Jens Bergström

Tribological simulation of aluminium hot extrusion

Abstract Hot extrusion is a very cost efficient method of aluminium forming. One limitation for further improvement of cost reduction is wear of the bearing surface on the die which deteriorates the dimensional tolerance and surface quality of the profile. Nitrided hot work tool steels are commonly used as die material but new surface treatments are being introduced. This paper describes a new block on disc test machine and a corresponding methodology for simulating tribological interactions on the die bearing surface. The applicability of the test is demonstrated in a comparison between traditional nitriding and a CVD coating by TiC+TiN. The same kind of wear mechanisms were found on actual dies as well as on tests specimens. Thus, the new test method proved to simulate realistic extrusion conditions. Chemical wear dominates the deterioration for the two surface treatments. Both in actual extrusion and in the laboratory tests, the CVD coating had a superior resistance to wear compared to the nitrided steel.

Physical vapour deposition duplex coatings for aluminium extrusion dies

Abstract Duplex coatings [plasma nitriding followed by physical vapour deposition (PVD) coating] have been given a lot of interest as surface treatment for forming and cutting tools in recent years. The advantage, as compared to conventional PVD coating, is a higher substrate hardness due to the nitriding prior to the PVD coating. The present investigation aimed at studying the effect of duplex treatment as compared to conventional PVD coating and nitriding, respectively, for aluminium extrusion dies. The tested surface conditions were CrN, TiN and TiAlN, all single-layered and duplex-treated. A salt-bath-nitrided specimen was investigated for comparison and reference purposes. Wear tests were undertaken using an extrusion simulation device. Prior to wear testing, coating properties were evaluated by scratch testing, surface roughness, hardness and residual stress measurements. Wear tests indicated that a PVD coating reduced the wear rate by about one order of magnitude as compared with the salt-bath-nitrided specimen. Duplex treatment resulted in a further reduction of wear rate by two to four orders of magnitude. TiAlN was shown to be the most wear-resistant of the tested coatings, both as a duplex coating and as a single layer. In addition, a field study with CrN duplex-treated extrusion dies was performed. The tools were worn by delamination of the coating followed by severe wear of the exposed substrate.

Influence of deposition temperature and time during PVD coating of CrN on corrosive wear in liquid aluminium

Erosive and corrosive wear are two major life-limiting factors in die casting dies. To resist the corrosive and erosive attack from molten metal flow the tool surface needs to be hard and chemically inert. It has been indicated that local coating defects rather than intrinsic deficiencies limit the potential gain of coatings on hot work tool steel exposed to liquid aluminium. This motivates a search for suitable protective coatings on the dies. A number of ceramic coatings are of interest. In this work, physically vapour deposited (PVD) CrN coatings were applied on hot work tool steel specimens and treated in an aluminium melt. Substrate temperature and deposition time were varied to give coatings with individual properties. Type and density of defects as well as thickness of the coatings were characterised for all coatings. The influence of substrate temperature and deposition time on coating characteristics and corrosion resistance was studied as well as the mechanism of corrosion damage. It is clearly demonstrated that liquid aluminium corrosion of CrN-coated tool steel is initiated at defects which penetrate through the coating, and localised corrosion pits are formed. Subsequently, the pits coalesce and the corrosive attack aggravates. Consequently, the corrosion resistance is improved by reducing the density of defects through the coating, which, for PVD CrN, was achieved by increasing the coating thickness.

New physical vapour deposition coatings applied to extrusion dies

Abstract The bearing surface of dies for extrusion of aluminium is subjected to very difficult wear conditions. To minimize wear and thereby obtaining a longer service life, these dies are typically made of hot work tool steel and surface treated by nitriding. In the present study both commercial and experimental physical vapour deposition (PVD) coatings are evaluated in a device which simulates the bearing wear in an extrusion die. The tested surfaces include hot work tool steel, coated with TiN, CrN, (Ti,Al)N and TiB 2 , respectively. To simulate the extruded material an aluminium cylinder heated to 550°C is used as counter body in a block-on-ring configuration. For comparison, the tool steel was also tested in uncoated conditions. Prior to wear testing, the test surfaces were characterized by roughness, hardness, thickness and intrinsic stress. It is shown that coatings of CrN, and especially (Ti,Al)N and TiB 2 have the potential to effectively reduce the wear in extrusion dies. These coatings are mechanically, thermally and chemically stable when sliding against hot aluminium, in contrast with uncoated steel. It was also observed that the mechanical strength of the TiN coating gradually degenerated during this test, probably due to poor chemical inertness when exposed to hot aluminium.

