A. Dubois

Identification of Coulomb's friction coefficient in real contact conditions applied to a wire drawing process

Abstract Coulombs friction coefficient in cold wire drawing is determined via new upsetting sliding test procedure which takes into account the bulk properties in the contact surface vicinity of both coating and substrate. The test is performed on the real workpiece directly from the drawing plant, thereby the physical and chemical properties of the interface are respected. The test parameters are defined from two simple graphs in order to simulate as well the effective local strain and the contact pressure of the studied forming process. Thus, both the mechanical contact conditions and the associated mechanisms of adhesion or separation and fracture are representative. During the test both the normal and the tangential forces are recorded and the mechanical analysis of the contact area easily enables the calculation of the mean Coulombs friction coefficient. This result is directly usable for the finite element simulation of the drawing process and leads to a very good agreement in regards to the experimental drawing force. Moreover, the analysis of the deformed part of the tested workpiece enables the selection of lubricant coatings and also prevention of some eventual defects.

A selection methodology for lubricating oils in cold metal forming processes

A selection methodology for the evaluation of lubricating oil performances has been developed in order to help engineers in the definition of the right lubrication strategy. The methodology is exposed through a cold forward extrusion process with a 75% reduction in area. Specific friction tests are performed according to the physical and mechanical contact conditions characterising the extrusion process. Four lubricating oils available on the market are investigated. Three criteria are proposed to characterise the performances: the friction coefficient, the occurrence of two types of surface defects, and the resulting roughness of the specimen. The tribological behaviours are related to the physicochemical characteristics of lubricating oils. Moreover, the reaction mechanisms of the different additives and the influences on the regimes of lubrication are discussed. Finally, the optimum lubricating oil with regards to the forward extrusion process is selected.

The effects of processing bath parameters on the quality and performance of zinc phosphate stearate coatings

Abstract In cold metal-forging processes, low-friction coatings often provide the main part of the lubrication. In many cases, zinc phosphate stearate (ZPS) coatings are used. Many parameters affect the coatings quality and performance. In this paper the typical Brinell indentation test is used to answer two industrial requirements: first, to reveal the sensitivity of the coating with regard to the ageing of the processing baths and phosphating time and, second, to make clear the correlation between the bulk properties of the coated surface and the tribological performance of the ZPS coatings. It is shown that a single configuration of the Brinell indentation test and a statistical analysis of the measurements provides answers to these questions.

Three selection criteria for the cold metal forming lubricating oils containing extreme pressure agents

Abstract The performances of four commercial lubricating oils containing extreme pressure agents were investigated using the Upsetting Sliding Test (UST). This friction test takes account of the mechanical (contact pressure and sliding velocity) as well as the physical contact conditions (materials, roughness and temperature) which characterise cold forming processes such as drawing or extrusion. In this study the parameters of the UST are adjusted to simulate a forward extrusion process with a 75% reduction in the area. Three selection criteria are proposed: (a) the friction coefficient; (b) the occurrence of defects (scratches and cracks); and (c) the resulting roughness. It is shown that these criteria indicate the ability of the lubricant to reduce friction stresses, delay the forming limits, maintain boundary or mixed lubrication regimes and improve the surface finish.

A new methodology to analyse iron fines during steel cold rolling processes

Abstract To optimise cold rolling process by numerical simulations, tribology, rheology and boundary conditions are needed. Thus, a methodology for accurate identification of contact conditions during cold rolling has been developed. It involves the upsetting rolling test (URT). During this specific test, the work roll slips against the strip and generates a plastic zone. The adjustment of the test is defined to simulate as well-contact pressure and effective plastic strain observed during the forming process. The mean Coulomb’s friction coefficient calculated from the URT analysis leads to a very good agreement between experimental and numerical results. Furthermore, it allows the updating of the contact conditions on numerical simulation. In addition, the developed methodology and the specific device (URT) have been used to demonstrate the mechanism of iron residues during cold rolling. We demonstrate the influence of rolling parameters in the iron residue formation. The conclusions could be directly applied on the industrial process and so improve the strip cleanliness.

