Properties of Large‐Scale Structure Workpieces in High‐Pressure Sheet Metal Forming of Tailor Rolled Blanks

In order to manufacture components optimised in regard to lightweight construction, the use of innovative forming processes like high‐pressure sheet metal forming (HBU) in combination with the use of tailor rolled blanks (TRB) as innovative semi‐finished material is a promising solution. Fundamental investigations on the HBU of TRB have been carried out in a joint research project at the Institute of Forming Technology and Lightweight Construction (IUL), University of Dortmund, and the Institute of Metal Forming (IBF), RWTH Aachen. The experiments performed with cylindrical parts have provided basic knowledge on the sheet material flow and resulting part properties. To achieve sufficient process reliability, a non‐adjustable as well as an adjustable seal system have been tested and proved to be suitable solutions, depending on thickness ratio and thickness gradient within the TRB. In order to demonstrate the lightweight potential of this process chain, a forming tool for an automotive body structure has been designed and tested. The experiments have shown that this large‐scale structure can well be manufactured in the HBU process from a TRB.     

Forming Limit and Thickness Transition Zone Movement for Tailor Rolled Blank during Drawing Process

The process of automobile lightweight can be promoted by the application of tailor rolled blank(TRB)in the automobile industry.Therefore,research on the formability of TRB is of good practical significance and application value because of the enormous potential of TRB in the aspect of automobile lightweight.Aiming at the present condition of lack of researches on the influence of characteristic parameters on TRB drawing process,the drawing formability of TRB was studied with a combination method of simulation and experiment by taking square box as the research object.Firstly,drawing simulation and experiment of TRB were carried out.Then,effects of thickness transition zone(TTZ)position and blank size on the drawing formability of TRB were analyzed.Forming limit and TTZ movement for TRB square box during the drawing process were respectively discussed,when transition zones of TRB were located at different positions and blanks were of different sizes.The results indicate that lubrication condition exerts greater influence on TRB forming limit in comparison with TTZ movement,and the smaller blank size and TTZ being located at the blank center or slightly offset to the thinner side are preferable for acquiring greater forming limit and smaller TTZ movement.     

Transverse Bending Characteristics in U-channel Forming of Tailor Rolled Blank

Research on the formability of tailor rolled blank (TRB)is of good practical significance and application value because of the enormous potential of TRB in the aspect of automobile lightweight.However,the forming of TRB is problematic because of the varying properties;especially,springback is a main challenge.The transverse bending (bending axis is perpendicular to the rolling direction)of TRB U-channel was studied through simulation and experiment.The forming characteristics of TRB U-channel during transverse bending were analyzed.The mecha-nisms of forming defects,including bending springback and thickness transition zone (TTZ)movement,were re-vealed.On this basis,effects of blank geometric parameters on springback and TTZ movement were discussed.The results indicate that springback and TTZ movement happen during transverse bending of TRB U-channel.Nonuni-form stress distribution is the most fundamental reason for the occurrence of springback of TRB during transverse bending.Annealing can eliminate nonuniform stress distribution,and thus diminish springback of TRB,especially springback on the thinner side.Therefore,springback of the whole TRB becomes more uniform.However,annealing can increase the TTZ movement.Blank thickness and TTZ position are the main factors affecting the formability of TRB U-channel during transverse bending.     

Numerical Simulation and Experimental Investigation of Springback in U-Channel Forming of Tailor Rolled Blank

 In order to grasp the springback rule of TRB (tailor rolled blank) parts after forming, the springback behavior of TRB was investigated by integrating such three means as theoretical research, numerical simulation and stamping experiments. Fundamental theories of springback were analyzed. The stamping and springback processes of annealed 1.2/2.0 mm TRB, 1.2 mm and 2.0 mm plates for U-channel were simulated, and the simulation results were compared with the experiments. The results indicate that the springback of TRB falls in between those of the 1.2 mm and 2.0 mm plates. It is desirable for the TRB U-channel to have die clearance of 1.1 times maximum blank thickness and friction coefficient of about 0.12, and longer thickness transition zone is preferable. The simulation data demonstrate reasonably good agreement with the experiments.     

