STRUCTURE AND PROPERTIES OF HYBRID COATINGS DEPOSITED ON REVERSIBLE ROTATING EXTRUSION DIES

Krzysztof Lukaszkowicz1, Jozef Sondor2, Paweł Ostachowski3
1Institute of Materials and Biomaterials Engineering, Silesian University of Technology, Konarskiego St. 18A, 44-100 Gliwice, Poland, Tel.: +48322371245, e-mail: krzysztof.lukaszkowicz@polsl.pl
2LISS, a.s., Dopravni 2603, 756-61 Roznov p.R., Czech Republic, Tel.: +420571842693, e-mail: j.sondor@liss.cz
3 Department of Structure and Mechanics of Solids, AGH University of Science and Technology, al. Mickiewicza 30, 30-590 Krakow, Poland

Abstract
The paper presents the research results on the structure, mechanical and tribological properties of the coating system deposited by PVD ARC-cathodes, PVD sputtering, and PACVD technology respectively on the dies made from the X40CrMoV5-1 hot work tool steel. The CrAlSiN+DLC, AlTiCrN+DLC, CrN+DLC and CrAlSiN+MoS2 coatings were investigated. It was most significant to compare the life of dies deposited with coatings in operating conditions. Studies into the kinetics of KOBO extrusion (with reversible rotating dies) and into the impact of the process conditions on the life of dies and mechanical properties of the products fabricated, have become reasons for selecting the parameters of the experiments carried out for the purpose of this paper. Tests were undertaken, though, in extremely difficult conditions unmet (but likely to arise) in industrial practice and in laboratory tests as a result of using high extrusion speeds and large degrees of processing. An EN AW-7075 alloy belonging to the group of hard-deformable material was used for extrusion test. A 3-fold increase in service life as compared to dies used normally in extrusion processes, i.e. quenched and tempered and nitrided was found during the experiments held for dies with CrAlSiN+DLC and AlTiCrN+DLC coatings produced on their surface. A surface quality of the extruded alloy was very good.

INTRODUCTION
Plastic working is one of those industries undergoing most intensive growth. The dynamic development of theory and manufacturing technologies has been seen in this field. It is now commonly thought that the most economical method of plastic working is extrusion [1]. It is essential in material engineering to produce materials with a nanocrystalline structure ensuring favourable mechanical and functional properties of products. Numerous methods have been developed for metals leading to a strong refinement of grains as a consequence of intensive plastic deformation. The methods are jointly referred to as Severe Plastic Deformation (SPD) [2]. One of the most innovative methods of plastic working is the KOBO technology the idea of which is based on a cyclic change of a metal deformation path by introducing an additional, reverse, cyclically changing influence of working tools on metal and enables both, forging, rolling, drawing, and extrusion [3, 4]. Tribological wear occurring in extrusion, particularly at a higher temperature, is very complex and usually encompasses the processes of friction, abrasive wear, abrasive-adhesion wear, adhesion wear, thermal wear and deformation. They do not occur individually in the majority of cases, and - by creating hybrid mechanisms - affect the tool surface aggregately [5]. As some metals and their alloys, especially aluminium, copper, titanium and platinum are strongly susceptible to adhering to steel surfaces of dies, a service life of dies is considerably shortened stemming from adhesion or abrasive-adhesion wear. As a result of high pressures, local joints and adhesions between contacting surfaces lead to the displacement of the material particles within the material volume, and not directly on the surface during their relative movement. A material loss in a die's working section is possible as a result of detaching the joints. Materials' susceptibility to tacking increases at a higher temperature, and surface seizing is observed in exceptionally severe working conditions caused by a sudden increase in the resistance of sliding friction, which may result from the system's lost thermal and mechanical balance.

