the molybdenum crucible with limiting drawing ratio of 1.94 was obtained, which indicated that the vacuum thermal drawing technology provided an effective solution for the formability improvement of difficult-to-deformation materials with strictly protecting against oxidation.

High-strength 7075-T651 aluminum alloy was selected as the research object, and the random-shots FE model was developed; the influences of shots number and spatial location on residual stress field (RSF) were discussed; the influence of analysis step time (AST) on the stability of RSF was analyzed, and the influences of coverage rate and surface roughness of component on the distribution of RSF were investigated. The results indicate that the generated RSF is seriously affected by subsequent shots number and location; RSF tends to be stable when AST adds to 0.075 ms for 45 shots FE model, and the AST values of 15, 25, 65 and 85 shots FE model are 0.065, 0.0683, 0.0817 and 0.0883 ms respectively. For shot peening with different coverage rates, σ_srs and σ_mcrs increase with the variation of shots number from 15 to 65, then decrease when shots number is 85, variation of Z_m is not obvious, Z₀ increases with the increment of shots number. For shot peening of component with different surface roughness R_a, variation of σ_srs has no obvious regularity, and σ_mcrs, Z_m and Z₀ decrease with the increment of R_a.

Deep drawing provides a great application potential for the manufacturing of parts with complex shapes, even when it is scaled to the micro range. Two different foils out of the Al-Zr alloy with a thickness of about 15 µm were manufactured using a magnetron-sputtering process by applying substrate temperatures of 310 K and 433 K, respectively. These foils were used as blank material in micro deep drawing with a punch diameter of 0.75 mm to investigate the formability. Even though the materials show small ultimate strains in tensile tests, deep drawing was carried out successfully. Limit drawing ratios of 1.8 and at least 1.7 were reached for the material produced with a substrate temperature of 310 K and 433 K, respectively. These are even better results than those realized with Al-sc alloys in former investigations. Comparison with deep drawing results of pure aluminum produced by conventional rolling shows the same achievable limit drawing ratio. This illustrates the good suitability of magnetron sputtering as a foil manufacturing process for micro sheet forming operations.

In the hydraulic bulge tests, dies with both circular and elliptical apertures were used. A recently proposed methodology was used to determine the equivalent stress-strain curves by bulging through elliptical dies. This methodology combines an analytical approach with the experimental data measured by an ARAMIS system. In the hydraulic bulge test using elliptical dies, as the die ellipticity ratio decreases, the equivalent stress-strain curves tend to move away from the curve obtained from bulging through circular die. The forming limit diagram in the range of positive minor strain of a DC04 sheet steel is also determined by using bulge test.

To explore the hydroforming possibility of thin-walled tubular component with rib, mechanical analysis and finite element analysis (FEA) were conducted to investigate the rib buckling mechanics conditions. Based on lath-beam assumption, buckling Euler force was derived for rib which is restrained at one end and free at the other. The results indicate that it is possible to achieve a sound thin-walled tubular with rib component through hydroforming process and its expiation ratio can reach 20%. According to FEA, the effects of rib height and inner radius on buckling degree were studied and the threshold values of both rib height and inner radius were given. Simulation results indicate that there is a certain value for buckling and the threshold values of rib height and inner radius to thickness are 14 and 30. The formability deteriorates as rib height and rib inner radius increase. At last, the hydroforming limit diagram was drawn for hydroforming thin-walled tubular component with rib of 1Cr18Ni9. The results are useful for further study of hydroforming regularity of thin-walled tubular component with rib.

The springback mechanism for a large-diameter thin-walled CT20 titanium alloy tube with an outside diameter of 85 mm and a wall thickness of 2.5 mm (denoted as D85 mm×t2.5 mm) was investigated using numerical simulation. The results show that the areas among the crest lines and the neutral layer unload significantly, whereas the areas adjacent to the neutral layer unload indistinctively. These differences lead to a reverse loading in certain areas of bent tube and an obvious tensile-compressive stress concentration in the area adjacent to the neutral layer after springback. The variation rules of springback angle and springback radius with bending radius and bending angle were studied. These data were used to propose a springback compensation method, which was verified to be effective in an example study.

