Abstract: In metal cutting, a severe thermo-mechanical load collective determines the friction and wear behavior at the tool-chip interface. The inaccessibility of this interface complicates studies and thus the understanding of tribological effects in metal cutting. During a tool’s lifetime, local friction conditions change drastically as coatings and tool geometry wear down. This paper shall provide a comprehensive overview of current methods to understand and describe friction conditions in metal cutting and how cutting induced surface layer states may influence the friction and wear behavior of the finished workpiece.
Abstract: This paper draws from the existing processes and applications of joining by plastic deformation to a comprehensive overview of a new set of processes that have been recently developed by the authors. The presentation includes solutions for connecting tubes, sheets and tubes to sheets and provides information on the tooling systems, operating variables, deformation mechanics and workability limits. Results from analytical modelling, finite element analysis and experimentation give support to the presentation and prove the feasibility of the new joining by plastic deformation processes for connecting tubes, sheets and tubes to sheets made from dissimilar materials, at room temperature, without having to use addition materials or adhesives. The resulting joints are easy to disassembly at the end of live, thereby allowing recyclability of the individual parts.
Abstract: Since the beginning of the 90’s, research activities focused on friction and wear in metal forming have been developed at the LAMIH UMR CNRS 8201 in Valenciennes. Specific methodologies have been designed to optimize a given forming process (bulk forming process or sheet forming process). These methodologies involve prototype benches which have been built to reproduce contact conditions encountered in manufacturing plants by taking specimens and contactors from the real industrial workpieces and tools. The evaluation of the friction coefficient added to the fine analysis of the surfaces have helped us to better understand friction and wear during processes. These facilities have been settled by numerical simulation at meso and macro scales by means of finite element methods. So, this paper is the sum up of the output of these methodologies with a specific focus on wear and lubrication, at room and hot temperatures.
Abstract: Over the last few years lightweight construction became increasingly important in modern cars. Motivated by reducing greenhouse gas emission the car industry is currently working on different approaches to decrease the weight of structural body parts. In this regard, a reduced sheet thickness of these components and therefore a reduced overall weight can be achieved by using high-strength steels. Hot stamping has been established as a suitable manufacturing process for these steel grades, in which a hot austenitic blank is formed and quenched simultaneously. The high strength of the formed parts is realized by the phase transformation of an austenitic to a martensitic structure during hot stamping. Due to the alternating thermo-mechanical loads, which occur during forming and quenching, the hot stamping tools are highly stressed. In addition, when the blank slides over the surface of its counterpart, a substantial adhesive wear occurs, which is the predominant wear mechanism in hot stamping. The aim of this study is, to increase the wear resistance of the tools by modifying the surface. In this context, the chemical affinity between the interacting components need to be reduced in order to decrease the adhesive wear on the hot stamping tool, which is possible by alloying the base material. For this reason, the wear development is investigated for samples alloyed with different materials with a modified pin-on-disc test. This experimental setup enables a continuous contact of the tool with the blank and thermal alternating stress of the pin. The contact area is investigated with a laser-scanning microscope to qualify the tool surface before and after the experiments by measuring the tool topographies. The results of an unalloyed and alloyed tool will be compared with each other to evaluate the wear behavior. In order to quantify the amount of wear the wear volume will be calculated with an algorithm of the software WinSam. The experiments will be carried out under process like conditions to ensure transferability to the real hot forming process.
Abstract: Liquid lubrication guarantees high precision and surface quality of workpieces in industrial forming processes. In the case of aluminum cold extrusion, wear and cold welding due to direct contact of tool and workpiece are usually prevented by the extensive use of lubricants. Since the use of lubricants is economically and ecologically unfavorable, surface treatments of tools by, e.g. laser polishing and/or coatings are in the focus of current investigations to substitute these lubricants and establish so called “dry metal forming” processes. The material AISI D2, a ledeburitic 12% chromium steel which is known to have a significant amount of chromium carbide precipitations, is widely used in cold extrusion for forming tools. The large fraction of chromium carbide precipitations, however, hinder the formation of a dense self-assembled monolayer (SAM) that is necessary to avoid direct contact of reactive aluminum with surface oxides of the tool. Therefore, a homogeneous distribution of the chemical elements with a smaller fraction or no chromium carbides in the steel matrix, particularly in the tool surface, is aimed for. Using laser polishing, the surface layer is molten by continuous or pulsed laser radiation. Within the melt pool, the elementary distribution is homogenized as a result of thermal convection and diffusion processes, as well as a smoothed surface and a grain refinement are achieved. Consequently, the effects of the surface treatment by laser polishing on the area coverage of self-assembled monolayers are investigated. Thus, a combined surface treatment by laser polishing and functionalization with a dense self-assembled monolayer shall reduce overall adhesive wear. For this investigation, several specimens of conventional manufactured and powder metallurgical molten AISI D2 are laser polished using continuous or pulsed laser radiation or a combination of both. The resulting surfaces are investigated by microscopy and spectroscopic techniques to analyze the surface topography and the elemental distribution near to the surface. These results are compared to those of conventionally hand-polished specimens. Furthermore, the influence of the element homogenization and grain refinement on the area coverage of self-assembled monolayers is explored. First results show that laser polishing of AISI D2 is suitable to achieve a reduction of grain size and a more homogeneous distribution of chromium carbides within the surface layer.
