Galling reduction through die surface treatment - testing three treatments for sheet metal stamping

This brief article describes how tool galling occurs in stamping and describes results of tests on how TiCN, Hard-Cr, and CrCN were used on sample parts to help reduce galling.

Contact pressure evolution and its relation to wear in sheet metal forming

For a given sheet metal forming process, an accurate determination of the contact pressure distribution is an essential step towards the estimation of tool life. This investigation utilizes finite element (FE) analysis to model and explain the evolution and distribution of contact pressure over the die radius, throughout the duration of a channel forming process. It was found that a typical two-peak steady-state contact pressure response exists for the majority of the process. However, this was preceded by an initial transient response, characterized by extremely large and localized contact pressures, which were more than double the magnitude of the steady-state peak pressure. The validity of the predicted contact pressure behavior was assessed via detailed numerical analysis and by examining the wear response of an experimental stamping operation. The experimental results revealed that the high contact pressure zones of the transient response corresponded to a severe galling wear mechanism. Therefore, the transient response may be of primary significance to the tool wear response; thus questioning the applicability of traditional bending-under-tension wear tests for sheet metal stamping processes. Finally, a parametric study was conducted, examining the influence of the major process parameters on the steady-state and peak transient contact pressures, using the developed FE model. It was found that the bend ratio and the blank material ultimate tensile strength had the most influence on the peak contact pressures. The main process-related parameters, friction coefficient and blank holder force, were found to have only a minor influence.

Evaluating lubricants in sheet metal forming

The sheet metal forming process basically involves the shaping of sheet metal of various thickness and material properties into the desired contours. This metal forming process has been extensively used by the automotive industry to manufacture both car panels and parts. Over the years numerous investigations have been conducted on various aspects of the manufacturing process with varied success. In recent years the requirements on the sheet metal forming industry have headed towards improved stability in the forming process while lowering environmental burdens. Therefore the overall aim of this research was to identify a technique for developing lubricant formulations that are insensitive to the sheet metal forming process. Due to the expense of running experiments on production presses and to improve time efficiency of the process the evaluation procedure was required to be performed in a laboratory. Preliminary investigations in the friction/lubricant system identified several laboratory tests capable of measuring lubricant performance and their interaction with process variables. However, little was found on the correlation between laboratory tests and production performance of lubricants. Therefore the focus of the research switched to identifying links between the performance of lubricants in a production environment and laboratory tests. To reduce the influence of external parameters all significant process variables were identified and included in the correlation study to ensure that lubricant formulations could be desensitised to all significant variables. The significant process variables were found to be sensitive to die position, for instance: contact pressure, blank coating of the strips and surface roughness of the dies were found significant for the flat areas of the die while no variables affected friction when polished drawbeads were used. The next phase was to identify the interaction between the significant variables and the main lubricant ingredient groups. Only the fatty material ingredient group (responsible for the formation of boundary lubricant regimes) was found to significantly influence friction with no interaction between the ingredient groups. The influence of varying this ingredient group was then investigated in a production part and compared to laboratory results. The correlation between production performance and laboratory tests was found to be test dependant. With both the Flat Face Friction test and the Drawbead Simulator unaffected by changes in the lubricant formulation, while the Flat Bottom Cup test showing similar results as the production trial. It is believed that the lack of correlation between the friction tests and the production performance of the lubricant is due to the absence of bulk plastic deformation of the strip. For this reason the Ohio State University (OSU) friction test was incorporated in the lubricant evaluation procedure along with a Flat Bottom Cup test. Finally, it is strongly believed that if the lubricant evaluation procedure highlighted in this research is followed then lubricant formulations can be developed confidently in the laboratory.

Sheared edge quality in the trimming of sheet metals

Examines the sheared edge quality of different automotive sheet metals resulting from the trimming process using experimental tooling and finite element modelling. The significant differences in burr, sliver and edge profile formation have been explained in terms of the strain distribution and tensile properties of the materials.

Tool wear analysis in sheet metal stamping

This thesis advances the understanding of die wear in sheet metal stamping. It was found that transient conditions exist at the die radius, resulting in severe contact pressures that are critical to the wear behaviour. The findings challenge applicability of traditional wear tests and models for sheet metal stamping processes.

