Laurent Dubar

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.

Dynamic Evolution of Interface Roughness During Friction and Wear Processes

Dynamic evolution of surface roughness and influence of initial roughness (S(a) = 0.282-6.73 µm) during friction and wear processes has been analyzed experimentally. The mirror polished and rough surfaces (28 samples in total) have been prepared by surface polishing on Ti-6Al-4V and AISI 1045 samples. Friction and wear have been tested in classical sphere/plane configuration using linear reciprocating tribometer with very small displacement from 130 to 200 µm. After an initial period of rapid degradation, dynamic evolution of surface roughness converges to certain level specific to a given tribosystem. However, roughness at such dynamic interface is still increasing and analysis of initial roughness influence revealed that to certain extent, a rheology effect of interface can be observed and dynamic evolution of roughness will depend on initial condition and history of interface roughness evolution. Multiscale analysis shows that morphology created in wear process is composed from nano, micro, and macro scale roughness. Therefore, mechanical parts working under very severe contact conditions, like rotor/blade contact, screws, clutch, etc. with poor initial surface finishing are susceptible to have much shorter lifetime than a quality finished parts.

Numerical simulation of lubrication mechanisms at mesoscopic scale

The mechanisms of liquid lubrication in metal forming are studied at a mesoscopic scale, adopting a 2D sequential fluid‐solid weak coupling approach earlier developed in the first author’s laboratory. This approach involves two computation steps. The first one is a fully coupled fluid‐structure Finite Element computation, where cavities in the surface are plastically deformed leading to the pressurization of the entrapped fluid. The second step computes the fluid exchange between cavities through the plateaus, one cavity at a time. This second step consists in solution of the local Couette’s equation, at the asperity level, in order to quantify the fluid leakage in the cavity/plateau network using the lubricant pressure computed previously.The numerical simulation is validated by experimental tests in plane strain strip reduction of aluminium sheet provided with model cavities in form of pyramidal indentations. The tests are performed with variable reduction and drawing speed under controlled front and ba...

Tribological Performances of Two White Lubricants in Hot Steel Forging

The tasks of a lubricant used in the hot forging of steel is to reduce friction, cool down tool surfaces, and limit tool surfaces pollution by wear debris and oxides scales transferred from work piece surfaces. The most widely used lubricants in hot forging are dispersions of graphite particles in water or in oil. The graphite is involved to reduce friction. The liquid is used, first, to carry the graphite solid particles to the tool surfaces and, second, to reduce tool surface temperature. But graphite and vapours resulting from the lubricant deposition on hot surfaces lead to dirty workspaces. Some new lubricants are then developed in order to reduce friction in a cleaner way. When they are graphite free, those new lubricants are called “white lubricants”. The aim of the present work is to test two different white lubricants. The first one is a mineral salt; the second one is an organic salt. Lubricant performances are classified using the Warm and Hot Upsetting Sliding Test (WHUST). This friction bench simulates tests with contact pressure, sliding velocity, contactor and specimen temperatures similar to industrial ones. Before performing friction tests, work piece are heated up to 1200°C, contactors are heated up to 200°C, and lubricants are sprayed on contactor surfaces. Then the contactor slides against the specimen with a constant penetration, leaving a residual deformed track on its surface. Direct WHUST results are tangential and normal loads measured on contactor, surface roughness and chemical compositions on specimen and contactor surfaces. “Wear markers” are derived from those direct results, and provide useful information on the ability of the tested lubricants to reduce friction and protect tool surfaces. In the present study, tests are performed with different sliding speed and different contact pressure. The two white lubricants are compared to a generic graphite in water dispersion. Results show the tested white lubricants lead to coefficient of friction in the same range of the graphite lubricant one, but white lubricants lose their ability to reduce friction as soon as the sliding lengths becomes greater to 10 mm, where graphite lubricants can undergo sliding length greater than 30 mm.

Friction and wear mechanisms study on a newly developed High Speed Tribometer

This paper presents a newly pin on disc tribometer which enables us to test friction materials with fixed boundary conditions instead of usually constant normal load. The disc is driven by means of a high speed machining device reaching 15000 rpm in rotational speed and the pin is instrumented to get mechanical and thermal data. A thermomechanical study led on this tribometer with phenolic pin against steel disc is then presented. Friction coefficient values are analyzed regarding contact conditions and worn surfaces are inspected by optical microscopy and profilometer analysis.

Friction and Wear Mechanisms of Phenolic-Based Materials on High Speed Tribometer

This work takes place in the understanding of the friction and wear mechanisms occurring in reinforced phenolic materials, widely used in organic braking pads. As the matrix is filled with a large variety of particles, the phenomena in the contact zone are complex and multi-physic. In a first approach the reinforcement is restricted to spherical steel particles with diameters in the range of the fibbers size. The influence of the sliding speed, the mean normal pressure and the contact temperature are examined and the benefits of using this kind of particle is as well discussed. The tribological tests are performed on a newly developed High Speed Tribometer designed to reproduce braking conditions. The results show that temperature is the most influential parameter, leading to a decrease of the friction coefficient. They further indicate that reinforcement pushes the loss of efficiency to a higher temperature. Optical observations and profilometer analysis show that the wear mechanisms are clearly dependent on friction conditions. These results improve our knowledge of wear debris formation and conditions leading to particle debonding in phenolic matrix material.

Finite element modelling of orthogonal cutting: sensitivity analysis of material and contact parameters

This paper presents a finite element model of orthogonal cutting developed with Abaqus/explicit software. An Arbitrary Lagrangian-Eulerian (ALE) formulation is used to predict chip formation, temperature, chip-tool contact length, chip thickness, and cutting forces. This numerical model makes possible qualitative analysis of input parameters related to cutting process and frictional models. A sensitivity allows the identification of significant input parameters (coefficients of the Johnson-Cook law, contact and thermal parameters) and their tolerance limits identification. This study draws to input parameters which have to be determined accurately in order to improve numerical approaches of machining.

Estimation of three‐dimensional distribution of heat flux on the pin frictional surface during a pin on disc test

We present the identification of heat flux dissipated at pin face on newly developed pin on disc tribometer. The repartition on contact surface is obtained by inverse heat conduction algorithm in two steps. Mean heat flux is firstly calculated by function specification method in one dimensional model then heat repartition at the surface is obtained with a Levenberg‐Marquardt optimisation algorithm on three dimensional finite element analysis. Experiments are performed on steel disc against phenolic resin pin commonly used for braking applications. Results show that heat flux distribution is dependent on pressure and sliding speed conditions.

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.

Friction and Wear in Metal Forming: 25 Years at LAMIH UMR CNRS 8201

Since the beginning of the 90’s, research activities focused on friction and wear in metal forming have been developed at the LAMIH UMR CNRS 8201 in Valenciennes. Specific methodologies have been designed to optimize a given forming process (bulk forming process or sheet forming process). These methodologies involve prototype benches which have been built to reproduce contact conditions encountered in manufacturing plants by taking specimens and contactors from the real industrial workpieces and tools. The evaluation of the friction coefficient added to the fine analysis of the surfaces have helped us to better understand friction and wear during processes. These facilities have been settled by numerical simulation at meso and macro scales by means of finite element methods. So, this paper is the sum up of the output of these methodologies with a specific focus on wear and lubrication, at room and hot temperatures.