Abstract: There are a number of ways to reduce friction such as lubrication, improving the surface roughness, reducing the acting surface pressure, surface texturing and reducing the contact area. In this study the effect of the contact size and surface pressure on process limits of a forming process is investigated. For this purpose draw-bending of U-Channel is subjected for testing of the effect of surface reduction by means of micro surface texturing and macro structuring. The results show that micro surface texturing and macro structuring of forming tools can reduce the friction force in forming processes. A combination of these approaches can be applied in tooling design to minimize the friction forces and ultimately realize a lubricant-free forming process.
Abstract: The aim of this study was to investigate the influence of laser surface texturing on fatigue life and tribological properties. Textures were introduced before and after hard TiAlN coating deposition in order to investigate the sequence of surface texturing process. It was found that the sequence of surface texturing has an effect on tribological and fatigue life behaviour. If laser texturing is done after coating deposition, friction is lower because of the better wettability of steel in comparison with coating, but suffers in fatigue life properties.
Abstract: Deep rolling is an established mechanical surface treatment technology based on local plastic deformation of the surface layer. By these means, residual stresses, and strain hardening are induced into the surface layer as well as its surface structure is smoothed. Vibrorolling is a derivate technology of deep rolling characterized by sinusoidal rolling lanes. Due to process kinematics of vibrorolling the surface layer is incrementally deformed multiple times in different directions. As a result, a more intensive plastic deformation of the surface layer is achieved and potentially tribologically active surface structures are produced. To investigate and compare the effects of both surface treatment technologies on the tribological behavior of a processed component, a friction and wear analysis under lubricated conditions was conducted in this work. Friction and wear behavior of untreated, deep rolled, and vibrorolled specimens using a pin-on-cylinder tribometer was conducted. Hardness, roughness, and geometrical measurements of the wear traces were used to characterize the specimens. Additionally, qualitative assessments of the wear traces using scanning electron microscopy imaging were made. The measurements were performed before, during, and after the friction and wear analysis. Furthermore, contact forces between a tribometer pin and the workpiece were determined to analyze the development of contact shear stresses. Based on the conducted investigations, the effects of deep rolling and vibrorolling on the friction and wear behavior of the treated specimens are discussed and explanations for the observed phenomena are formulated in this work.
Abstract: Aluminum alloys are materials that have a strong tendency to galling when they are cold formed. Caused by a breakdown of the lubricant film between the part and the tool, galling can have dramatic consequences on the forming operation: scratches and cracks in the surface of the piece, clogging and deterioration of tools, etc. The present work studies the galling mechanisms of the aluminum alloy 6082 during its cold forming. Trials involving the Upsetting-Sliding Test (UST) are performed first. The UST is a test bench able to simulate in laboratory conditions the contact encountered at the part/tool interface of industrial processes. Trials are achieved under varying contact pressure and lubrication. UST results show that galling is strongly influenced by tool roughness and is not accompanied by a significant increase of friction. Three sets of finite element computation of the UST are then run to predict galling onset. Lubricant and adhesion forces are not modelled in this simplified approach: only the mechanical aspects are taken into account, the chemical ones are implicitly taken into account by coefficients of friction. The Lemaitre’s and the Xue’s damage models are compared. Results show that the Lemaitre model needs the tool roughness to be modeled to detect the galling onset. The Xue model is able to detect the occurrence of galling without modelling roughness. This result is due to the used of the Lode angle with enable the calculation of damage under low stress triaxiality.