The Gurson-Tvergaard-Needleman model (GTN model) was employed to analyze bursting behavior in the hydroforming of stainless steel T-shape. A free-bulging test combined with simulation was conducted to determine the critical porosity and the failure porosity in GTN model. The effects of the forming pressure and the axial feeding on damage development were investigated and the influences of stress triaxiality and the plastic strain on porosity variation were also studied. The results show that a higher forming pressure or a less axial feeding will lead to bursting failure. The stresses of the top of protrusion are in bi-axial tension state, while the stresses of the side wall of main tube are in hoop tension state and axial compression state, respectively. The plastic strain has a more significant influence on the porosity than the stress triaxiality under the lower internal pressure; however, the stress triaxiality will govern the growth of porosity under the higher internal pressure. The simulation results give a good agreement with the experimentally determined thickness, and the maximum thickness-thinning rate is about 36%.

Pre-bulging effect on the sheet hydroforming process of irregular box with unequal height and flat bottom was investigated by means of numerical simulation and experiment. The influences of pre-bulging height and pre-bulging pressure on the forming were discussed respectively. The pressure loading path was optimized. The results show that pre-bulging has important influences on the forming results. Too high pre-bulging height causes the crack and obvious crease at the punch nose near the lowest corner. Too low pre-bulging height causes fracture at the punch nose of the highest corner. Pre-bulging pressure has a smaller effect on the fracture at the punch nose of the highest corner when the pre-bulging height is reasonable. But over high pre-bulging pressure causes crack and obvious crease at the punch nose near the lowest corner. Failure can be avoided by using reasonable pre-bulging height and pre-bulging pressure.

A ‘bone-like’ hollow component with curved axis and complicated cross-sectional geometry was used as the object. The experimental setup for the hollow component by welded double sheet hydroforming was designed and used for the forming of the part. The formability influenced by blank shape and size, loading path combined by the clamping force and liquid pressure was explored, then the reasonable process parameters avoiding defects were obtained. The results show that the reasonable blank shape can not only save materials and reduce costs but also avoid fracture at the corner caused by the large expansion rate and wrinkle in the transition zone of different cross sections. The loading path has a serious effect on the material flow in the pre-forming stage. The complex hollow shell structure with curved axis and varied cross sections can be fabricated by double sheet hydroforming process more easily and efficiently than traditional stamping and welding method. The process can also solve the problem of the limitation of expansion rate by tube hydroforming for the structure with a large difference of cross sections.

New advanced algorithms for the detection of detailed features in parts formed by single point incremental forming (SPIF) were developed. The features were detected in STL part specifications that took into account the geometry, curvature, location, orientation and process parameters to detect 33 different features within an expert CAPP system for SPIF. The detection process was facilitated by using multi-level edge segmentation routines that first created a frame of edge features. Within this frame, the remaining features were then detected using region growing algorithms. The results show successful detection for a number of test cases. A case study for a double curved hemisphere illustrates the generation of optimal tool paths using compensation for the detected features in the part. These tool paths lead to the improvement in the accuracy of the formed sheet metal parts.

Ultrasonic methods of measuring the thin thickness in a non-parallel irregular surface sample require quite a high time resolution of the detection device. Ultrasonic resonance method based on spectral analysis was used for accurate thickness measurement of this kind irregular aluminum alloy sample. Aiming at performing successive measurement of the thicknesses of different positions of one sample, non-contact detection mode was selected by using 20 MHz water immersion ultrasonic focused transducer and ultrasonic PAC testing system with 3D location device. Multiple resonant thickness algorithm was set up, so that the thickness value can be calculated by multiple resonant frequencies. In order to obtain the precise multiple resonant frequencies, in spectral analysis, surface wave in time domain signal needs to be removed and the appropriate window of time domain signal was selected. A high measurement accuracy of ±15 μm can be achieved using this method. In addition, the effect of the transducer location error was investigated and the allowed transducer offset ±500 μm was determined.

The setting round process is one of the most important processes to ensure the quality of large pipe products. The setting round for pipe-end is a local elasto-plasticity deformation process, so it is difficult to quantificationally analyze the springback law because of the influence of rigidity. The physical experiments were used to study the equivalence relation between the setting round process for pipe-end and the circle-to-oval process for pipe-end, and the similarity relation between the circle-to-oval process for pipe-end and the circle-to-oval process for short pipes. The results indicate that the reductions are equal both in the setting round process and the circle-to-oval process for pipe-end; the relationship between the ovality after unloading and the relative reduction is linear in the circle-to-oval process both for pipe-end and the whole pipe, and the horizontal axis intercepts of the two lines are equal. Based on the relation above, the two-step control strategy of over-bending setting round for pipe-end was built, which can control the residual ovality of pipe within 0.5%.