November 2020 Volume 2

FORGING RESEARCH AND TECHNOLOGY

Enhancing Tool Life Via Manipulation of the Elastic Strain Field of the Dies During Forging By Gracious Ngaile, Hao Pang, Daniel Santiago, Bharatharajan Subramaniaraman, Department of Mechanical and Aerospace Engineering, North Carolina State University, Raleigh, NC

Abstract Forging tools are often subjected to a combination of cyclic thermo-mechanical, chemical, and tribological loads. Strategies for minimizing these loads are critical for preventing premature tool failure and increasing productivity. This paper examines the energy flow in a typical forging press and explores the possibilities of harnessing the potential energy stored in the press system during the forging cycle. By concentrating the potential energy in a certain part of the tooling, it is possible to conceive a forging die design architecture that minimizes the residual contact pressure at the die workpiece interface during the ejection stroke. The principle behind this design concept is that during the forging stroke, a tapered die canmove in the direction of the forging load, thus concentrating the potential energy on the die insert by inducing negative radial elastic strain. When the extrusion load is removed, the elastic strain energy (potential energy) stored in the die is released, thus repositioning the die to its initial state. With this design architecture, the workpiece can be ejected at minimal load. The proposed concept was validated using Finite Element (FE) warm forging simulations for a constant velocity joint.These simulations showed that in addition to reducing residual contact pressure, which enhances tribological conditions, the new die design can lower die stresses, thus increasing die fatigue life. 1. Introduction Tool life is a major factor in the cost of forgings, productivity, and part integrity. Forging and extrusion tools are usually subjected to mechanical, thermal, chemical, and tribological loads. Subjecting dies and punches to severe loading will bring about their eventual failure. The three common modes of failure are tool wear, fatigue fracture, and plastic deformation. In hot forging, tool wear accounts for approximately 70%, whereas the primary mode of failure in cold forging is fatigue fracture [1-4]. Many parameters influence die failure in cold, warm, and hot forming. The complex interaction of these parameters makes enhancement of tool life a challenge, and strategies to increase tool life can vary greatly from one forging operation to another.

There are numerous strategies for reducing tool failure currently used in practice. These strategies include (i) improved lubrication and cooling systems, (ii) selection of the tool materials and adequate thermal treatments; surface treatment of the tools via thermochemical reactions such as nitriding, which involves diffusion of atomic nitrogen in the steel surface, (iii) utilization of hard materials like ceramics, (iv) inducing compressive stresses on the die surface by shot peening or roller burnishing, (v) utilization of stress rings for increasing the stiffness of the die assembly, (vi) tool shape optimization, which at present is carried out with the aid of numerical simulations and (vii) modern tool work supervision, where certain parameters can be monitored on the forging machine [5-8]. Some of the strategies highlighted above for tool life enhancement are briefly summarized below. Lubricants play a critical role in the success of the forging operation. They reduce the friction stress at the tool-workpiece interface, thereby reducing the forming loads and energy expended. Effective lubrication is also critical for the longevity of the tool and surface integrity of the forgings. A wide range of lubricants based on mineral oils and grease is available for simple forging operations. However, for severe cold forging operations, which exhibit high interface pressures and larger surface expansion, there are few effective lubricants. The most commonly used lubrication systems for steel and aluminum in industry are based on conversion coatings [9,12]. Lubrication for hot forging and extrusion operations presents a different set of challenges to those observed in cold forming. For example, high thermal loading at the die-workpiece interface in hot forming precludes many lubricants. A good lubricant for hot forging should not only lubricate but also cool the die.This is crucial because the dies need to be kept below a certain temperature to prevent die thermo softening, which can substantially reduce too life. To that effect, the most widely used lubricant in hot forging is water-based graphite. Although it pollutes the environment, graphite provides good anti-galling performance, due to its lower friction and heat resistance. In the past two decades, several white lubricants have been developed in the effort to replace graphite [13]. A detailed review of white lubricants used in hot forging and extrusion can be found in [14].

FIA MAGAZINE | NOVEMBER 2020 32