Sheet Forming

Introduction

CMFI has one of the best sheet forming research groups in Australia. Sheet forming is where a flat blank sheet is plastically deformed or drawn into a specific shape via a punch and a die. A majority of parts are formed using this process, and it is a major process within the automotive industry.

The sheet forming group has had a number of projects:

ARC Linkage - Light weight automotive structures (2004-2009)

This project has investigated the potential introduction of Advanced High Strength Steels (AHSS) in the automotive environment. The project has included the comparison of stamping of Falcon cross member using HSLA, Dual Phase, TRIP steels. In Australia, most automotive companies are now considering advanced high strength, formable steels for weight reduction. These steels are extremely complex with little published research in key areas of their application. The objective is to develop a detailed understanding of the forming and post forming properties of a range of these steels and from this to design and produce new lighter weight automotive structures. This work will include improved finite element models to predict the shape as well as a detailed understanding of how to manufacture components from these steels at high production rates and the mechanical properties after manufacturing.

Ford URP - Predicting the tool life issues for AHSS stampings (2005 - 2007)

This project has investigated developing a model to determine tool life given the stamping of AHSS. Automotive companies around the world are increasingly using AHSS (advanced high strength steels) to achieve vehicle weight reductions without compromising crash and durability performance. Forming components from these materials requires greater press force which generates higher loads in the tools. The result has been a dramatic increase in tool wear problems leading to increased tool maintenance and unscheduled stoppages in the plant. Expensive die steel inserts, complex tool coatings and heat treatments can be used to combat these problems but can significantly increase tooling cost. Currently there is no method available to reliably predict the likelihood of problems occurring so that the most cost effective solution can be selected for each specific case. The ideal tooling solution is the least expensive tool material and treatment that will allow the plant to run at their required production rate without compromising the quality of the part. This requires an understanding of the loading conditions between the tool and blank coupled with a database of tooling options from which the best solution can be selected. Ford, Volvo and Mazda have completed trials for a combination of sheet materials and a range of tool coatings, materials and post machining treatments. The results from their trials can form the basis for this database and the output from FE forming simulations must be evaluated to determine if this could be used to characterize the loading conditions between the blank and the tool surface.

Studying variation in stamping

The inherent variability in incoming material and process conditions in sheet metal forming makes quality control and the maintenance of consistency extremely difficult. Current FE-based simulations can predict whether a particular sheet metal component can be formed with given tooling, but because of their numerical deterministic nature, they are ill suited to predict the variability in an actual production process; although there are now some simulation packages that are addressing the stochastic nature of the production process. This project is attempting to develop stochastic models for stamping.

Forming of sandwich (Steel and Aluminium laminates) sheets

In recent years, automobile manufacturers have developed Steel-Polymer-Steel (SPS) laminates for automobile parts such as fenders, doors and interior panels. Compared with homogeneous metal sheets, SPS laminates offer a significantly lower density and better sound and vibration damping characteristics. Compared to other light weight solutions they also have the advantage of a good surface finish and can maintain a bending rigidity which is almost equal to that of a simple metal sheet of the same total thickness. Despite the superior mechanical properties of SPS laminates, they have not gained widespread use in the automobile industry because of geometric distortion and defects arising from the forming process. Several geometrical defects are observed which do not occur with homogeneous material, especially in bending. This project has concentrated on developing analytical and finite element models of laminate behaviour, and validating them against experimental data.

Sheet forming equipment:

Erichsen forming machine
Mechanical 120T press
Charpy tester for bending energy absorption

Key paper:

Contact pressure evolution and its relation to wear in sheet metal forming, Michael P. Pereira, Wenyi Yan and Bernard F. Rolfe.
Brief summary: An accurate prediction of the contact pressure distribution at the tooling surface, is an essential step towards the estimation of tool life for sheet metal forming process. This paper utilises finite element analysis to model and explain the evolution and distribution of contact pressure, throughout the duration of a channel forming process. A transient response, characterised by large and localised contact pressure peaks over the die radius, was identified and later proved to be important to the wear response. These results have direct implications on the applicability of traditional sheet metal stamping wear tests, which do not capture this critical contact pressure response.

Fig. Evolution of contact pressure over the die radius as the punch travels upwards