Date of Award

1-1-2022

Publication Type

Thesis

Degree Name

M.A.Sc.

Department

Mechanical, Automotive, and Materials Engineering

Keywords

Diamond-like carbon, Galling, Lubrication, Material transfer, Shear-punch tests, Sheet metal forming

Supervisor

A. Alpas

Supervisor

D. Green

Rights

info:eu-repo/semantics/embargoedAccess

Abstract

When sheets of aluminum alloys are pierced or trimmed, tool failure occurs by transfer of material from the sheet to the surface of the tool. This phenomenon referred to as galling adversely affects sheared edge quality and increases energy consumption. In this study, an instrumented pneumatic press was designed and built to conduct shear–punch tests on 2mm-thick AA5754-O sheets to investigate the progression of galling on uncoated and NH-DLC-coated AISI M2 steel punching tools during dry and lubricated punching. The punching tests were performed using die-punch clearances of 2.0%, 4.0%, and 6.0% of the lower die diameter. Cumulative galling volumes were measured using a non-contact optical surface profilometer, and the galling rate was estimated. The galling initially occurred at a high rate; then progressed with a lower constant rate. Lubricating the aluminum sheet with an oil-based lubricant that is the common industrial lubrication method (LUB1) mitigated the galling rate compared to the dry punching. A new lubrication method was proposed in this study to further enhance the galling performance that is lubricating both the sheet and the punch (LUB2). Punching force-displacement curves indicated a higher amount of energy expended to shear AA5754-O sheets in dry and LUB1 compared to LUB2 punching that is suggested to be due to the higher galling resulting in higher friction forces at the interface. To further improve the galling mitigation behavior of the tribosystem, NH-DLC-coated tools were used for punching. SEM images taken from the tip of the punch combined with EDS analysis showed the incidence of coating removal in dry punching after 20 strokes resulting in damage to the coating. In LUB1, Raman spectroscopy suggested sliding-induced structural changes of the coating after 30 strokes. In LUB2 punching, however, a combination of NH-DLC coating and the LUB2 lubrication method eliminated the galling, so that no aluminum transfer was detected by SEM and white light interferometry after 500 strokes, and the coating structure remained intact according to the Raman spectroscopy.

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