A probabilistic study of failures of solid and hollow dies in hot aluminum extrusion

Abstract

Hot metal forming (rolling, forging, extrusion, wire drawing) constitutes a very large proportion of manufacturing activity. Of all the equipment and tooling involved in a hot forming process, the most critical component is usually considered to be the die due to its superior precision and reliability requirement and the associated high cost. Dies and ancillary tooling are exposed to high pressures/forces, elevated temperatures, mechanical and thermal fatigue. Cost and engineering difficulty are then obviously high because of factors such as special material and processing, very fine tolerances, and high demands on repeated thermo-mechanical performance. Critical to any study involving efficiency, productivity, or overall economy of any hot forming operation is thus an analysis of tooling performance in terms of die life and reliability assessment and prediction.The current paper presents results of an ongoing study about the probabilistic behavior of service life of tooling in commercial aluminum extrusion, and the relationship (if any) between die reliability and profile complexity. A total of 595 die failures involving 17 different die profiles (24 profiles, considering different extrusion ratios) are studied, in collaboration with a local industrial setup. All dies are made of H-13 steel, while the billet material is Al-6063 in all the cases. Considering die life to be a random variable, an attempt is made in the first half of the paper to fit a reasonable probability distribution to the 24 different data sets. Rather than adopting a non-parametric approach, a number of probability distributions most popularly employed in reliability analyses are explored. The second half of the paper evaluates shape complexity of the die profiles studied according to three most prevalent definitions, and explores any possible patterns between die complexity and die reliability.