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.
The authors demonstrate that functionalizing tool steel die surfaces with an octadecylphosphonic acid molecular monolayer decreases friction during Al cold forming. Specifically, molecular functionalization leads to a 1.9-fold decrease in time-averaged torque during tribological compression-torsion wear tests. Electron spectroscopy suggests that weak van der Waals interaction between aluminum and the distal CH3 termini of the phosphonic acid molecules anchored to the steel surface via P–O bridges lubricate the aluminum–steel interface. The observation of this effect at contact-pressures of ≥75 MPa underscores the tremendous potential of molecular functionalization for devising industrial metal forming processes without the use of liquid lubricants.
Gaseous medium is being used for sheet metal forming at elevated temperatures, especially for lightweighting purposes. These processes enable forming of high strength alloys of a wide range of thickness due to low material flow stress as well as improved formability. In these processes, the resulting component properties are an interplay of numerous parameters. Instead of cost and time intensive experiments, FEM aids an effective and economic process optimization and enables a better understanding of the influence of process parameters on the component properties. In the current study, the importance of appropriate discretization of the workpiece within a gas-based hot sheet metal forming process is investigated based on a laboratory scale component. AA6010 sheet metal blanks of different thicknesses are studied numerically and experimentally. Simulations with different types of elements are performed and the evolution of process parameters as well as their influence on the final component thickness are analysed. Different element types resulted in noticeable difference in the simulation results and this difference also varies with the initial sheet thickness. Upon further experimental validation, the suitable element type for workpiece discretization is suggested, which enables practitioners to get reliable results via FE simulation of these processes.
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