轧制差厚板成形极限场

摘要：轧制差厚板板料厚度和材料性能分布不均匀,准确预测差厚板零件的成形极限变得十分困难。首先将差厚板的过渡区离散成若干等厚板的组合,对每一个厚度下的板料进行不同路径的凸模胀形数值模拟,拟合出不同厚度板料的成形极限曲线,对厚度进行插值获取差厚板的成形极限场。最后,采用成形极限场对差厚板筒形件的成形极限进行预测。研究结果表明：采用数值模拟、拟合以及插值的方法得到的成形极限场,可以较为准确地预测实际差厚零件的成形极限。     

Tailored Profiles Made of Tailor Rolled Strips by Roll Forming – Part 2 of 2

The main scope of the presented work is to demonstrate the potential of load optimized tubes with a varying thickness distribution in circumferential direction produced by roll forming. As initial material a so called Tailor Rolled Strip (TRS) sheet metal coil produced by Strip Profile Rolling (SPR) method was used instead of plain sheet. The TRS sheet metal is manufactured in a continuously working process by rolling one or more groves in transverse direction into the sheet metal coil. In this paper, the secondary forming of the TRS sheet metal to TRS tubes is investigated by means of FE‐simulations and roll forming experiments. To simulate the manufacturing process of the TRS tube by FEM, an integrated consideration of the process is necessary because of the large local strain hardening in the groves of the initial SPR sheet metal. In experimental roll forming operations welded tubes could be manufactured successfully. The geometrical and material properties of these tubes are analyzed. The reprocessing of TRS tubes by hydroforming is investigated by means of tube bursting tests. It has been found that an additional annealing process is necessary to achieve deformations in the grooved area during the hydroforming process.     

Influencing factors of wrinkle defects of tailor rolled blank for square box in drawing process

Tailor rolled blank(TRB) is inclined to wrinkle during the forming process because of the nonuniformity of blank thicknesses and mechanical properties. The wrinkle defect of TRB square box in deep drawing was studied by numerical simulation and stamping experiment. The mechanism for the wrinkle defect of TRB square box was discussed, and effects of blank size, thickness transition zone length and position, and blank thickness on the wrinkle defect of TRB were analyzed by taking thickness strain as an evaluating indicator. Finally, the measures restricting the wrinkle defects were presented. The results indicate that the spots inclined to wrinkle for TRB square box are the flange on the thinner side, the flange on the thicker side, and the flange in the thickness transition zone. The smaller the blank size and the thickness transition zone length are, the harder TRB is to wrinkle. Greater or smaller thickness difference can both restrain the wrinkle phenomenon, and thickness transition zone center offset from the blank center may lead to the occurrence of wrinkle.     

New types of coating systems for steel sheets by high‐rate evaporation in combination with plasma processes

High‐rate evaporation in combination with plasma processes is a promising approach to obtain new types of steel sheet coating with improved corrosion resistance and application properties. To estimate the potential for the application of PVD‐coatings (physical vapour deposition) different coating systems for steel sheet as well as for hot‐dip or electro‐galvanized steel sheet were designed. The samples were produced on a laboratory scale using PVD processes with very high deposition rates (in the order of 1 μm s^‐1) as well as high‐power plasma processes for the pre‐treatment. The relationship between the composition, microstructure and properties of the coating systems, in particular concerning corrosion protection, abrasion during forming, phosphating and paint adhesion, were studied. It was found that the corrosion resistance of galvanized steel sheets can be considerably improved by vapour deposition of metal or inorganic films with a thickness of several hundred nanometers. Investigations on vapour deposition of titanium and stainless steel coatings on steel sheets, for applications in a severely corrosive environment, showed that the corrosion resistance in relation to the coating thickness can be significantly enhanced by means of plasma activation during the vapour deposition process. Finally, an outlook on possible industrial applications including an estimation of the process costs will be presented. For certain coating systems the results look promising. Consequently, these particular coating systems will be investigated in more detail by means of using a large‐scale in‐line deposition plant for metallic strips and sheets.     

