Advanced Materials Research Vols. 966-967

Abstract: Hot stamping of quenchenable high strength steels represents the most promising forming technology for the manufacturing of safety and crash relevant car parts. In such process, the manganese-boron steel 22MnB5 is homogeneously austenitized, formed and subsequently quenched in one process step, so that complex geometric structural components can be formed with an ultimate tensile strength up to 1500 MPa. Due to the high temperature, no lubrication is used during the process with consequent high wear and reduced service-life of the dies. Commercial available steel blanks usually present an Al-Si coating that has been specifically developed as a protection from oxidation and decarburization and, at the same time, has proved positive influence also on the decreasing the friction at interface between the blank and the dies during the forming. Although such coating is generally accepted as the only lubrication medium in hot stamping, its performances are considered not appropriate for a good lubrication. The paper presents the comparison of the standard Al-Si coating and a new Zn coating when applied to metal sheets in hot stamping. A novel apparatus to investigate the tribological conditions during sheet metal working processes is presented. In addition to the control of mechanical (i.e. normal pressure) and kinematic parameters (i.e. sliding speed, sliding length), the developed testing machine permits to reproduce the thermal fields and monitor the thermal conditions of the sheet and tool materials. Experiments were carried out on Zn coated 22MnB5 sheets in the range between 700° and 950°C and compared with the performances of the commercial Al-Si coating. The coating performances are investigated for different heating temperatures and soaking times.

Abstract: Increasing focus on environmental issues in industrial production has urged sheet metal stamping companies to look for new tribo-systems in order to substitute hazardous lubricants such as chlorinated paraffin oils. The efficiency of chlorinated paraffin is due to the fact that the lubricant reacts chemically with the tool and workpiece material forming thin films, which adhere strongly to the surfaces and reduce the tendency to metal-metal contact and material pick-up. Production tests of new, environmentally benign tribo-systems are, however, costly and laboratory tests are preferred as a preliminary simulative method to investigate alternative tribo-systems. The present paper presents a case study where an industrial process, consisting of deep drawing with two subsequent re-drawings, was selected and four potential new tribo-systems were tested including different workpiece materials, i.e. AHSSs and stainless steels. The performance of the tribo-systems was analyzed in the laboratory by means of a newly developed simulative test as well as in an industrial production process. The results obtained show a good agreement between the laboratory test and the industrial production process regarding the tribological performance, i.e. tendency to material pick-up and galling, of the evaluated tribo-systems. Moreover the SEM analysis shows that different workpiece materials result in different types of material pick-up.

Abstract: This paper presents recent investigations in Micro-Plasto-Hydrodynamic (MPH) lubrication. Industrial evidences of the existence of MPH lubrication mechanism for cold rolling processes are presented. A new lubrication model developed for strip drawing processes is then applied to predict the MPH lubrication initiation and MPH lubrication extension along the tool-piece solid contacts initially in boundary lubrication regime. Finally, it is shown how this new MPH lubrication model can be implemented in a cold rolling model to maximize mills capabilities, determine optimum rolling oils properties and predict roughness transfer.

Abstract: Knowing the flow curve of a sheet metal, strain distribution on a sheet strip may be used to roughly but quickly evaluate the Coulomb friction coefficient. Strain distribution on the strip being stretched on a cylindrical surface of interest may be measured by an optical strain measurement system. This could be used to estimate the stresses on the specimen. The capstan equation is then used to roughly evaluate the coefficient of friction acting between the sheet strip and the cylindrical surface. Validation of the approach is done using the simulation of the process. The corresponding experiments can be performed easily on a sheet metal testing device equipped with an optical strain measurement system, which is commonly used for the experimental evaluation of FLCs.