A turning point for improving the operating properties of dies was the introduction of their surface modification methods, in particular through thermochemical treatment [6], low-temperature physical vapour deposition (PVD) of surface layers [7], or using hybrid technologies being a combination of the aforementioned methods [8]. Its limitations should, however, be pointed out when underlining an incontestable value of this approach, linked to the fact that nitrided layers produced on dies do not ensure suitably long service life and results repeatability. It also becomes a general issue that adhesion of the extruded material to the die cannot be prevented.

EXPERIMENTAL PROCEDURE
A material for the research consisted of specimens dimensioned 305 mm and extrusion dies used in the KOBO method made of hot-work X40CrMoV5-1 tool steel deposited with hard nanocrystalline and low-friction layers produced with the PVD and CVD technique. Prior to the coatings production process, the specimens were ground and polished to obtain the roughness of Ra≤0.03 μm, and then washed in organic solvents and alkali detergent solutions, including the use of ultrasound aid. The so prepared specimens were placed into a work chamber of a coating deposition device.

The production process of hybrid two-layer coatings of the hard nitride layer – low-friction layer DLC type was performed with a π80 and π300+DLC unit by PLATIT. A MoS2 layer was produced in a reactive magnetron sputtering process with a PL200 device by PLATIT.

The assumed shape of dies for testing coatings' properties in operational conditions – extrusion with the KOBO method – is shown in Fig. 1.

(a) 

(b) 

Fig. 1. KOBO method extrusion dies: a) general view; b) front view

The purpose of a numerical analysis was to determine a distribution of stresses and strains occurring in a die and of the layers produced on its surface during extrusion with a reversely rotating die. The analysis was performed with ANSYS 12.1 software. A model used for an MES analysis consisted of the following geometric elements: extruded material, low-friction layer, nitride layer, forming die. Such model geometry adopted resulted from the method of discretisation, type of finite elements and a need to reduce its number. A die calculation model with coatings applied and the extruded material consists of 3829 nodes and 16214 elements. Non-linear properties of the extruded material were used due to a non-linear character of the process. Frictional contact of 0.1 was assumed at the contact surface of the extruded material (hard workable EN AW-7075 aluminum alloy) and a low-friction layer.

Service life tests of dies for plastic working of non-ferrous metals with coatings deposited thereon in working conditions – the extrusion of rods made of selected metals and non-ferrous metal alloys (aluminium with commercial purity of 99.5%, copper alloy with phosphorus Cu6.5P, aluminium alloy EN AW-7075) were carried out on a hydraulic press with a nominal pressure of 1 MN with the KOBO method. The resistance of coatings to tribological wear and mechanical properties, dimensional accuracy and surface quality of the rods produced in extrusion were taken into account in particular. The extrusion process implementation diagram with a reversely rotating die (KOBO method) is shown in Fig. 2.


Fig. 2. Extrusion diagram with reversely rotating die (KOBO method): 1 – extruded material, 2 – recipient, 3 – punch, 4 – reversely rotating die


The fractographic tests of coatings were made on transverse fractures in a scanning electron microscope SUPRA 35 by ZEISS. Diffraction investigations and coating structure investigations were conducted using a scanning-electron microscope (S/TEM) Titan 80-300 by FEI. The adhesion of the coatings to the substrate material was evaluated by a scratch test.

RESULTS AND DISCUSSION
The distribution of stresses in a die was determined in the first stage, within a nitride and low-friction layer formed on its surface, as well as in the deformed material (EN AW-7075 aluminium alloy) during extrusion with the KOBO method with a reversely rotating die. A value of stresses reduced acc. to the Huber-Mises hypothesis in the places with the highest concentration of the analysed system elements was determined (Fig. 3). An analysis of the results obtained reveals that the maximum value of reduced stresses initiated with the activity of external forces during extrusion for an external low-friction layer is 700 MPa, while for a hard nitride layer – 2600 MPa. The maximum values of stresses for the both cases were situated at the edges of grooves made on the front die surface, the purpose of which is to increase deformation intensity (shearing, torsion) of the extruded material by transmitting a torque moment from a die onto the material.