Effects of temperature in relation to sheet metal stamping

The demand to reduce the use of lubricants and increase tool life in sheet metal stamping has resulted in increased research on the sliding contact between the tool and the sheet materials. Unlubricated sliding wear tests for soft carbon steel sliding on D2 tool steel were performed using a pin-on-disk tribometer. The results revealed that temperature has an influencing role in the wear of tool steel and that material transfer between tool and sheet can be minimized at a certain temperature range in sheet metal stamping.

Tool wear in sheet metal stamping

This paper discusses our recent research on wear at the die radius in sheet metal stamping. According to wear theory, contact pressure and sliding distance are the two dominant factors in determining sliding wear. We applied the finite element analysis to accurately quantify the contact pressure and sliding distance at the die radius in sheet metal stamping. The results were then applied to analyze sliding wear at the die radius. We found that a typical two-peak steady-state contact pressure response exists during a channel forming process. The steady-state contact pressure response was preceded by an initial transient response, which produced extremely large and localized contact pressures. We proposed a method to numerically quantify the sliding distance, which was applied to examine the contact sliding distance at the die radius. Correlating the contact pressure and sliding distance, a new insight into the wear/galling that occurs at the die radius in sheet metal stamping was gained. The results show that the region close to zero degrees on the die radius is likely to experience the most wear, with the identified transient stage contributing to a large proportion of the total wear.

Temperature conditions during 'cold' sheet metal stamping

This paper investigates the friction and deformation-induced heating that occurs during the stamping of high strength sheet steels, under room temperature conditions. A thermo-mechanical finite element model of a typical plane strain stamping process was developed to understand the temperature conditions experienced within the die and blank material; and this was validated against experimental measurements. A high level of correlation was achieved between the finite element model and experimental data for a range of operating conditions and parameters. The model showed that the heat generated during realistic production conditions can result in high temperatures of up to 108 °C and 181 °C in the blank and die materials, respectively, for what was traditionally expected to be 'cold' forming conditions. It was identified that frictional heating was primarily responsible for the peak temperatures at the die surface, whilst the peak blank temperatures were caused by a combination of frictional and deformation induced heating. The results provide new insights into the local conditions within the blank and die, and are of direct relevance to sheet formability and tool wear performance during industrial stamping processes. © 2014 Elsevier B.V. All rights reserved.

Path independent limiting criteria in sheet metal forming

The Forming Limit Diagram (FLD) is a conventional failure diagram to estimate necking limits of sheet metal for proportional loading conditions. Previous studies reveal that the FLD is not suitable for predicting the influence of nonlinear strain paths. This paper presents methodical comparison among all common available strain path independent strain/stress based limiting criteria. All the strain path independent strain based limiting criteria (Traditional Failure Diagram (TFD), Extended Forming Limit Diagram (XFLD), Extended Stress Ratio Based Forming Limit Diagram (ESRFLD), and Polar Effective Plastic StrainDiagram (PEPSD)) and stress based limiting criteria (Traditional Stress based Failure Diagram (TFSD), Stress Based Forming Limit Diagram (FLSD), Stress Ratio and Stress Based Forming Limit Diagram (SRFLSD), Extended Stress Based Forming Limit Diagram (XFLSD), and Polar Effective Stress Diagram (PESSD)) are approximately path-independent for smaller amount of pre-straining and path dependent for large pre-straining conditions. From advance image correlation technique precisely determination of local strains near necked area is possible today. However direct determination of local stresses near necked area is not possible. Therefore, local stresses and equivalent stress are determined by employing both yield criterion and strain-hardening law. Similarly equivalent strain is calculated by the use of yield criterion. Therefore, the choice of yield criterion has an impact on the results for TFD, XFLD, ESRFLD and PEPSD. However, selections of both yield criterion and strain-hardening law have substantial influence on the results for TFSD, FLSD, SRFLSD, XFLSD and PESSD. The inherent calculation error can be minimized by generation of experimental data for each material and then selection of representable yield criterion and strain-hardening law. Improvement of experimental techniques and generation of rigorous material data bank in various strain paths may help researchers to diagnose and resolve the issue. TFD, TFSD and XFLSD have inherent variables to take care the effect of through thickness stress, however rigorous experimental verification is needed before the field application.