Abstract: Conventionally manufactured cold work tool steel is often used in sheet metal forming as die material. Due to the forging process, the as-cast network structure of carbides is broken into elongated particles. Depending on the tool cross-section, the orientation and shape of carbides in the active tool surface is different. In the present research, the influence of tool steel hard phase orientation and shape on galling was investigated. D2 type tool steel was cut in three different orientations and tested in lubricated sliding conditions against AISI 304 austenitic stainless steel. Tests were performed using a Slider-On-Flat-Surface and galling was detected by changes in friction and post-test microscopy. The lubricant was Castrol FST8 using 5 g/m² sheet material. Results showed a strong correlation between sliding distance to galling and tool steel hard phase orientation and shape at low loads, whereas high load contact resulted in early galling in all cases. Material transfer was observed mainly to the tool steel matrix. The worst performance was observed for specimens cut so that the tool steel hard phase, M₇C₃ carbides in the D2 steel, were oriented along the sliding direction, which resulted in longer open tool matrix areas contacting the sheet material.

Abstract: Hot sheet metal forming is gaining in importance in many fields, because of its capability to produce more complex components than possible with cold forming. Hot forming is also used to influence the final material properties with the hot forming of manganese-boron steels being a good example. One of the major challenges in hot forming is the tribological conditions between the tool and sheet material at the required high temperatures. This article will discuss the influence of different tool material coatings, ranging from PVD to mechanically bonded ceramic coatings, on the tribological conditions during forming. It will also shed light on how these coatings influence the heat transfer between the component ́s material and the tool material.

Abstract: In present time the FE-simulation of body parts in sheet metal drawing processes has to be continuously improved to increase the prediction accuracy. The objective of this paper is to describe a methodology to improve the simulation by mapping the complex tribological conditions of the process in the simulation. This methodology focuses on the dependency of influencing parameters on the friction coefficient. The influence respectively the dependency of these factors has been investigated, both in an experiment of a plane strip drawing test and in a FE-simulation of this test. With the sensitivity analysis in the numerical simulation it is possible to compare the implemented Coulomb-friction-model against the experimental results. The results of the experimental and numerical investigation show the need of further investigations on a more geometrical complex test to prove if it is possible to transfer the knowledge and dependencies that has been found in the experiment with the strip drawing test to a more geometrical complex test.

Abstract: A new upsetting-extrusion type tribometer is developed to investigate the performance of a lubrication coating on the side surface of a billet in multi-stage cold forging. In this tribometer, the lubrication coating on the billet surface is first destroyed by the upsetting process and then evaluated by the extrusion process. The frictional shear factor of the lubrication coating is obtained by plotting the measured extrusion load and the position after the extrusion of a centerline drawn on the billet in advance on the calibration curve obtained by FEM. Experimental results using a zinc phosphate coating and a dry in-place type coating showed that the reduced peak height Rpk is more appropriate than the maximum height Rz to express the effect of the surface roughness of tool on galling generation. When no galling occurs, the frictional shear factor hardly varies with the type of the lubrication coating and the surface expansion ratio. The anti-galling ability of the dry in-place type coating is greatly improved by a two-stage shot blast before the lubrication coating and reaches a level better than the zinc phosphate coating.

Abstract: A novel design of extrusion tooling is proposed. This process segments the traditional die set into three separate pieces, a punch, a wall die, and a bottom die. Additionally, the wall die is given the freedom to move along the axis of the punch and a high pressure lubricant is supplied to the interface between the wall and bottom dies. Preliminary tests of the segmented tooling show that, if the tribological conditions are severe enough, the forming load will decrease in comparison to a conventional extrusion system.

Abstract: Although the aluminum fluoride conversion coating has been a popular lubricant for aluminum cold forging in Japan, it has considerable environmental risks and troubles in cost. Double-layer-type environmentally-friendly solid lubricant films have been developed as the substitute. One of the lubricants was evaluated by a friction test based on the combined forward spline-backward can extrusion. The lubricating ability of the double-layer-type lubricant changed through the year. It showed best ability in the rainy season in Japan. Next, the double-layer-type lubricant was applied on three kinds of aluminum alloys to examine the influence of flow stress of materials on the lubricating properties. The lubricating abilities were almost same for each material in the rainy season. Those of the hardest material were higher than others in the winter. Machined and wet-blasted workpieces were tested to investigate the effect of preprocess. A remarkable friction reduction effect by preprocess was observed with the hardest material. The effect of preprocess was not significant for the materials with lower flow stress.