轧制差厚板纵向弯曲回弹规律分析

为了推动轧制差厚板在汽车梁结构件上的应用,以U型件为对象,研究了轧制差厚板的纵向弯曲回弹特性。首先完成了差厚板U型零件纵向弯曲成形数值模拟,分析了差厚板的回弹趋势,讨论了差厚板的应力分布,揭示了差厚板弯曲回弹规律,探讨了差厚板等效应力的影响因素,并通过试验对回弹仿真结果进行验证。结果表明,不均匀的应力分布是纵向弯曲的差厚板U型件沿弯曲轴方向上回弹不一致的根本原因,退火处理能够减小差厚板卸载前后的应力差,从而实现抑制差厚板回弹的作用。差厚板的板料尺寸、厚度、过渡区长度均会对差厚板的等效应力造成较大影响,从而改变差厚板的回弹大小及分布。另外,差厚板零件不同厚度部位的回弹相互牵制,使得各部分的回弹量趋向一致,从而导致差厚板的回弹量均介于薄、厚等厚板之间。     

Material Flow Control in High Pressure Sheet Metal Forming of Large Area Parts with Complex Geometry Details

Working media based forming processes show advantages compared to the conventional deep drawing in the range of sheet metal parts with complex geometry details. By High Pressure Sheet Metal Forming (HBU), complex parts can be formed with reduced tool costs, fewer process steps, and improved part properties, particularly by the use of high strength steels. In order to use these advantages to full capacity, the material flow into the area of the geometry details needs to be optimised. The key element for the material flow control is a multi‐point blank holder. In combination with flange draw‐in sensors, a closed loop flange draw‐in control can be built up which guarantees a reproducible material flow and, consequently, defined part properties. Furthermore, a favourable pre‐distribution of sheet metal material can be reached which leads to a widening of the process limits. Considering a large area sheet metal part with a complex door handle element as example, strategies for the material flow control will be discussed in this paper. The conclusions are based on FE‐simulations as well as experimental findings.     

轧制差厚板横向弯曲成形仿真与试验

为了抑制轧制差厚板在横向弯曲成形过程中的缺陷问题,采用仿真和试验方法研究轧制差厚板U型件的横向弯曲过程。分析差厚板U型件在横向弯曲过程中的成形特点,探讨弯曲回弹以及过渡区移动等缺陷的发生机理。结果表明,横向弯曲的差厚板U型件在成形过程中除了会产生回弹缺陷外,还会发生厚度过渡区移动;差厚板薄、厚板侧的厚度以及性能差异是差厚板产生缺陷的根本原因;采用退火处理能够减小差厚板的回弹量,但是会导致过渡区移动量增大;板料尺寸越大,回弹量与过渡区移动量也随之增大。     

3D打印钛合金薄壁构件的研究进展

钛合金薄壁构件具有质量轻、结构紧凑等优势,然而因其轴向尺寸大、壁厚薄和形状复杂等几何特征,传统成形技术在成形薄壁构件时流程长、工艺复杂,严重限制了钛合金薄壁构件的应用。金属粉床3D打印技术可快速成形复杂异形零部件。为此,对电子束选区熔化技术(SEBM)和激光选区熔化技术(SLM)的成形能力和成形钛合金薄壁构件的微观组织、力学性能和表面粗糙度进行综述,并分析3D打印高性能精密复杂整体钛合金薄壁构件的发展趋势,为轻量化钛合金薄壁构件在高端装备上的应用提供参考。     

Air Bending Process for Load Optimised Profiles

In this paper the air bending of Tailor Rolled Blanks (TRB) to load optimised profiles is investigated. A new flexible modular tool system has been developed in order to locally adapt the die height to the spring back which is more pronounced in thinner areas in comparison to thicker areas of the TRB. With the new tool system the dies can be modulated according to the thickness distribution of the Tailor Rolled Blank, whereas the amount of die lifting depends on the thickness difference and the corresponding variance of mechanical properties. In bending tests with several specimens having a constant thickness a linear relation between the increasing part angle and a stepwise increased die lifting was observed. With this linear dependency the amount of die lifting can be calculated and thus applied on bending of TRB. Special consideration is required for the die adjustment in the area of the linear thickness transition, therefore three different die arrangements have been investigated. The results of different bending tests are compared and evaluated in order to apply the new technique to form a new car body floor structure in profile intensive construction. By using load optimised profiles in combination with a steel 22MnB5 which has been heat‐treated at defined sections the survival space of a car passenger in a side pole impact has been enormously increased by more than 40 %.     

