Analysis of the tribological behaviour of polymer composite tool materials for sheet metal forming

Today's automotive industry shifts its focus on customised production, facing an increasing demand for medium and small batch production, where cost-effective manufacturability of sheet metal forming dies comes into the foreground. Filled polymers offer possibilities to fulfil such requirements in the ambit of prototype tools manufacturing or in small batch production of sheet metal components. This paper presents investigations dealing with tribological and tool design aspects of using polymeric materials for sheet metal forming purposes. Friction and wear behaviour of two polymer composites on sheet metal counterface materials have been investigated. A new testing method for wear evaluation of polymeric materials for sheet metal forming using a Strip Drawing Test facility is presented and discussed. A method to predict lifetime of polymeric stamping dies using the linear wear–distance relation W_l/s measured with the new testing method is also proposed. Significant improvements in friction and wear performance of polymer composites have been observed using sheet materials with structured surfaces. A theoretical model for abrasive friction and wear of polymer composites on sheet metal counterface material pairs has been developed, based on the results obtained by measurement of friction and wear.     

Selection of tool materials and surface treatments for improved galling performance in sheet metal forming

Galling is a known failure mechanism in automotive stamping. It results in increased cost of die maintenance and scrap rate of products. In this study, rectangular pan and U-channel stamping experiments are used to (1) investigate the effect of stress states on galling performance in sheet metal forming, (2) select proper tool materials and surface treatments for improved galling properties, and (3) differentiate galling performances of bare and coated steel sheets. The results indicate that problems with galling are of major interest at the regions where sheet materials deform under the action of compressive stress. For the four investigated tool materials, Mo–Cr alloy cast iron shows the best galling performance. A combination of hardening, surface polishing, and Cr coating is suggested as the optimum tool treatment in the forming of bare high-strength steel. Hot-dip galvanized steel shows better galling behaviors than galvannealed and bare steel sheets. Galling performances of hot-dip galvanized and bare steels are improved with increased hardness of the forming tool. However, galvannealed steel results in severe galling when the hardness of the forming tool is very high.     

Comparison of two test methods for evaluation of forming tool materials

Two test methods often used to simulate the tribological performance of forming tool materials are compared in this investigation through an evaluation of the friction and wear properties of four tool steels in dry sliding. One test (slider‐on‐flat‐surface (SOFS)) utilises a vertical disc sliding on a horizontal flat test surface, and the other (load scanner (LS) tests) utilises two crossed cylindrical rods. The test conditions were selected as equal as possible for the two tests, and the following conclusions are made. Somewhat unexpectedly, the friction and wear results differed substantially between the two tests. The SOFS test showed a better potential to evaluate wear resistance since one test sample is in continuous contact with the other. The LS test can generate higher contact pressures since the two rods contact each other in an unworn condition throughout the whole test stroke. LS indicate that two hard and smooth tool steels tested against each other generally give low friction and good galling resistance. The two hard couples tested sustained the highest loads without any detectable surface damage. For the same combinations of hard steels, SOFS gave a higher friction due to the wear of the disc. The carbides in the disc material resist wear better than the matrix and will consequently wear the disc by abrasion, which adds to the friction. The above conclusions are drawn from a rather limited examination using only one set of test parameters. In fact, the two tests are both very flexible as to the way they can be used, and they both have advantages and limitations in tribological studies. They should rather be considered complementary than competitive. Copyright © 2008 John Wiley & Sons, Ltd.     

Modeling of tool wear by diffusion in metal cutting

A modeling of diffusion wear is proposed for high speed cutting processes. Diffusion at the tool–chip interface is controlled by the contact temperature. Cratering and tool life are analyzed in terms of cutting conditions (cutting velocity, feed, rake angle), friction characteristics and material properties. Optimal cutting conditions can be found with respect to tool life and volume of material removal. Comparison with experimental observations show the potentiality of the modeling.     

High-temperature abrasive wear testing of potential tool materials for thixoforming of steels

High temperature abrasive wear performance of Inconel 617, Stellite 6 alloys and X32CrMoV33 hot work tool steel was investigated. The wear resistance of the latter is degraded at 750 °C due to its inferior oxidation resistance. Extensive oxidation co-occuring with abrasive wear at 750 °C leads to substantial material loss due to the lack of a protective oxide scale, sufficiently ductile to sustain the abrasion without extensive spalling. The wear resistance of the Inconel 617 and Stellite 6 alloys, on the other hand, improves at 750 °C owing to protective oxides that sustain the abrasion without spalling.     

On the model for identification of wear law parameters of viscoelastic materials based on ball-on-disk wear tests

An analytical model of contact interaction between spherical indenter and rotating disk of viscoelastic material (ball-on-disk configuration) is considered. The numerical analysis for influence of load–speed regime on wear rate of disk is carried out. It is shown that adequate choice of contact interaction model which takes into account rheological behavior of the disk material is important in identification of wear law parameters based on the testing results.     

大型板材模具设计所需的回弹逆解算法

在分析理想弹塑性材料弯曲回弹的基础上,提出了一种板料弯曲回弹的计算方法,推导出根据板料强度、厚度和最终成形半径直接计算弯曲半径的逆解公式.这一公式对板料成形中模具修正有一定的借鉴作用.     

Effect of tool wear on delamination in drilling composite materials

Among all machining operations, drilling using twist drill is the most frequently applied for secondary machining of composite materials owing to the need for structure joining. Delamination is mostly considered as the principal failure model in drilling of composite materials. Drill wear is a serious concern in hole-making industry, as it is necessary to prevent damage of cutting tools, machine tools and workpieces. The industrial experience shows the worn drill causes more delamination. This paper presents a comprehensive analysis of delamination caused by the drill wear for twist drill in drilling carbon fiber-reinforced composite materials. The critical thrust force at the onset of delamination for worn drill is predicted and compared with that of ideal drill. The experimental results demonstrate that though the critical thrust force is higher with increasing wear ratio, the delamination becomes more liable to occur because the actual thrust force increases to larger extent, as the thrust factor (Z) illustrates. Compared to sharp drill, the worn twist drill allows for lower feed rate below which the delamination damage can be avoided.     

