Experimental Analysis on the Frictional Behaviour of Drawbeads in Sheet Metal Forming

In sheet metal forming, drawbeads are commonly used to control uneven material flow, which may cause defects such as wrinkles, fractures, surface distortion and springback. Although friction may not directly change the limiting strain of steel sheets, the tribological conditions in the contact zone between the sheet surface and the tool surface play an important role in determining the limits of the forming process. Friction in the drawbead contact zones affects the flow of the material in the tool and is used deliberately to control the stamping process. Therefore in this study, the frictional behaviour of drawbeads is experimentally investigated by the drawbead friction test. To characterize the effect of processing variables on the friction coefficients, tests are performed for various sheets, lubricants and bead materials suffering different surface treatments. The results obtained from the drawbead friction test show that the friction and drawing characteristics of deforming sheets were strongly influenced by the strength of sheet, viscosity of lubricant and hardness of bead surface.     

High Temperature Tribological Studies on Surface Engineered Tool Steel and High Strength Boron Steel

The popularity of hot sheet metal forming processes in the recent years has necessitated research efforts to improve tool life and control the friction level during hot forming operations. In this work, the tribological properties of tool steel and ultra high strength boron steel (UHSS) pairs at elevated temperatures have been studied by using a special hot sheet metal forming test rig that closely simulates the conditions prevalent in the real process. This test involves linear unidirectional sliding of a preheated UHSS sheet between two tool steel specimens where new workpiece material is continuously in contact with the tool surface. The study is aimed at investigating different surface treatments/coatings applied on either the tool or sheet surface or on both. The results have shown that it is possible to control the coefficients of friction through surface treatments and coatings of the tool and workpiece materials. The application of a coating onto the sheet material has a greater influence on the friction compared to changing the tool steel surface. After running‐in, the investigated tool steel variants show almost similar frictional behaviour when sliding against the same sheet material. Although coating the UHSS sheet reduces friction, it abrades the tool surface and also results in transfer of the sheet coating material to the tool surface.     

Process Modeling for Precision Forming of Discrete Parts – State of the Technology and Critical Issues

This paper presents an overview on the application of FE simulation as a virtual manufacturing tool in designing manufacturing processes for precision parts. The processes discussed include forging, sheet metal forming and hydroforming. Determination of reliable input parameters to simulate a process is a key element in successful application of process simulation for process design in all the mentioned areas. These issues are discussed in detail. Practical examples of application of FE simulation are presented for improvement of the existing metal forming process and/or designing new metal forming process for manufacturing discrete precision parts in forging, sheet metal forming and hydroforming.     

固体润滑助剂对镀铝锌耐指纹钢板表面摩擦特性的影响

在常温及高温条件下,利用表面性能分析仪和拉延设备考察了三种固体润滑助剂（聚乙烯润滑助剂、聚四氟乙烯润滑助剂及聚乙烯/四氟润滑助剂）对热镀铝锌耐指纹钢板表面摩擦特性的影响,并采用三维视频显微镜分析了镀铝锌耐指纹钢板表面划痕状态及深度。结果显示,在常温条件下,固体润滑助剂的加入不仅降低了耐指纹钢板表面的动摩擦因数,而且还提高了耐指纹钢板表面的抗磨损性和加工成型性。在高温条件下,聚乙烯润滑助剂软化导致耐指纹钢板表面润滑性能降低,并影响其加工成型性。聚四氟乙烯及聚乙烯/四氟润滑助剂的软化点较高,在高温下能够保持润滑粒子表面硬度及润滑性能,从而显著提高耐指纹钢板在高温下的表面润滑性能。     

A Simple and Low-Cost Lubrication Method for Improvement in the Surface Quality of Incremental Sheet Metal Forming

The incremental sheet forming technology as a flexible forming process is often used to fabricate parts with small batches or individuation. However, the lubrication of this technology is usually difficult and complex due to the rigidity in the contact between tool and sheet. In this work, a novel lubrication method with graphite grease/oil is adopted to fabricate parts with aluminum in incremental sheet forming, which is simple and low-cost compared to previous methods. Meanwhile, other two lubrication methods, such as oil lubrication and graphite lubrication, are adopted as control groups. The surface roughness profile and the surface morphology obtained by the new method are both better than that obtained by control groups. Moreover, machining and step-down traces of the forming surface of specimens fabricated, thickness distribution, and forming force could be availably improved with the novel lubrication method.     

