Date of Award

2-1-2022

Publication Type

Dissertation

Degree Name

Ph.D.

Department

Mechanical, Automotive, and Materials Engineering

Keywords

Deformation and fracture, Die, High strength steel, Trimming, Wear

Supervisor

A.Alpas

Supervisor

D. Green

Rights

info:eu-repo/semantics/embargoedAccess

Abstract

Trimming is a shearing operation that removes excess material from stamped parts, and minimizing the associated tool wear will help to reduce manufacturing costs and improve the edge quality of trimmed parts. The objectives of this research are to (i) identify the damage mechanisms in trim die materials, (ii) characterize the damage in the vicinity of the cutting edge of trim dies and determine their evolution with increasing numbers of trimming cycles, (iii) study the micromechanisms of wear-induced plastic deformation and damage at the sheared edge of AHSS. The mechanical properties of the trim die materials (D2, Uddeholm Carmo and Caldie) were determined. It was found that Caldie, with nano-sized carbides uniformly distributed in the prior austenite grains, exhibited as much as 169% and 281% increase in stress and strain, respectively in tension compared to D2. The fracture surfaces of Carmo and Caldie exhibited dimples and local quasi-cleavage fracture.

Worn D2 trim dies were removed from an industrial-scale semi-production trimming facility after predetermined numbers of trimming cycles of DP980 strips. White light interferometry was used to measure the wear parameters as a function of the number of trimming cycles at the same locations on the trim edge of industrial scale dies. A systematic method was developed to measure the volumetric loss on the industrial scale trim dies. Damage features found on the upper and lower sliding planes consisted of sliding-induced wear in addition to fracture-induced chipping. Chipping was the dominant damage mechanism on the upper and lower impact planes. Fracture induced chipping was more prevalent on the lower sliding plane, whereas sliding induced wear became more pervasive on the upper sliding plane as trimming progressed. In terms of trimmed parts, the severity of plastic deformation beneath the sheared edge was estimated using martensite plate displacements in the ferrite matrix as metallographic markers. Maintenance of the trim die was recommended after 60,000 trimming cycles as wear-induced damage on trim die edge affected the tensile ductility of the trimmed parts (the fracture strain decreased from 14% after 40,000 trimming cycles to 9% after 80,000 trimming cycles).

A laboratory-scale trimming die was developed with a built-in load cell and laser transducer to generate the load/displacement curves during trimming. Partial trimming trials were conducted by using sharp die edge either with a support pad under the scrap piece (robust trimming) or without it (conventional trimming) in order to investigate the damage mechanisms in the sheared sheet specimens progressively. A new metallographic sample preparation methodology was applied to preserve the progressive plastic deformation within the shear affected zone (SAZ) for analysis. In conventional trimming, the shearing mechanisms changed as the clearance was increased to 30%, the crack initiated from the upper surface and propagated through to the lower surface resulting in a large, tensile-type burr. However, in robust trimming by using sharp trim die edge, the crack initiated first from the lower surface which led to a significant reduction of the height of the burr by 4.6 and 16.3 times at 20% and 30% clearances, respectively.

From an engineering point of view, this study determined the mechanical and tribological processes that damage the edge of trim dies, systematically quantified the wear losses on industrial-scale trim die edges, and correlated the trim die wear with the loss of ductility of the sheared edge of the part itself. The study on progression of the damage within the SAZ during trimming process by using the instrumented lab-scale trimming facility will further improve the sheared edge quality and promote the implementation of AHSS in the automotive industry which will help to further reduce the weight of vehicles.

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