Mechanisms of Transfer of Aluminium to PVD-Coated Forming Tools

Wear, 2021

Hot stamping is characterised by severe contact conditions, especially when forming aluminium components. In order to improve the tool lifetime, process economy, and component quality, understanding the initiation mechanisms behind aluminium transfer onto the tool surface at high temperatures is critical. To date, the tribological interaction between tools and aluminium sheets at high temperature has received limited attention. Lubricants, combined with surface engineering techniques (e.g. coatings, nitriding and surface topography control), show great potential for reducing the severity of material transfer at high temperatures. However, there is still, limited knowledge about their interaction and performance in this tribological context. In this study, high temperature tribological tests were carried out to characterise the synergetic effects of surface coatings/treatments with and without lubrication on friction and wear. A commercially available lubricant was evaluated when used in combination with uncoated, nitrided and CrWN- or DLC ta-C-based PVD coated tool steel. The tests were carried out on a hot strip drawing tribometer, employing an open contact configuration representative of the hot stamping contact conditions at two different temperatures. The counter-material was a 6082 aluminium alloy, heated up following a thermal cycle relevant for the hot stamping process. The results showed that the tribological response was highly dependent on the retention of the lubricant in the contact and the type of surface modification technique. The results show that bonding of the lubricant to the tool surface is critical. In the case of lubricant failure, severe adhesive wear and aluminium transfer onto the tool surface occurred, correlated with an increase in friction. The use of different surface engineering methods led to different results: lower friction levels could be reached when combining use of lubricant and PVD coatings compared to using uncoated or plasma nitrided tool steel. In this study, the best combination to minimise aluminium transfer and friction is the association of the lubricant with CrWN PVD coating in this study.

Metallurgical Science and Tecnology, 2013

The behoviour of PVD cootings in olumínium die<osting ond sÍeel forging wos investigoted. As for os the die<osting process is concerned, three series of core pins surfoce heoted by meons of the Balînit PVD technology with CrC, CrN or IrAlN were exomined. For steel forging dies o PUSK PVD process wos used lo cool o cold extrusion punch withTiN while worn ond hot forming tools were CrC cooted with lhe some Íechnology. The onolyses corried oul on îhe CrC, cooÍed core pins showed o greot tendency to the formoÍion of multiple Ioyers of Al olloy which protected the steel from erosion effects. However, thís behoviour was detrimental for the surfoce roughness of workpíece.s. Ihe IAIN coofed píns were subiected lo o service life olnost double with respect of thol of the other seríes of core pins.They demonstroted to be porticularly resistant to weor ond lo oluminíum soldering. Lîmiled soldering was also observed on CrN pins. However, discontinuities ot the steel/coolíng interface mode ...

Friction, 2020

Aluminium alloys are commonly used as lightweight materials in the automotive industry. This non-ferrous family of metallic alloys offers a high versatility of properties and designs. To reduce weight and improve safety, high strength-to-weight ratio alloys (e.g. 6XXX and 7XXX), are increasingly implemented in vehicles. However, these alloys exhibit low formability and experience considerable springback during cold forming, and are therefore hot formed. During forming, severe adhesion (i.e. galling) of aluminium onto the die surface takes place. This phenomenon has a detrimental effect on the surface properties, geometrical tolerances of the formed parts and maintenance of the dies. The effect of surface engineering as well as lubricant chemistry on galling has not been sufficiently investigated. Diamond-like carbon (DLC) and CrN physical vapour deposition (PVD) coated steel have been studied to reduce aluminium transfer. However, the interaction between lubricants and PVD coatings during hot forming of aluminium alloys is not yet fully understood. The present study thus aims to characterise the high temperature tribological behaviour of selected PVD coatings and lubricants during sliding against aluminium alloy. The objectives are to first select promising lubricant-coating combinations and then to study their tribological response in a high-temperature reciprocating friction and wear tester. Dry and lubricated tests were carried out at 300 °C using a commercial polymer lubricant. Tests using DLC, CrN, CrTiN, and CrAlN coated tool steel were compared to uncoated tool steel reference tests. The initial and worn test specimen surfaces were analysed with a 3-dimensional (3D) optical profiler, scanning electron microscope (SEM) and energy dispersive X-ray spectroscope (EDS) as to understand the wear mechanisms. The results showed formation of tribolayers in the contact zone, reducing both friction and wear. The stability of these layers highly depends on both the coatings’ roughness and chemical affinity towards aluminium. The DLC and CrN coatings combined with the polymer lubricant were the most effective in reducing aluminium transfer.

Wear, 2008

In this work, the tribological properties of tool coatings applied by physical vapour deposition are studied. A first aspect is the resistance to galling and abrasive wear, which are investigated by means of a flat/cylindrical multifrottement test. The observations for the evolution of the friction coefficient discriminate the various coatings as regards the onset of galling. In addition, topographic inspection of the tools and the steel strips yields crucial information about galling scratches and wear tracks supporting a possible quantitative ranking of the different tribological systems. Subsequently, the effect of tool coatings in laboratory deep drawing operations is considered. The modification of the tribological conditions by applying a coating to the tool has a striking influence on the feasibility window to make cups. These observations are described in detail, while also the correlation with the roughness of the coated tools is briefly discussed.