Advanced Processing and Properties of Thixotropic Formed Components Based on Partially Melted Magnesium

Contrary to the manufacture of aluminium components in the semi‐solid, thixotropic state the production of magnesium based components by semi‐solid techniques is still uncommon. For this reason, the advantage of this production method is analysed with regard to the commercial magnesium alloy AZ80. The objective of this research is semi‐solid‐casting (SSC) of AZ80 for the production of a light weight component in near‐net‐shape quality and with advanced properties. Using extruded primary feedstock material, the behaviour and the advantages are investigated. Billets with a weight of up to 2 kg are heated up into a semi‐solid state. To avoid any risk of self‐ignition of the material an automatic, temperature controlled induction heating system is used. To achieve an optimum homogeneous grain structure the induction heating power is varied making use of a process control system based on power‐time‐curves. The heated billets are transported in the soft semi‐solid condition from the induction heating system to a die casting machine to produce components with wall thickness’ between 2 and 10 mm. After forming of the components, the influence of heat treatment on the grain structure and especially on the mechanical properties is determined to provide parts with optimised characteristics. To compare the properties of the special globular grain and microstructure, the results of various static and dynamic tests are analysed. It is found that components can be manufactured with a magnesium alloy in a thixotropic state in near‐net‐shape quality, with low porosity and with excellent mechanical properties like elongation of up to 15%.     

Semi‐Solid Processing of Metal Alloys

Semi‐solid metal casting is an innovative technology for the production of near‐net‐shape parts with demanding mechanical properties. The paper describes different processing routes and materials for semi‐solid‐metal casting (SSM), which have been investigated and also partially developed at the Foundry‐Institute of Aachen University. The standard thixocasting process for aluminium, highly reactive magnesium alloys and steel alloys with high melting points was investigated under variation of a wide range of process parameters. Specially adapted pre‐material production and reheating methods were developed for different materials and their application and future potential is pointed out. The thixocasting experiments were executed on a modified high pressure die‐casting machine with a specially designed “step‐die” providing wall thicknesses from 0.5 to 25 mm. The mechanical properties were tested in dependence of the wall thickness and the metal velocity. The results of these examination show high tensile strength values in combination with very good elongations. The rheocasting process is a new SSM‐forming method with liquid melt as feed‐stock and a high recycling potential. The research results of RCP‐technology (Rheo‐Container‐Process) invented at the Foundry‐Institute and of the Cooling‐Channel‐Process for aluminium and magnesium alloys are promising and are presented in this paper. Studies on semi‐solid processing of magnesium alloys and mixtures of them were conducted by Thixomolding^TM. To establish the most adequate process parameters, the temperature and the mixture relations were varied. Using a mould for tensile test specimens, the mechanical properties and the microstructure evolution could be evaluated. The chemical composition of the different phases was determined using SEM and EDX technologies. Evaluations of the flowing properties were conducted using a spiral mould with a total length of 2m and a cross section of 20mm x 1.5mm.     

Production of Structure Components with Selective Properties by means of Action Media based Cold Forming

Minimising a metallic component's weight can be achieved by either using lightweight alloys or by improving the component's properties. In both cases, the material formability affects the utilisability for mass production processes. Most of the high‐strength materials show a material‐restricted formability and are difficult to forge. The formability of a material is described by its maximum forming limit. Large plastic strains can lead to mechanical damage within the material. A promising approach of handling low ductile, high‐strength alloys in a forming process is deformation under superimposed hydrostatic pressure by active media. In the present study, the influence of superimposed hydrostatic pressure on the flow stress is analysed as well as the forming ability for different sample geometries at different hydrostatic pressure and temperature levels. The experimental results show that the superimposed pressure has no influence on the plastic deformation, nor does a pressure dependent near‐surface material hardening occur. Nevertheless, the formability is improved with increasing hydrostatic pressure. The relative gain at room temperature and increase in the superimposed pressure from 0 bar to 600 bar for tested materials was at least 140 % and max. 220 %. Therefore, a cold forming process under superimposed pressure is developed to produce structure components with selective properties. For example, the gain in formability will be used to enlarge local plastic strains to higher limits resulting in higher local strain hardening and hardness. This offers new design possibilities with selectively adjusted local structure or structure component properties, especially adapted to their technical application. Additionally, by applying damage models, finite‐element analysis is used in order to predict damage occurring in the cold forming process under superimposed hydrostatic pressure for various sample geometries.     