(a)

(b)

(c)

(d)

Fig. 3. Distribution of reduced stresses: a) in the extruded material, b) in a die, c) in a hard internal layer, d) in an external low-friction layer
The maximum value of reduced stresses for the analysed die made of hot-work X40CrMoV5-1 tool steel used in the EN AW-7075 alloy extrusion process with the KOBO methods spanned 1100-1200 MPa. Similar to the analysed layers, the maximum values of stresses were situated at the peripheries of grooves made on its front surface. A yield strength value for typical grades of steels used for extruding dies indicates that a die material's plastic deformation up to the temperature of 300-400°C is impossible due to acceptable stress values initiated during extrusion, not exceeding a yield point value for steels used for dies. A substantial plastic deformation occurs during cold extrusion with the KOBO method accompanied by a charge's temperature increase up to ca. 300°C which does not affect strength properties of the steel the die was made of, either. Nevertheless, a charge's temperature grows above 400°C during hot extrusion. However, a small heat conductivity coefficient of the external layers applied and their high oxidisation resistance at increased temperature are effectively reducing the impact of external temperature on the die material.

Investigations into the service life of dies for plastic working of non-ferrous metals with coatings deposited on their surface were carried out during extrusion with a reversely rotating die (KOBO) due to extremely hard working conditions. It was found as a result of preliminary investigations that the life of dies coated with monolayer nanocrystalline coatings grows only to a small extent as compared to dies used currently in the extrusion process, worked conventionally as a result of thermal or thermochemical treatment. An extruded material's susceptibility to sticking to the die surface was a major problem for monolayer nitride coatings. However, the results attained for two-layer coatings (with an additionally deposited low-friction layer) have shown it is appropriate to conduct additional experiments. An EN AW-7075 aluminium alloy was subjected to extruding because of its hard formability.

Investigations for dies used traditionally in the extrusion process were additionally undertaken for comparative reasons, whose functional properties are enhanced in thermal treatment (quenching and high tempering) of thermochemical treatment (nitriding). The first stage was EN AW-7075 aluminium alloy extrusion with constant die rotation frequency for a quantitative assessment of the dies’ service life. The following process conditions were established: constant extrusion speed of υ=0.5 mm/s; constant charge geometry of 40x40 mm; constant processing ratio of λ=100 – with 4 mm product diameter; constant die rotation frequency of f=5 Hz; constant torsional angle γ=±8°; constant start charge and recipient temperature of 24°C; variable extrusion force. The impact of temperature, speed and force of extrusion on the wear of coatings was evaluated in the second stage. The following process conditions were established: constant charge geometry of 40x40 mm; constant processing ratio of λ=44.4 – with 6 mm product diameter; constant extrusion force of approx. 1 MN; constant torsional angle of γ=±8°; constant start charge and recipient temperature of 400°C; free product cooling in air; constant die rotation frequency of f=5 Hz; variable die rotation frequency υ to maintain constant force.

An extrusion process with the KOBO method at constant speed and dies rotation frequency was carried out to assess quantitatively a service life of dies. Operational tests held in laboratory conditions enabled to compare the life of dies coated with the layers produced and classic dies subjected to a nitriding or quenching and tempering process only. The extrusion process can be interrupted by even a small recess or damage of the deposited protection coating or nitride layer and produce a product of an unacceptable quality. Hence a surface condition of the extruded product and wear of the front surface and a device calibrating strip were used as evaluation criteria for a die's service life. Table 1 presents a die in the initial condition after 5 extrusion tests and after 10 tests completed (optionally after fewer tests due to die wear according to the service life evaluation criteria applied).



