Numerical Investigation of the Material Behaviour during Compression Tests for Samples with Rough Surfaces Represented in Different Geometry Scale Factors

Abstract

In modern technological operation, the original sample could be provided by so-called pre-operation or pre-forming follows prior the main operation. In such case, the surface roughness of the tool set is directly owned by the surface of the pre-formed workpiece, as less rigid body compared with the tool set. In metal forming practice there are mainly several pre-forming processes, among them roll forging, cross-wedge rolling and electro-upsetting. In any non-convenient forging operation route, where pre-forming is done by casting, powder sintering or any additive technology (e.g. SLS, EBM), the surface roughness and wellness obtained pre-formed workpiece could influence stronger on the quality of the end part after main forging operation. In order to investigate the different materials’ behaviours depends on the surface artificial roughness the finite-element analysis (FEA) was carried out for the simple compression tests, thereby the cylindrical, made from aluminium and titanium alloys, and ring specimens, made from steel, were compressed between two flat dies with modelled zero and non-zero roughness at room and elevated temperatures. The behaviour of the lubricant (fluid phase) positioned between two neighboured peaks of the roughed surface was investigated in one direction material flow test as well. The results have shown the strong pressure increase in the bottom area of the peaks, which correspond the practice case, when the fluid phase is not completely evaporated after lubrication. Further, it can course the different microstructure evolution during hot forging operation and during cold bulk forming operation, additional surface pressure can result the material hardening near to the contact zone (not investigated here). In commonly, in observed results the peaks of the roughness can be easily broken due to enormous tangential stresses, as it is well known from the fundamental investigations. Moreover, the computation of geometry scaled tools and workpieces of the ring compression and one-direction material flow tests resulted the exponential dependency against the deformation forces.

The numerical analysis of selected defects in forging processes

“…Also traditional approach for modelling of ductile fracture indication is of limited use in case of instability of metal flow, which does not produce apparent visually detectable defects. Supplement to standard and user-defined functions of commercial codes are coefficients combining auto-calculated parameters, which make it possible to interpret the indication with accordance to one's needs, for example relative density for porous materials, coefficients for closing pores in ingots, surface representation, abrasive die wear factor or thermal parameters (Wojtaszek & Bednarek, 2011;Chyła et al, 2011;Skubisz et al, 2008a;Petrov et al, 2016;Hawryluk et al, 2018;Skubisz et al, 2012;Stebunov et al 2011). Altogether, they form a versatile design and diagnostics tool to be used in determination and optimization of forging process parameters.…”