Benefits of Die Wall Lubrication for Powder Compaction

The achievement of high density at reasonable cost would be a definite advantage for the production of P/M components requiring high static and dynamic properties. The use of the electrostatic die wall lubrication technique appears as a very attractive route to promote densification. By reducing the level of friction at die walls, this technique should have the potential to also reduce density variations in powder compacts, particularly for parts having a large sliding surface or a long die fill. This paper presents two case studies of parts having high aspect ratio compacted using die wall lubrication either on a laboratory press or on an industrial mechanical press. Special attention is paid to the axial density distribution of the green compacts as a function of the compacting pressure and the type of lubrication (admixed or die wall lubrication). INTRODUCTION Since the beginning of the nineties, many authors have demonstrated the advantages of using die wall lubrication (DWL) associated with a decrease of the admixed lubricant content to improve density and mechanical properties of P/M parts [1, 2, 3, 4]. By maintaining good lubrication at die walls during the compaction and the ejection of parts, this technique was shown to be very efficient, in particular when combined with warm pressing to increase the density of parts. However, until now, literature focused mainly on the use of very high compacting pressure to reach the highest possible density for relatively simple parts with a low aspect ratio. With the development of die wall lubrication systems that enable the application of external lubricants on die walls in industrial conditions even for long die fill parts [5, 6], it becomes interesting to investigate the benefits of DWL specifically for parts having a large sliding surface or a long die fill. A point to consider regarding the compaction of complex parts is the amount of admixed lubricant required to press the parts. Recent studies have shown that, using DWL, the content of admixed lubricant required to optimize the densification and achieve a good particle rearrangement during pressing was very low, well under 0.2 wt% [7, 8, 9]. However, this was done on simple parts like tensile or transverse rupture specimens. Studies conducted at the Industrial Materials Institute (IMI) in collaboration with QMP, have shown that as the aspect ratio of a part is increased, the optimum level of admixed lubricant is not only dictated by the ultimate reachable density (% Pore Free Density), but also by the level of friction at die walls. Indeed, if the reduction of the admixed lubricant is required to avoid the inhibition of compaction at high pressure due to the volume occupied by the lubricant, the admixed lubricant still play an important role on wall lubrication when combined with a die wall lubrication system. More precisely, a high die fill part easily processed with a mix containing 0.2 wt% of admixed lubricant and with DWL became practically impossible to process with the same mix containing only 0.1 wt% lubricant due to higher ejection forces and bad surface finish (galling). Therefore, an optimum content of admixed lubricant must be determined as a function of the complexity of parts and the lubrication performance at the die walls. In addition to the possibility of applying higher compacting pressures while maintaining adequate ejection pressures, another significant advantage of DWL for parts having a long die fill might be the reduction of the pressure loss along the part height due to less friction on die walls. This reduction should bring a decrease of the axial density variation and an increase of the average green density of the part. Besides, it is worth mentioning that density variations in a powder compact may not only be generated by friction at the die walls, but also by the apparent density variations of the powder mix in the die due to poor die filling, magnetized die, mix segregation, or by a poor particles rearrangement or non optimal tool movements during compaction. The objective of this study was to quantify the effect of DWL on the reduction of axial density gradient in a high aspect ratio part and, consequently, on the increase of the average part density. EXPERIMENTAL PROCEDURE Laboratory Scale Evaluation All experiments of powder compaction at the laboratory scale were carried out by using a highly compressible water-atomized iron powder (ATOMET 1001, supplied by Quebec Metal Powders Limited) with different quantities of admixed zinc stearate (ZnSt) lubricant. Weight fraction of lubricant content ranged from 0 to 2 wt%. The behavior of the powder during compaction was evaluated using the Powder Testing Centre (model PTC-03DT). This apparatus consists of an instrumented cylindrical die operating in a single action mode. This lab press allows continuous recording all along the compaction and ejection process, of the punch displacement, the applied pressure and the pressure transmitted to the stationary punch. To evaluate the effect of the lubrication mode, a device was designed to apply a thin layer of lubricant on the die wall. The principle used by this apparatus is based on tribostatic electrical charge produced on lubricant particles when they are carried by a flow of air through a small Teflon tube. Cylinders having a height of ~ 8 mm (aspect ratio of 3.36 compared to 1.4 for a standard 1⁄4” TRS bar) were compacted at 25°C and 45 tsi in a D2high speed steel die having a diameter of 9.525 mm at a compacting rate of 1 mm/sec. Powder mixtures containing from 0.5 to 2 wt% ZnSt were compacted without die wall lubrication (DWL), while powder mixtures containing from 0 to 0.75 wt% ZnSt were compacted with ZnSt external lubricant applied on the die walls. At least 5 specimens were pressed for each material and condition. For the different admixed lubricant contents and DWL conditions, friction at the die walls was evaluated from the pressure drop between the pressure applied on the moving lower punch and the pressure transmitted to the stationary upper punch (Figure 1). Compacting pressure 0 10 20 30 40 50 D en si ty