A Study on Improving Tool Life of Automotive Axle using Direct Metal Deposition Technology

Abstract

The working condition of hot forging die is very terrible, such as high temperature, high load, repeated thermal and loads. The life of hot forging die is generally lower than cold forging die, the cost of the workpieces and the economic efficiency of the manufactory are affected directly. Expecially for developing counties, average life of hot forging die is far lower than developed countries. It is essential to enhance the service life of hot forging die and to reduce cost of the pieces for hot forging industry．

The hot forging die simultaneously withstands the repetitive thermal load and the mechanical load, which can cause thermal stress and mechanical stress respectively. The thermal load is the main reason to cause damage of hot forging die. It is of great social and economic significance to study the surface hardening of tools and reduce the thermal stress precess.

This dissertation focuses on the early failure of the hot forging die for automotive axles, mainly by increasing the high-temperature strength of die to prevent thermal softening of tool surface. The surface hardening technology used in this study is direct metal deposition technology to deposit a high-performance metal on the surface of the traditional die that is easy for wear. However, thermal cracks are frequently generated on the deposited areas due to thermal stress from different material properties. A thermal stress control layer (TSCL) is designed to reduce thermal stress and increase fatigue life as a buffer in the vicinity of the joining region between the hardfacing layer and the base metal. TSCL and hardfacing layer are deposited through layer-by-layer way on the substrate using direct metal deposition technology. The TSCL to be produced by mixing of Stellite21 and SKD61 is designed with thicknesses of 0 mm, 1 mm, 1.5 mm, and 2 mm separately. The effect of thermal stress in the transition regions is investigated after adding TSCL. The optimal design of TSCL is selected by the change of thickness and composition proportion.

Stellite21 superalloy deposited on the hot forging die must undergo effects of repetitive thermal stress and mechanical stress during forging process. The hardening mechanism of Stellite21 is studied by microhardness tester. The etched Stellite21 samples are observed under the optical microscope. The microstructure analyses of Stellite21 are carried out on a TESCAN MIRA3 scanning electron microscope (SEM) with energy dispersive X-ray (EDX) spectrum. The phases present in the specimens are examined with an X-ray technique, using Cu K_radiation. So that the evolution microstructure and properties of Stellite21 are explained through before and after forging.