Carbon based tool coatings as an approach for environmentally friendly metal forming processes

The deep-drawing of austenitic stainless steel AISI 304 provides a considerable challenge for production engineers. A major problem in such processes is adhesive tool wear which is often prevented using chlorous mineral oils. The aim of this work was to replace these hazardous lubricants with biodegradable, rape oil based fluids. For that purpose a modified triglyceride named HIGTO(I) is used in combination with a graded zirconium carbide (ZrC_g) physical vapour deposition (PVD) tool coating chosen because of their good performance in previous studies. The results from deep-drawing tests with uncoated dies show that HIGTO(I) itself is not able to replace chlorous deep-drawing oils. But in combination with HIGTO(I) as a lubricant and ZrC_g as tool coating deep-drawing forces are reduced and adhesive tool wear is prevented. This combination even exceeds the performance of chlorous deep-drawing oils. The wear effects, which appeared in different tests, could be explained by an analysis of tool loads and process temperatures, calculated using FE-simulations.     

Application of thin layer activation to measuring tool wear in metal cutting

Thin layer activation (tla) as a method for measuring tool wear is discussed in the context of previously available methods and shown to be more accurate, quicker and less costly. The concept of tool wear resistance, a function of cutting time and wear, obtained experimentally by tla, is introduced and shown to be a useful aid to determining tool life     

硬质合金圆柄止动槽成型刀设计

针对加工止动槽用硬质合金焊接刀刃磨时,其宽度难以控制、对刀困难、无法加工倒角等问题,设计了圆柄止动槽成型刀代替硬质合金焊接刀,只需刃磨排屑槽,节约了磨刀时间,保证了止动槽宽度尺寸,节省了倒角工序,提高可产品质量和经济效益。     

Modified membrane finite element formulation for sheet metal forming analysis of planar anisotropic materials

A finite element formulation is derived for sheet metal forming analysis of planar anisotropic materials. The formulation incorporates membrane elements whereas it takes the bending effect into account explicitly. The strain energy term in the formulation is decomposed into the membrane energy term for mean stretching and the bending energy term for pure bending. This procedure needs careful evaluation for the orientation of the anisotropic axes. The formulation is then combined with an effective algorithm to calculate distribution of the blank holding force in each step according to the thickness in the flange region. The calculation employs a special relation between the thickness and the blank holding force. The simulation examples demonstrate the validity and versatility of the developed computer code by showing that the thickness variation in the flange region redistributes the blank holding force during the deformation. The present algorithm can predict accurate deformed shapes and thickness strain distribution with the anisotropy of materials and the variable blank holding force.     

Friction and wear tests for elastomers

A common use of elastomers in business machines is as rollers, controlling the motion of such items as checks, paper forms and printer ribbons. In these applications both the friction and the wear resistance characteristics of the elastomer are of major significance. In this paper, two tests, developed to perform such characterizations against paper and ribbon surfaces, are described. Two case histories in which these test methods were effectively employed in resolving design problems are discussed. In addition, data for several elastomers are presented and compared.     

A general forming limit criterion for sheet metal forming

The forming limit of sheet metal is defined to be the state at which a localized thinning of the sheet initiates during forming, ultimately leading to a split in the sheet. The forming limit is conventionally described as a curve in a plot of major strain vs. minor strain. This curve was originally proposed to characterize the general forming limit of sheet metal, but it has been subsequently observed that this criterion is valid only for the case of proportional loading. Nevertheless, due to the convenience of measuring strain and the lack of a better criterion, the strain- based forming limit curve continues to play a primary role in judging forming severity. In this paper it is shown that the forming limit for both proportional loading and non-proportional loading can be explained from a single criterion which is based on the state of stress rather than the state of strain. This proposed criteria is validated using data from several non-proportional loading paths previously reported in the literature for both aluminum and steel alloys. In addition to significantly improving the gauging of forming severity, the new stress-based criterion is as easy to use as the strain-based criterion in the validation of die designs by the finite element method. However, it presents a challenge to the experimentalist and the stamping plant because the state of stress cannot be directly measured. This paper will also discuss several methods to deal with this challenge so that the more general measure of forming severity, as determined by the state of stress, can be determined in the stamping plant.     

Galling resistance evaluation of tool steels by two different laboratory test methods for sheet metal forming

Adhesive accumulation of work material on the tool surface is today a major problem in many sheet metal‐forming applications. Different laboratory test methods are used to investigate galling with respect to different tool materials, lubricants and process conditions. In the present study, the galling resistance of a modern nitrogen‐alloyed powder metallurgy tool steel and an conventional ingot cast D2 type tool steel was evaluated under lubricated sliding against ferritic stainless steel sheets using a commercial pin‐on‐disc (POD) and an in‐house made slider‐on‐flat‐surface (SOFS) tribotester. The investigated tool steels ranked similarly in terms of galling resistanc in both test methods. However, sliding distances to galling were longer for the SOFS equipment due to continuous sliding on new lubricated sheet surface. Best performance was demonstrated by the powder metallurgy tool steel treated to 65 HRC. Differences in friction behaviour and galling initiation were analysed on the basis of the two different working conditions, i.e. open (SOFS) and closed (POD) tribosystems. Copyright © 2012 John Wiley & Sons, Ltd.