Niels Bay

A theoretically determined model for friction in metal working processes

Abstract A general theory for friction in metal working processes is developed based upon the slip-line theory as a model of analysis. The real area of contact α and the nominal friction stress τn are determined as functions of the nominal normal pressure q/2k and the friction factor m. The results show how the real area of contact increases and approaches the apparent, as the normal pressure increases. Furthermore it is found that Amontons law is valid only until q/2k = 1.3 irrespective of the m- m value . Curves of τn/k as functions of q/2k and m show that τn/k approaches the friction factor m as q/2k tends to infinity. This means that Amontons law in the case of small m-values (m

Single‐point incremental forming and formability—failure diagrams

In recent work, the present authors constructed a closed‐form analytical model that is capable of dealing with the fundamentals of single‐point incremental forming (SPIF) and explaining the experimental and numerical results published in the literature over the past couple of years. The model is based on membrane analysis with in‐plane contact frictional forces but is limited to plane strain, rotationally symmetric conditions. The aim of the present paper is twofold: first, to extend the previous closed‐form analytical model into a theoretical framework that can easily be applied to the different modes of deformation that are commonly found in general single‐point incremental forming processes and, second, to investigate the formability limits of SPIF in terms of ductile damage mechanics and the question of whether necking does, or does not, precede fracture. Experimentation by the present authors, together with data retrieved from the literature, confirms that the proposed theoretical framework is capable of successfully addressing the influence of the major parameters of the SPIF process. It is demonstrated that neck formation is suppressed in SPIF, so that traditional forming limit diagrams are inapplicable to describe failure. Instead fracture forming limit diagrams should be employed.

Friction stress and normal stress in bulk metal-forming processes

Abstract When analysing pressure distributions in bulk metal-forming processes, it is conventional to adopt either Amontons friction law, τ = μ p , or the law of constant friction stress. Neither of these two laws however, is applicable universally. In this paper a general friction model including both laws is presented. Using a slip-line analysis of the plastic deformation of surface asperities, the influence on friction stress of normal pressure, friction factor and surface roughness (defined by asperity slope) is expressed analytically. Examples of applications of the friction model when analysing bulk metal-forming processes are given: the tool/workpiece interface pressure and friction stress distributions are estimated for simple upsetting, forward bar extrusion and plate rolling. The calculated pressure distibutions show good accordance with those measured experimentally.

The state of the art in cold forging lubrication

Abstract The manufacture of components in steel, aluminium and copper alloys by cold forging production has increased ever since the 1950s. Typical processes are forward rod extrusion and backward can extrusion, upsetting, ironing, tube extrusion and radial extrusion. The tribological conditions in cold forging are extremely severe due to large surface expansion and normal pressure in the tool/workpiece interface combined with elevated tool temperatures. Except for the more simple cold forging operations successful production therefore requires advanced lubrication systems. The present paper gives a detailed description of the state of art for lubricant systems for cold forging of C-steels and low alloy steels as well as aluminium alloys including all the basic operations such as cleaning of the slugs, application of eventual conversion coating and lubrication. As regardscold forging of steel the conversion coatings are based on zinc phosphates but different requirements to the coating properties have to be met in different cold forging operations. This is obtained by adopting different oxidants leading to different composition, layer thickness and morphology of the conversion coatings. Concerning aluminium unalloyed and softer alloys like the AA 1000, 3000 and 6000 series can be cold forged with either grease, oil or zinc stearate whereas the harder alloys from series AA 2000, 5000 and 7000 require a conversion coating to carry the lubricant. Three different types of conversion coating are described, i.e. phosphate coating, calcium aluminate coating and aluminium fluoride coating. Alternative lubricants and their application are also described.

Revisiting single-point incremental forming and formability/failure diagrams by means of finite elements and experimentation:

In a previously published work, the current authors presented an analytical framework, built upon the combined utilization of membrane analysis and ductile damage mechanics, that is capable of modelling the fundamentals of single-point incremental forming (SPIF) of metallic sheets. The analytical framework accounts for the influence of major process parameters and their mutual interaction to be studied both qualitatively and quantitatively. It enables the conclusion to be drawn that the probable mode of material failure in SPIF is consistent with stretching, rather than shearing being the governing mode of deformation. The study of the morphology of the cracks combined with the experimentally observed suppression of neck formation enabled the authors to conclude that traditional forming limit curves are inapplicable for describing failure. Instead, fracture forming limit curves should be employed to evaluate the overall formability of the process. The aim of this paper is twofold: (a) to compare the mechanics of deformation of SPIF, namely the distribution of stresses and strains derived from the analytical framework with numerical estimates provided by finite element modelling; and (b) to compare the forming limits determined by the analytical framework with experimental values. It is shown that agreement between analytical, finite element, and experimental results is good, implying that the previously proposed analytical framework can be utilized to explain the mechanics of deformation and the forming limits of SPIF.