The surface of dies, in grooves and at peripheries, is gradually wearing in subsequent tests for classical quenched and tempered and nitrided dies during an extrusion process. Quenched and tempered and nitrided dies are wearing very quickly (4th pass). This substantially influences a final product's quality. The dies with CrN+DLC and CrAlSiN+MoS2 coatings deposited onto their surface exhibit a twice higher service life. Extrusion force decreases over the subsequent passes, and the gradual wear of coatings becomes visible, especially at the peripheries, front surface and dies’ grooves. Numerous grooves and scratches are visible on the surface of the extruded material manifesting that the calibrating band is not smooth (7th pass). A 3-fold increase in service life as compared to dies used normally in extrusion processes, i.e. quenched and tempered and nitrided, was found for dies with CrAlSiN+DLC and AlTiCrN+DLC coatings produced on their surface. A surface quality of the extruded alloy is very good. Fine scratches on a product surface appear only during the 10th pass as the surface of dies is wearing and as an alloy sticks to a calibrating band. Test with a temperature of 400C with a constant horizontal force of 1 MN were carried out to appraise the effect of temperature, speed and force of extrusion on the wear of coatings. Dies with the eye diameter of 6 mm (processing ratio of λ=44.4) were used to increase speed controllability. The tests performed at this stage point out that a CrN+DLC coating does not exhibit ant signs of wear. This can be caused by the lowest heat conductivity of the CrN layer for all the analysed nanocrystalline nitride layers produced in a PVD process. The signs of wear were noticed on the front surface for other coatings. A die with a CrAlSiN+DLC coating is characterised by hindered process initiation and a very high final extrusion force. An extrusion process with the KOBO method at a higher temperature is characterised, for an AlTiCrN+DLC coating, by the smallest final extrusion force and very high punch transfer speed. Wires for extrusion in all the examined cases were characterised by a good surface quality only in the initial stage of extruding. When the process is held at a temperature of 400C with final speeds over 3 mm/s, the end pieces of wire break as a result.

The fractographic tests made with the electron scanning microscope (Fig. 4) allow asserting that the tested coatings, depending on the applied system of layers, indicate a double-layer structure consisting of a hard nitride layer and a low-friction layer. It was also found that a chromium-based transition layer exists well bound with a substrate that was fabricated to improve the coatings’ adhesion to a hot-work tool steel substrate. The individual layers are deposited uniformly and tightly adhere to each other and to the substrate material. Morphology of the surface of fractures in the tested coatings is characterized by a compact structure.


Fig. 4. Fracture image of: a) AlTiCrN/DLC, b) CrAlSiN/DLC coating deposited onto the X40CrMoV5-1 substrate

Tests were carried out, using the transmission electron microscope, in order to determine the microstructure and size of crystallites in the layers produced and to examine the character of transition zones between the substrate and the coating, as well as between the individual layers in the coatings. The size and shape of grains in the deposited layers was determined using the dark field technique and based on electron diffractions obtained signifying an amorphous or nanocrystalline structure of the analysed layers. The results of the tests obtained using the transmission electron microscopy confirmed the amorphous character of a low-friction DLC layer. The electron diffraction patterns obtained have shown the considerable broadening of diffraction rings (Fig. 5a). It was found by examining thin lamellas from the cross section of CrAlSiN layer produced by PVD technique that the layer features a compact structure with high homogeneity and a grain size is less than 10 nm (Fig. 5b). It can be concluded already based on TEM images in the bright field that the layers have a nanocrystalline structure. Dark areas appearing on the light field image are crystallites that are oriented close to the axis of bands relative to an electron beam. Observations in the dark field and the diffraction images made for increasingly smaller areas confirm a nanocrystalline structure of the examined nitride layers.

The critical load LC1 and LC2 values were determined with a scratch test with a growing load allowing to determine the values of the force causing coating damages. An aggregate list of the tests’ results is presented in Tab. 2. The highest values of the critical load LC1 and LC2 account for, respectively, 36 and 76 N, and, therefore, the best coating adhesion to the substrate was achieved for CrAlSiN+DLC coatings. The other critical load values measured, signifying coating adhesion to the substrate, do not exceed 70 N.





