A Comprehensive Study of Hole Punching for AHSS

The work discussed in this presentation was partially supported by the A/SP Stamping Team using funds from the Auto/Steel

Partnership.

Introduction

Experimental Procedure

• Tool Setup

• Experiment Variables and Materials

Results and Discussion

• Punching Force Studies

• Dimensional Studies

• Tool Protections

• Cutting Edge Qualities

FEA Simulations

Summary & Future works

Sample size: 254mm×254mm

• Punch rate: 10 mm/s

• Punch shapes: flat, conical, rooftop

• Punch tipping angle: 7°

Conical shaped punch induces large deformation within the cutting area.

• The punch load is quite uniform due to gradual shearing process , similar to scissor cutting for the rooftop punch

For all cases, the maximum punch load decreases as cutting clearance increases, but the difference is trivial (about 3 to 4%).

• The rooftop punch leads to significant force reduction and it is more effective on AHSS.

• The hole punching force coefficient can be calculated as

This definition is similar to the shear strength index. More dependencies are considered during the evaluation.

 The hole punching force coefficient is negatively correlated to the material strength.

• Mild steel → 1.0; AHSS: 0.7 ~ 0.8

 Dimensional accuracy of punched holes is important in the sheet metal forming.

• Dimensional measurements were repeated for three times for each punch configurations (punch shape, material, and cutting clearance).

Conical shape leads to an uniform enlargement for diameter due to the stress release and consequent spring back.

• The holes punched with rooftop shape exhibited oval shape with minor axis along the ridge direction.

Snap-through load, i.e. reverse tonnage, leads to severe press machine damage.

• Rooftop punch can provide an effective solution for press machine protection and noise reduction.

• The cutting surface was examined using optical microscope with 200X magnification.

• In-plane hole expansion tests were conducted to evaluate the edge damage due to the punch geometry during the punching stage.

• The conical shaped tool can produce a punched hole with higher edge stretchability, while rooftop punch results in the most severe edge damage.

 In-plane hole expansion tests will be continued to study the sheared edge damage mechanism.

• A numerical damage model will be developed to simulate the edge cracking.

• The punch shape and geometry will be optimized to achieve the goals of load reduction and dimensional accuracy.