Influence of Mandrel's Surface and Material on the Mechanical Properties of Joints Produced by Electromagnetic Compression

Electromagnetic compression of tubular profiles with high electrical conductivity is an innovative joining process for the manufacturing of lightweight structures. The mandrel's material has an influence on the transferable loads which is affected by the Young's modulus as well as the strength of the material. This was investigated, on the one hand, by changing the mandrel's material and, on the other hand, by using the same mandrel material with differing strength. Furthermore, taking conventional interference fits into account, the contact area's influence on the joint's quality seems to be of significance, as e.g. the contact area and the friction coefficient between the joining partners proportionally determine an allowed axial load or torsional momentum. Therefore, different contact area surfaces were prepared by shot peening and different machining operations and strategies. The mandrel's surfaces were modified by shot peening with glass beads and Al₂O₃ particles. An alternative preparation was performed using simultaneous five‐axis milling, because potential joining partners in lightweight frame structures within the Transregional Collaborative Research Centre SFB/TR10 would be manufactured similarly. After that, the manufactured surfaces were characterized by measuring the surface roughness and using confocal whitelight microscopy. Afterwards the modified mandrels were joined by electromagnetic compression. The influence of different mandrel's surface conditions on the joint's mechanical properties was analysed by tensile tests. Finally, conclusions and design rules for the manufacturing of joints by electromagnetic compression are given.     

Computer Aided Product Optimization of High‐Pressure Sheet Metal Formed Tailor Rolled Blanks

In view of today's increasing economic pressure the industry has to rationalize in order to remain internationally competitive. Achieving higher product quality using less material and machine‐man‐hours is one possibility to reach this goal. The high‐pressure sheet metal forming of tailor rolled blanks (TRB) allows to produce optimized components specially developed for their future function and which cannot be made from conventionally rolled sheet metal. This paper aims at showing that the two processes, i. e. flexible rolling and high‐pressure sheet metal forming (HPSMF), can be well represented in finite element simulations. By linking the finite element models with a combinatory optimizing tool, it is possible to simulate and optimize the entire process chain. Two example components were used to illustrate the principle of the optimization.     

3D‐FE Modelling and Simulation of Multi‐way Loading Process for Multi‐ported Valves

Multi‐ported valves are widely used in the marine, sanitary, petrochemical and power industry. Multi‐way loading forming technology provides an efficient approach for integral forming of high strength multi‐ported valves, such as tee pipe coupling, high‐pressure cross valves, large‐scale complex valves, and so on. Since the multi‐way loading process is a very complicated plastic forming process due to the complexity of loading path, finite element numerical simulation is adopted to investigate the multi‐way loading process in order to predict and control the multi‐ported valve forming process. A reasonable model of the process is developed under DFEORM‐3D environment based on the coupled thermo‐mechanical finite element method. Then the reliability of the model is validated with respect to geometry development and forming defects. Numerical simulations of multi‐way loading forming for a tee valve and a cross valve have been carried out via using the developed model. Further, the forming processes of tee valve and cross valve have been compared. Moreover, the modelling method is also suitable for multi‐way loading processes of other complex components.     

Micro Semi‐solid Manufacturing ‐ A New Technological Approach towards Miniaturisation

One of the most important prerequisites to meet the increasing demand for efficient technologies for micro‐part production is constituted by the ability to overcome existing process limitations by new innovative technological approaches. By the introduction of a new process variant based on a hybrid material condition between solid and liquid state, such an approach is presented. This micro semi‐solid manufacturing technology, so‐called Micro‐Thixoforming, was initiated, on the one hand, by being aware of the technological limits of existing micro‐forming and micro‐casting processes and, on the other hand, by a comprehensive understanding of the special rheological mechanisms of metallic materials in semi‐solid state finally establishing the desired potential to follow the trend towards miniaturisation with drastically reduced process restrictions. However, this promising potential can only be successfully exploited when the initial idea, which is based on phenomenological considerations, can be transferred to a process technology with sufficient practical relevance. Therefore, the presented new integrated process concept for Micro‐Thixoforming is particularly characterised by the application of unconventional solutions for the main process steps: raw material conditioning, thermal pre‐processing, semi‐solid forming and thermal post‐processing. To give an indication of the innovative character of the chosen practical solutions liquid metal jet technology, LASER‐induced plasma shockwaves and high pressure water jet should be mentioned. However, what is even more important in this context is the ability not only to realise a process concept but, beyond that, to recognise the further potential regarding new strategies for material design arising from the availability of this process. Such a strategy e.g. consists of utilising the well‐known segregation effect, which notably often is negatively associated with semi‐solid forming. However, for the envisaged technological approach, controlled segregation aims at a defined adjustment of functionally graded properties for the produced micro part.