Wear, 2009

Cold forming of aluminium is a group of very efficient methods, which are successfully used in a number of industrial applications. Two of the major factors limiting the tool life and restricting the shapes and sizes possible to produce, are galling, i.e. transfer of work material to the tool surface, and high stresses occurring when forming complex shapes. Both phenomena are closely related to the friction and adhesion in the tool to workpiece interface. The present paper investigates the influence of several surface parameters to the tendency to galling. This is done by forming aluminium using tool steel in a geometrically simplified lab test. The test scans over a wide load interval while monitoring the coefficient of friction. The corresponding transfer of work material to the tool surface is studied in the SEM after testing. The test is focused on the initial tool contact and also on the number of contacts before a critical friction level is reached. The test setup comprises two crossed cylinders in sliding contact, one made of tool steel and one of work material. Three commercial tool materials were included, each prepared to two surface finishes. The aluminium workpiece cylinders of AA6082 were prepared by two different pre-treatments, solid lubrication followed by soft annealing and pickling, respectively. The respective importance of the tool material, preparation of the tool surface finish and the aluminium surface pre-treatment are compared with respect to initial galling tendencies and friction stability. The practical implications for real forming applications are discussed.

Journal of Materials Processing Technology, 2006

This paper reports an investigation on the tribological properties of Al thin foils (0.2 mm nominal thickness) in sliding contact with PVD-coated carbide forming-tools, with reference to the applications in the manufacture of miniature/micro-products, particularly through micro-forming. The study addresses the evolution of the friction coefficient between both surfaces under dry lubrication conditions, its influence on the surface quality of the processed material and the damage produced on the tool due to sticking phenomena. Uncoated, CrN-coated and WC-C-coated tools were tested, using a pin-on-disc configuration to monitor the evolution of the relative friction coefficients under different contact pressures. It was shown that the contact sliding of Al foils on the untreated carbide tool produced a significant transfer of Al to the tool, leading to a rapid degradation of both tool and foil surfaces, even faster than that produced on bulk aluminium. Conversely, the sticking of Al was retarded using low friction magnetron sputtered WC-C-coated carbides. Other PVD coatings such as CrN failed under similar operation conditions. Aluminium transfer to the WC-C-coated tool surfaces during sliding seemed to be a non-continuous process, which appeared after certain sliding distances. Temperature accelerated the Al transfer to the WC-C tool surfaces, probably due to the increase of the thermoplastic behaviour of Al above 100 • C, which led to an increase of the adhesive interaction with the WC-C coating.

International Journal of Material Forming, 2010

Semi-solid processing of metals combines the advantages of forging and casting for the shaping of metallic components. Having already matured into an industrial practice for Al and Mg alloys, this innovative forming technology could upgrade the market and provide lightweighting for forged steel parts. However, with process temperatures above 1250°C, the surface-to-interior temperature differentials in steel thixoforming dies are much larger than with Al and Mg. Combined with the erosive wear caused by abrasion and impact of the already solid particles in the slurries and high temperature oxidation of the die surface, these cyclic thermal stresses confer very specific requirements on tool materials. Hot work tool steel dies proved to be entirely inadequate when thixoforming steels, due basically to the limited temper resistance of the commercially available grades. Physical Vapour Deposition (PVD) hard coatings, shown to have beneficial impact on the lifetime of pressure die casting tools, could offer a solution. X32CrMoV33 hot work tool steel widely used in the manufacture of conventional forging dies was coated with PVD AlTiN and AlTiON in the present work. The performance of the coated samples was subsequently tested under thermal fatigue conditions encountered in the thixoforming of steels.

Friction, 2020

The use of high strength aluminium alloys, such as 6XXX and 7XXX series, is continuously increasing for automotive applications in view of their good strength-to-weight ratio. Their formability at room temperature is limited and they are thus often formed at high temperatures to enable production of complex geometries. Critical challenges during hot forming of aluminium are the occurrence of severe adhesion and material transfer onto the forming tools. This negatively affects the tool life and the quality of the produced parts. In general, the main mechanisms involved in the occurrence of material transfer of aluminium alloys at high temperature are still not clearly understood. Therefore, this study is focussed on understanding of the friction and wear behaviour during interaction of Al6016 alloy and three different tool steels in as-received and polished state. The tribotests were carried out under dry and lubricated conditions, with two distinct lubricants, using a reciprocating friction and wear tester. The worn surfaces were analysed using scanning electron microscopy (SEM) and energy dispersive X-ray spectroscopy (EDS). The results showed a high dependence of friction and wear behaviour on the tool steel roughness as well as on the stability of the lubricant films. Tribolayers were found to develop in the contact zone and their capacity to improve the tribological behaviour is seen to be drastically impacted by the surface roughness of the tool steel. When the tribolayers failed, severe adhesion took place and led to high and unstable friction as well as material transfer to the tool steel.

International Journal of Surface Science and Engineering, 2010

This paper presents the effects of surface coating on the wearing resistance of cold forging tools used for manufacturing fixing parts. The wear of test specimens was analysed with and without coating. The titanium carbonitride, titanium nitride and chrome nitride coating were obtained by Physical Vapour Deposition. The coatings were also tested for two different kinds of tools: a trimming die and an extrusion die used in manufacture of fixing parts. The wearing behaviour was observed at industrial scale during the manufacturing process. The best wearing resistance was found for titanium carbonitride coating in both laboratory and industrial scale.

Wear, 2006

The aim of the present work was to investigate and compare different hard coatings as to the tendency for work material adhesion and galling properties when applied on forming tool steel and sliding against different work materials. The surface coatings included were PVD deposited TiN, TiB 2 , VN, TaC and DLC coatings. They were all applied to cold work tool steel. Tribological evaluation was carried out in a load-scanning test rig, with the normal load being gradually increased during each test from 100 to 1300 N (1-3.5 GPa). The coated steel was tested against austenitic stainless steel and alloys of aluminium and titanium.