薄板冲压成形中摩擦测定装置伺服控制系统

主要介绍了液压伺服系统在汽车板冲压成形中表面摩擦测定装置上的应用 ,分析了系统各部工作原理及动态过程函数的组成。本系统对生产实际冲模中实现冲压过程自动控制具有一定的研究意义     

Gas‐pressure Forming of an AlMg‐Alloy Sheet at Elevated Temperatures

Forming of automotive leightweight parts using aluminium offers numerous advantages. Compared to other wrought aluminium alloys, in particular AlMg‐alloys generally show a good formability which is favourable for the production of complex parts. However, forming of Mg‐containing alloys at room temperature leads to yielding patterns preventing their implementation for class‐A‐surface applications. Furthermore, the formability of steel still exceeds that of AlMg‐alloys at room temperature. Thus, in the present study, sheet metal forming is applied at a temperature range that is typical for warm forming. It is supposed to profit from the advantages of warm forming like high achievable strains and improved surface quality of the formed part, while not having the disadvantages of long production times and high energy consumption, which is correlated with superplastic forming. Applying fluid‐based sheet metal forming in this paper, nitrogen is used as fluid working medium to satisfy the demand on high temperature resistance. Concerning the blank material used, formability of Mg‐containing aluminium alloys shows strong strain rate sensitivity at elevated temperatures. To figure out the optimal strain rates for this particular process, a control system for forming processes is developed within the scope of this paper. Additionally, FE‐simulations are carried out and adapted to the experiment, based on the generated process data. FE‐investigations include forming of domes (bulging) as well as shape‐defined forming, having the objective to increase formability in critical form elements by applying optimal strain rates. Here, a closed‐loop process control for gas‐pressure forming at elevated temperatures is to be developed in the next stages of the project.     

Effect of Tooling Temperature on the Transient Lubricant Behavior in Hot Metal Forming Processes

In hot metal forming processes, the temperature of the forming tool progressively increases under serial production conditions. Water-based two-phase lubricants may be applied to cool the forming tool and moderate temperature, in which the liquid agent would evaporate or decompose rapidly with dry matter deposited on the tooling surface during the dwelling time before the forming process commences. Herein, an interactive friction model for a two-phase lubricant is developed to predict the transient lubricant behaviors, i.e., predicting the effects of tool temperature and dwelling time on the friction coefficient evolution and lubricant breakdown. Friction tests between a warm pin and hot aluminum workpiece are conducted using the advanced friction testing system, TriboMate, to validate the modeling results.     

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.     

Induction Heat Treatment of Sheet‐Bulk Metal‐Formed Parts Assisted by Water–Air Spray Cooling

In order to produce components with massive secondary functional elements from sheet metal bulk forming operations, termed sheet‐bulk metal forming, can be applied. Owing to high, three‐dimensional stress and strain states present during sheet‐bulk metal forming, ductile damage occurs in the form of micro‐voids. Depending on the material flow properties, tensile residual stresses can also be present in the components' formed functional elements. During service, the components are subjected to cyclic loading via these functional elements, and tensile residual stresses exert an unfavorable influence on crack initiation and crack growth, and therefore on the fatigue life. Following the forming process, temperature and microstructurally related compressive residual stresses can be induced by local heat treating of the surface. These residual stresses can counteract potential crack initiation on the surface or in the subsurface regions. In the present study, the adjustability of the residual stress state is investigated using a workpiece manufactured by orbital cold‐forming, which possesses an accumulation of material in its edge region. Based on residual stress measurements in the workpiece's edge region using x‐ray diffractometry, it is possible to verify the compressive residual stresses adjusted by varying the cooling conditions.     

Lubricant behavior in the deformation source during the pressure treatment of metals

The flow of viscous liquid (used to simulate lubricant) as it is compressed with shear by parallel plates (surfaces) may be determined by the method here proposed. This method provides information regarding the flow kinematics and the contact forces. Practical application of the results permits estimation of the surface quality of the metal when hot-rolled pipe passes through rolling mills of various types.     

