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 1950's. 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.