To improve crash worthiness and fuel economy, the automotive industry is using increasingly Advanced High Strength Steels (AHSS). In addition having limited formability, compared to mild steels, AHSS require high pressures at the tool-workpiece interface. This leads to frequent lubricant film break down and galling as well as reduction in tool life, resulting in a considerable challenge to stamping engineers. Galling is a form of adhesive wear and the economic impact of galling upon stamping production is significant due to the increase of scrap rate and die maintenance cost.
The major objective of this study is to predict and reduce galling in forming galvanized AHSS. For this purpose, a preliminary model was developed to predict the severity of galling for given lubricants, die materials and coatings in forming galvanized AHSS.
In this study, Finite Element Analysis (FEA) of selected stamping operations with AHSS was conducted to determine the critical pressure conditions that exist in practical stamping. Thus, an attempt was made to develop laboratory tribotests, e.g. Twist Compression Test (TCT), Deep Drawing (DDT) and Strip Drawing Test (SDT) that emulate practical stamping conditions. By using these tribotests, the performance of lubricants, die materials and coatings was evaluated for various grades of AHSS in terms of coefficient of friction (COF) and galling. The results of this study helped to develop a preliminary model for predicting galling by considering the empirical relationships between important tribological parameters (e.g. interface pressure, sliding length, surface roughness, lubricant viscosity, hardness of zinc-coatings and tool coatings) and galling in forming galvanized AHSS. This model was applied to estimate the total number of parts that can be formed before severe galling occurs. Therefore, this model can offer a cost effective way to select practical and best combinations of lubricant, tool material and tool coating for reducing galling in forming galvanized AHSS. In addition, it can be further developed for use as a scheduling tool for the die maintenance before galling occurs, thus the unexpected downtime of stamping process can be eliminated.
|Commitee:||Bhushan, Bharat, Kinzel, Gary, Walter, Mark|
|School:||The Ohio State University|
|School Location:||United States -- Ohio|
|Source:||DAI-B 78/11(E), Dissertation Abstracts International|
|Keywords:||AHSS, Advanced high strength steels, Galling, Stamping|
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