Experimental and Numerical Failure Criteria for Sheet Metal Forming

For sheet metal forming, often the forming limit diagram (FLD) is used as failure criterion as it can be derived easily in experiments. It is based on the assumption that localization of strain in the sheet plane is responsible for crack initiation, but application of FLD is limited to linear strain paths. Hence, only forming processes with approximately the same deformation history as the experiments carried out for FLD determination should be evaluated by this criterion. Forming limit stress diagrams (FLSD) do not exhibit such strict limitations. They are based on the assumption that principal stresses in the sheet plane are responsible for crack initiation. As these stresses are usually calculated by FE analysis using elastic plastic material laws, strain hardening is considered. Two‐step forming tests as application examples prove the FLSD to be adequate for evaluation of non‐linear forming processes with alternating forming directions. Nevertheless, FLSD are derived in extensive investigations which makes them unattractive for most industrial applications. Furthermore, both FLD and FLSD do not consider the physical background of ductile crack initiation which is provoked by an interaction of local stress triaxiality and equivalent plastic strain. Hence, a reliable failure criterion should concentrate on these two parameters. The Gurson‐Tvergaard‐Needleman‐ (GTN‐) damage model can predict crack initiation during sheet metal forming. Application of the GTN model to 2 step forming tests with the bake hardening steel H220BD+Z showed good agreement to experimental results although a sensitivity of the model to mesh size and stress triaxiality is observed.     

Ensuring Specified Surface Roughness in the Production of Thin Cold-Rolled Steel Sheet

The formation of surface microrelief on rolled strip is considered. The influence of the surface state of the rolls, the strip reduction, the rate of reduction, the mechanical properties of the metal, the lubricant employed, and other rolling and tempering parameters on the roughness of cold-rolled steel strip is analyzed. Practical means of ensuring compliance with the surface-roughness standards for thin sheet are proposed.     

Development of a Robot‐Based Sheet Metal Forming Process

This paper describes a new sheet metal forming process for the production of sheet components for prototypes and small lot sizes. The generation of the shape is based on kinematics and is implemented by means of a new forming machine consisting of two industrial robots. Compared to conventional sheet metal forming machines, this newly developed forming process offers a high geometrical form flexibility, and comparatively small deformation forces enable high deformation degrees. The principle of the procedure is based on flexible shaping by means of a freely programmable path‐synchronous movement of the two robots. The final shape is produced by the incremental infeed of the forming tool in depth direction and its movement along the contour in lateral direction at each level of the depth direction. The supporting tool with its simple geometry is used to support the sheet metal and follows the forming tool at the rear side of the sheet metal. The sheet metal components manufactured in first attempts are of simple geometry like frustum and frustum of pyramids as well as spherical cups. Among other things the forming results are improved by an adjustment of the movement strategy, a variation of individual process parameters and geometric modifications of the tools. In addition to a measurement of the form deviations of the sheet with a Coordinate Measurement Machine, screened and deformed sheets are used for deformation analyses. Furthermore, the incremental forming process is analysed with assistance of the finite element method. In total the results show that a robot‐based sheet metal forming with kinematic shape generation is possible and leads to acceptable forming results. In order to be able to use the potential of this process, a goal‐oriented process design is as necessary as specific process knowledge. In order to achieve process stability and safety, the essential process parameters and the process boundaries have to be determined.     

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.     

冷轧普板压痕缺陷产生原因综述

冷轧板最主要的表面质量问题是表面缺陷,而压痕在冷轧板表面缺陷中占很大比重。针对攀钢冷轧产品出现的压痕机械损伤类缺陷,通过现场跟踪调查分析缺陷形成原因,制定控制措施,取得了较好的效果。     

推拉式机组酸洗板表面质量的控制

对酸洗板表面主要缺陷进行了介绍,分析了形成各种缺陷的成因及其对后续板材深加工的潜在影响,提出了质量控制措施.     

FD级镀锌钢卷的表面质量控制

FD级表面镀锌IF钢是表面质量要求最高的镀锌产品,表面质量控制难度大,合格率低。其主要缺陷之一“线状”缺陷是炼钢内生夹杂或保护渣卷入引起的。控制“线状”缺陷的主要措施是洁净度控制和机清（火焰清理）。为了对比洁净度控制和机清措施的有效性,进行了IF钢镀锌钢卷表面质量控制试验。结果表明,机清对提高FD命中率的作用大于洁净度控制措施;没有经过机清的板坯,钢水洁净度控制措施、连铸液位波动和三路氩气背压控制的影响是明显的。试验过程发现的“机清翘皮”缺陷,是机清板坯表面毛刺或凸棱经轧制形成的。“机清翘皮”下方没有夹杂物,有时能够发现少量FeO。修磨等提高机清后板坯表面质量的方法有助于消除机清“翘皮”缺陷。     

金属板料成形性能及其CAE分析

详细论述金属板料成形性能及其影响因素,并介绍成形缺陷的产生原因．同时探讨利用CAE技术,分析板料成形过程,达到提高质量,缩短设计周期的目的．