Fig. 5. Microstructure of the: a) DLC, b) CrAlSiN layer with corresponding SAED pattern

SUMMARY
The service and functional life of dies produced of hot-work X40CrMoV5-1 tool steel can be extended by modifying their surface layer with the vapour deposition technique, as proven in the framework of the comprehensive own research. A 3-fold increase in service life as compared to dies used normally in extrusion processes, i.e. quenched and tempered and nitrided was found during the experiments held for dies with CrAlSiN+DLC and AlTiCrN+DLC coatings produced on their surface. A surface quality of the extruded alloy is very good. The observations made indicated that abrasive wear and abrasive-adhesion wear were the main forms of the analysed dies' destruction.

The factors decisive for the suitability of the proposed coatings modifying the working surfaces of dies are, to a large extent, the chemical composition and structure of such coatings. Tests using the transmission electron microscopy confirmed an amorphous character of a low-friction DLC and MoS2 layers. With regard to the layers formed with the PVD technique, the size and shape of grains was determined based on the structure obtained using the dark field technique and based on electron diffractions obtained signifying a nanocrystalline structure of the analysed layers and a grain size between 5 to 10 nm. Small, crystalline grains sized several nanometres deposited in an amorphous Si3N4 matrix were observed for a CrAlSiN layer, which may signify the layer’s nanocomposite structure.

The improved adhesion of coatings to a substrate material and an extended life of dies during operation was ensured by creating layers on the surface of tools for plastic working of non-ferrous metals in the extrusion process meeting the accurately defined functions and at the same time forming correctly a joint zone between the substrate and the coating, as well as between the individual layers in coatings and an advantageous distribution of stresses.

REFERENCES
[1] P.K. Saha, Aluminium extrusion technology, ASM International, Materials Park, Ohio, 2000.
[2] R.Z. Valiev, Y. Estrin, Z. Horita, T. G. Langdom, M.J. Zehetbauer, Y.T. Zhu, Producing bulk ultrafine-grained materials by Severe Plastic Deformation, JOM 58 (2006) 33-39.
[3] W. Bochniak, A. Korbel, KOBO type forming: forging of metals under complex conditions of the process, Journal of Materials Processing Technology 134 (2003) 120-134.
[4] W. Bochniak, Teoretyczne i praktyczne aspekty plastycznego kształtowania metali, Wydawnictwa AGH, Kraków, 2009.
[5] M. Pellizzari, High temperature wear and friction behaviour of nitrided, PVD-duplex and CVD coated tool steel against 6082 Al alloy, Wear 271 (2011) 2089-2099.
[6] M. Tercejl, A. Smolej, P. Fajfar, R. Turk, Laboratory assessment of wear on nitrided surfaces of dies for hot extrusion of aluminium, Tribology International 40 (2007) 374-384.
[7] T. Bjork, M. Berger, R. Westergard, S. Hogmark, J. Bergstrom, New physical vapour deposition coatings applied to extrusion dies, Surface and Coatings Technology 146-147 (2001) 33-41.
[8] J. Smolik, A. Mazurkiewicz, Rozwój hybrydowych technologii powierzchniowych w oparciu o praktyczne zastosowania przemysłowe, Problemy Eksploatacji 3 (2010) 105-114.
 

Fig. 1. KOBO method extrusion dies: a) general view


Fig. 1. KOBO method extrusion dies: a) front view


Fig. 2. Extrusion diagram with reversely rotating die (KOBO method): 1 – extruded material, 2 – recipient, 3 – punch, 4 – reversely rotating die


Fig. 3. Distribution of reduced stresses: a) in the extruded material, b) in a die, c) in a hard internal layer, d) in an external low-friction layer


Fig. 4. Fracture image of: a) AlTiCrN/DLC, b) CrAlSiN/DLC coating deposited onto the X40CrMoV5-1 substrate


Fig. 5. Microstructure of the: a) DLC, b) CrAlSiN layer with corresponding SAED pattern



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