Two new methods for testing lubricants for cold forging

Abstract Two new variants of the twist-compression test are presented, enabling evaluation of solid-film lubricants in cold forging under controlled and variable surface enlargement, sliding length and normal pressure. Investigations have been performed with: AlMgSi1 as the work-piece material; cold-working steel, HSS and TiC/TiN coated HSS as the tool material; and various lubricants for aluminium. The friction stress has been estimated as a function of the tool material, the lubricant and the process parameters of surface enlargement, sliding length and normal pressure. Furthermore, lubricant performance has been investigated. The threshold surface enlargement/sliding length curve for the breakdown of the lubricant film has been found for the various tool materials and lubricants investigated.

Tool/workpiece interface stresses in simple upsetting

Abstract When analysing the stress distribution at the tool/workpiece interface in simple upsetting, it is conventional to assume either Amontons friction law or the law of constant friction stress. Neither of these two laws, however, is valid generally. Furthermore, the friction stress is assumed to jump discontinuously at the centre, which assumption has been proven, experimentally, to be incorrect. In this paper considerations are made of these two objections. A combination of a lower-bound analysis using the slab method with an axi-symmetric upper-bound analysis is found to be satisfactory when studying disk forging. The slab method combined with a general friction model developed by Wanheim and Bay, is used to estimate the local normal stress and frictional stress. A central region of sticking where the friction stress varies linearly with the radius is introduced into this analysis. The radius of the sticking region is found by an upper-bound analysis using the distribution of friction stress obtained by the slab method. It is shown that a sticking region is always present except in the case of zero friction, which explains experimental observations showing that the value of the friction stress increases linearly with the radius in the centre region. The calculated distribution of normal stress is compared with experimental measurements obtained using a special pressure sensor mounted in the lower anvil. The agreement between the results of theory and experiment is good.

Contact Modelling in Resistance Welding. Part 1: Algorithms and Numerical Verification

Abstract Finite element based numerical modelling programs for resistance welding are presently being improved as an important tool for development and optimization of complex joints. One of the remaining challenges in obtaining useful results is appropriate modelling of the contact between the parts. The contact problem in resistance welding includes not only mechanical contact but also thermal and electrical contact. In this paper such a contact model is developed for simulation of resistance spot and projection welding. The mechanical contact includes frictionless contact as well as contact with sticking and with sliding friction. The thermal and the electrical contact are modelled by introducing an artificial contact layer. After a description of the algorithms, several numerical examples are presented to validate the mechanical contact algorithm.

Contact Modelling in Resistance Welding. Part 2: Experimental Validation:

Abstract Contact algorithms in resistance welding presented in the previous paper are experimentally validated in the present paper. In order to verify the mechanical contact algorithm, two types of experiment, i.e. sandwich upsetting of circular, cylindrical specimens and compression tests of discs with a solid ring projection towards a flat ring, are carried out at room temperature. The complete algorithm, involving not only the mechanical model but also the thermal and electrical models, is validated by projection welding experiments. The experimental results are in good agreement with the simulation prediction, showing the validity of the algorithm.

A Study on DLC Tool Coating for Deep Drawing and Ironing of Stainless Steel

The trend in metal forming tribology is to develop new tribo-systems including new lubricants, tool materials and tool coatings in order to substitute environmentally hazardous lubricants by environmentally friendly tribo-systems. In preliminary testing the limits of lubrication of new tribo-systems for sheet forming production, it is advantageous to use dedicated simulative tribo-tests. This paper studies the influence of tool coatings on deep drawing operations using the Bending Under Tension (BUT) test and also under more severe tribological conditions by adopting the Strip Reduction Test (SRT) to replicate industrial ironing of deep drawn, stainless steel parts. Non-hazardous tribo-systems in form of a double layer Diamond-like coated tool applied under dry condition or with an environmentally friendly lubricant were investigated via emulating industrial process conditions in laboratory tests. Experiments revealed that the double layer coating worked successfully, i.e. with no sign of galling, when it was used with environmentally friendly lubricants, whereas the results were more prone to galling under dry condition.