A multi-step lubricant evaluation strategy for wire drawing–extrusion–cold heading sequence

The upsetting-sliding test was used to simulate the characteristic contact conditions of a cold heading sequence comprising two operations, wire drawing and direct extrusion. Friction conditions for each elementary forging operation were analysed according to the lubrication parameters, such as ageing of the soaping phosphate line, immersion time in the phosphating bath, and additional lubricants (drawing powder and cold heading oil). The results enabled: (i) better knowledge and control of the changing friction conditions as the forging procedures progress; (ii) predicting and remedying lubrication damage, and (iii) anticipating the geometric variations of the forged product according to the friction conditions.

Determination of a hardening behaviour law for a cold forging TiN-coated tool steel

Abstract Damage occurrence has been industrially observed in cold forging tooling even though stresses remain lower than the yield point derived from a conventional rigid perfectly plastic law. A hardening stress–strain law has therefore been determined for the TiN-coated AISI M2 tool steel to improve the knowledge of the tool behaviour. The methodology used to establish the hardening law is based on the Brinell indentation test, which is simulated under increasing load. For each increment of the load, the parameters of the behaviour law are adjusted in order to make the numerical diameter and the experimental results of the Brinell test converge. In parallel, scanning electron microscope observations on the Brinell indentations show the presence of cracks at high loads, justifying the slight decay of the behaviour law towards the non-coated specimen behaviour one. Finally, this behaviour law is applied to an industrial process showing plastic deformation on the surface of the tool.

Friction and wear in hot forging of steels

In the field of hot forging of steels, the mastering of wear phenomena enables to save cost production, especially concerning tools. Surfaces of tools are protected thanks to graphite. The existing lubrication processes are not very well known: amount and quality of lubricant, lubrication techniques have to be strongly optimized to delay wear phenomena occurrence. This optimization is linked with hot forging processes, the lubricant layers must be tested according to representative friction conditions. This paper presents the first part of a global study focused on wear phenomena encountered in hot forging of steels. The goal is the identification of reliable parameters, in order to bring knowledge and models of wear. A prototype testing stand developed in the authors’ laboratory is involved in this experimental analysis. This test is called Warm and Hot Upsetting Sliding Test (WHUST). The stand is composed of a heating induction system and a servo‐hydraulic system. Workpieces taken from production can be...

Quantitative Evolution of WC-Co Cemented Carbide Tool Surface during the Cold Forming of Steel Parts

The present paper investigates the evolution of the WC grain size and morphology with the number of produced parts. Four reduction dies used in cold forward extrusion of steel are taken from the production line: a brand new die and three dies which have respectively produced 100.000, 150.000 and 220.000 parts. 3D roughness measurements and SEM micrographs are performed on the contact surface of each die. Results of these analyses highlight that WC grains are subject to plastic strain and are removed from the surface as the number of produced parts increase, leading to a growth of WC free areas where steel adhesion may occur. When analyzing the size of the WC grains, it appears that the population of small grains increases with the number of produced parts until 150.000. Then the population of small grains decreases. A wear mechanism is proposed to explain this variation of WC grains size with the number of extruded parts.

Towards the Numerical Prediction of Galling Onset in Cold Forming

Aluminum alloys are materials that have a strong tendency to galling when they are cold formed. Caused by a breakdown of the lubricant film between the part and the tool, galling can have dramatic consequences on the forming operation: scratches and cracks in the surface of the piece, clogging and deterioration of tools, etc. The present work studies the galling mechanisms of the aluminum alloy 6082 during its cold forming. Trials involving the Upsetting-Sliding Test (UST) are performed first. The UST is a test bench able to simulate in laboratory conditions the contact encountered at the part/tool interface of industrial processes. Trials are achieved under varying contact pressure and lubrication. UST results show that galling is strongly influenced by tool roughness and is not accompanied by a significant increase of friction. Three sets of finite element computation of the UST are then run to predict galling onset. Lubricant and adhesion forces are not modelled in this simplified approach: only the mechanical aspects are taken into account, the chemical ones are implicitly taken into account by coefficients of friction. The Lemaitre’s and the Xue’s damage models are compared. Results show that the Lemaitre model needs the tool roughness to be modeled to detect the galling onset. The Xue model is able to detect the occurrence of galling without modelling roughness. This result is due to the used of the Lode angle with enable the calculation of damage under low stress triaxiality.