Wear of die materials in full scale plastic injection moulding of glass fibre reinforced polycarbonate

In this study an experimental methodology to study surface wear of injection moulds on a full scale polymer injection moulding machine is developed. A comparative study of four different tool materials for mould cavities is carried out in situ. The polymer injection conditions (geometry cavities, machine settings, polymer) are chosen in order to accelerate the degradation of the mould cavity surface. The reinforced polycarbonate resin Lexan 341R-739, containing 40% of weight of short glass fibres, well known for their abrasive character, was injected with jetting and normal injection conditions. Jetting conditions implies high pressure and velocity of the injected resin. The die wear is discussed in relation to the polymer flow conditions and the die materials.

A comparative study on the evaluation of the tribological behaviour of polymer/zinc coated steel sheets

Abstract A major factor affecting the formability of coated automative steel sheets is the interfacial frictional behaviour between the sheet and the forming die. Within this study, the suitability of standard laboratory-type tribological tests (pin-on-disc and block-on-cylinder test) to evaluate the tribological behaviour of uncoated and zinc and zinc/polymer coated steel sheets in dry sliding contact with cold working steel is investigated. The results obtained are compared to those obtained by a deep drawing experiment. Friction coefficients measured during the running-in stage of the pin-on-disc test seem to be well suited to distinguish between the different frictional behaviour of coated steel sheets.

Study of the Influence of Contact Geometry and Contact Pressure on Sliding Distance to Galling in the Slider-on-Flat-Surface Wear Tester

One of the major causes of tool failure in sheet metal forming is wear in the form of galling. Galling is gradual buildup of adhered sheet material on the tool and leads to unacceptable scratches on the sheet surface and to components that fail to meet tolerances. Because it is difficult to reproduce operational and interactional conditions in laboratory test equipments it is hard to test, model, and predict galling initiation. Here the authors examine how changes from elliptical to line contact geometry influenced galling initiation under dry sliding by using a slider-on-flat surface (SOFS) wear tester. A micro clean tool steel was tested against ferritic low-strength and martensitic high-strength steel sheets. The sliding distance to galling initiation was extracted from friction data and verified by scanning electron microscopy (SEM) observations. The presence of adhesive wear on worn tools after completed tests was used as a criterion. Experimental results showed that the elliptical contact causes galling quicker than the line contact. Applicability of experimental results depends on the relevance of test conditions, so contact pressures calculated for the described tests were compared to calculated contact pressures in a semi-industrial U-bending test and to literature data relevant to industrial applications. Good agreement between values observed for SOFS and for most selected industrial applications was found, which assume that contact pressures typical for most common industrial applications can be successfully simulated by selection of tool geometry and normal load in the SOFS tester.

Fatigue and Microstructure of Iron Based Sintered Alloys

Abstract Direct metal laser sintering is a relatively new fabrication technique for near net shaped parts. It is for instance extensively used for die parts when making industrial forming tools. Another, yet more unexplored process is the starch consolidation method using environmentally friendly starch and water to shape components. A powder suspension is transformed into a green body by starch swelling and subsequent drying with sufficient strength to be handled and high vacuum liquid phase sintered. Here, it is used as a way to form high speed steels. Both methods are aiming to enable near net shaping of components subjected to wear, fatigue or thermal loads. Results presenting microstructure and mechanical properties are presented. In particular is fatigue strength and short crack growth in multiphase and low porosity sintered metal treated. The fatigue behaviour was shown to be related to the interaction of crack growth and complex layered structures, where crack initiation, growth and path are depending on porosity, microstructural barriers and crack linkage. Crack growth acceleration and retardation interacting with major barriers was a main feature in the bending fatigue tests.

Detecting Large Inclusions in Steels: Evaluating Methods

The distributions of large non-metallic inclusions in two steel grades have been investigated using light optical microscopy, scanning electron microscopy and ultrasonic fatigue testing in the giga ...

Finite Element Simulation of the Tool Steel Stress Response As Used In a Hot Forging

Hot‐work forging tools are subjected to severe and complex loading conditions (cyclic stress/strain conditions, varying strain rates, varying temperature, environmental damage), leading to short die life. There is also a temperature and time dependant material response to consider. Presently, hot‐forging of a steel crankshaft is approached by a combined experimental and numerical simulation study of a hot‐work tool steel.The forging conditions were investigated by thermal measurements and damage analysis. Laboratory testing by isothermal fatigue in the temperature range 200 – 600 °C was performed. An elasto‐plastic kinematic and isotropic hardening model (according to Chaboche) was used to model the material behaviour where the material parameters were derived from the experimental part. Viscoplastic effects are not accounted for in the material model as they have minor influence on the stress‐strain relation in the experimental setup. The model was implemented in a FEM program (MSC.Marc) using a semi‐imp...