Investigation of the effect of forming parameters in incremental sheet forming using a micromechanics based damage model

Shakir Gatea,Graham Mccartney,Hengan Ou,Bin Lu,Jun Chen

doi:10.1007/s12289-018-1434-3

Abstract

The incremental sheet forming (ISF) process is considered as a feasible solution for forming a variety of small batch and even customised sheet components. The quality of an ISF product is affected by various process parameters, e.g. sheet material, step-down, feed rate, tool diameter and lubricant. To produce an ISF part of sufficient quality and accuracy without defects, optimal parameters of the ISF process should be selected. In the present work, experiments and FE analyses were conducted to evaluate the influence of the main ISF process parameters including the step-down, feed rate and tool diameter on the formability and fracture of two types of pure Ti (grade 1 and 2). The Gurson–Tvergaard-Needleman (GTN) damage constitutive model with consideration of stress triaxiality was developed to predict ductile fracture in the ISF process due to void nucleation, growth and coalescence. It was found that the ISF parameters have varying degrees of effect on the formability and fracture occurrence of the two types of pure Ti, and grade 2 pure Ti sheet is more sensitive than grade 1 Ti sheet to the forming parameters due to low ductility.

Highlights

Incremental sheet forming (ISF), often named single point incremental forming (SPIF), is considered as an innovative technology as compared to traditional sheet metal forming processes due to the fact that traditional sheet metal forming processes involve additional costs linked to the equipment and tooling to be employed and the set-up time
This work aims to provide a better understanding of the effect of three key forming parameters, namely the step down, feed rate and tool diameter, and their interactions on the formability and fracture in the ISF process of pure Ti grade 1 and 2
A parametric investigation was undertaken in order to provide a better understanding of the relationship between three key ISF parameters, namely the step-down, feed rate and tool diameter, and the formability and fracture occurrence of grade 1 and 2 pure Ti using experimental testing and FE analysis incorporating the stress triaxiality modified GTN damage model

Summary

Introduction

Incremental sheet forming (ISF), often named single point incremental forming (SPIF), is considered as an innovative technology as compared to traditional sheet metal forming processes due to the fact that traditional sheet metal forming processes involve additional costs linked to the equipment and tooling to be employed and the set-up time. ISF is a flexible process and the part can be plastically deformed by moving a hemispherical tool along a programmed path. Uniform plastic strain and thickness distribution can be obtained by reducing the stepdown and increasing the tool diameter and wall angle [2]. The stresses in the contact region between the tool and the sheet and in the wall of the ISF part can be reduced by decreasing the step-down [4]. Through an experimental investigation to measure the forming force in ISF, [5] showed that the step-down has little effect on the formability of the ISF process. The first design showed that the step-down had an effect on the ISF formability and that the likelihood of the part to deform was enhanced by decreasing the step-down.

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