Aluminium alloys show poor formability at room temperature, and the production of complex components requires a series of high temperature forming processes, such as warm and hot forging, extrusion and hot sheet metal forming. Forming aluminium in these conditions subjects the tools to severe adhesive wear and galling, leading to increased energy needs, shorter tool life, lower part quality and increased cost. In this work, the wear mechanisms generated by aluminium alloys on forming tools have been studied by means of linear reciprocating sliding tests. Aluminium alloy AA2017 balls were slid against DIN 1.2344 (AISI H13) tool steel samples with various surface finishes at temperatures up to 450 ºC. The main results show that the observed wear mechanisms are extremely dependent on the system temperature, ranging from pure abrasive wear to formation of layers of compacted aluminium debris and gross aluminium transfer in the form of lumps. On the other hand, tool surface finish has a limited effect on gross material transfer, but does affect the material transfer micromechanisms.

In this work, the mechanisms resulting in transfer of aluminium on hot forming tools have been analysed by means of two separate laboratory tests. The influence of chemical affinity in aluminium adhesion has been studied in contact tests, measuring the force used in pulling at low velocity an aluminium ball pressed against a tool surface. The role of mechanical interaction has been investigated through ball-on-disk sliding tests at high temperature, using tool steel disks with different surface finish against an aluminium counterpart.These tests have been used for the evaluation of different strategies in adhesive wear reduction, including different tool steels and surface modification, and to study the effect of surface finish on the material transfer mechanisms observed.

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Scratch testing is one of the most usual characterisation techniques for systems with thin hard coatings. Scratch tests are performed at run at room temperature, due to the risk of degradation of the indenter diamond tip at high temperature and added test complexity. However, results obtained at room temperature may differ from final system performance in high temperature applications, due to factors such as substrate thermal softening. In this work, a simple and a robust test for the mechanical characterization of hard PVD coatings at high temperature and open atmosphere has been developed, based on a modification of the scratch test where the diamond tip is replaced by a WC indenter. The test has been used to evaluate the effect of temperature up to 500 ºC on commercial PVD AlCrN coating deposited on tool steel subject to different surface treatments. Results show a decrease in the failure load Lc2 with increasing temperature, and a very noticeable effect of substrate nitriding. Highlights(i) A modified scratch test for high temperature applications has been developed (ii) Performance of AlCrN-coated 1.2344 has been studied up to 500ºC (iii) Effect of substrate nitriding on system performance up to 500ºC has been explored (iv) Results show the relationship between performance and substrate load bearing capacity ABSTRACTScratch testing is one of the most usual characterisation techniques for systems with thin hard coatings. Scratch tests are performed at run at room temperature, due to the risk of degradation of the indenter diamond tip at high temperature and added test complexity.However, results obtained at room temperature may differ from final system performance in high temperature applications, due to factors such as substrate thermal softening.In this work, a simple and a robust test for the mechanical characterization of hard PVD coatings at high temperature and open atmosphere has been developed, based on a modification of the scratch test where the diamond tip is replaced by a WC indenter. The test has been used to evaluate the effect of temperature up to 500 ºC on commercial PVD AlCrN coating deposited on tool steel subject to different surface treatments. Results show a decrease in the failure load Lc2 with increasing temperature, and a very noticeable effect of substrate nitriding.

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