On the Machinability of Ductile and Strain Hardening Materials - Models and Methods for Analyzing Machinability

Sammanfattning: As quality and performance demands on today’s products increases, more and more advanced materials are being used during modern production. The problem is however that this in turn place new demands on the machining processes utilized. Even though a significant amount of research has been published on the machining of these materials knowledge is still limited in several crucial areas. A problem with machining research is that it often relies heavily on quantitative data primarily obtained through experimental investigations. Due to the substantial amount of potentially different machining cases it could be difficult to generalize the obtained results to other scenarios. In this dissertation it has been attempted to model the investigated phenomena through using universal physical relationships. Even though this might result in a larger modeling error for the specific case investigated the author sees a great advantage of being able to have a physical explanation to the obtained results. The aim of this dissertation has been to increase the knowledge on, and to a certain extent predict, the machinability of some common ductile and strain hardening materials. The research has focused on evaluating duplex stainless steel, Ti6Al4V and Alloy 718. However, the proposed models have been constructed in a way as to aid future implementation for other workpiece materials. A central pillar of the research has been the influence of the stagnation point and the related minimum chip thickness. This aspect influences all machining operations and could potentially have a significant impact on the machinability, not least for ductile and strain hardening materials. During this research it was found that even though cutting conditions have a major influence on the value of the minimum chip thickness, material factors such as ductility and strain hardening should not be neglected as these also influence the obtained value. In turn, it was found that the minimum chip thickness could to a certain extent be used to explain the obtained workpiece surface roughness. Also, the tool surface roughness was found to have a determinate influence on the mechanics of the machining process. During the present research it was also found that it is difficult to predict the tool life using conventional models for the investigated materials, essentially due to their high strength at elevated temperatures, adhesive behavior during machining, and low thermal conductivity. The influence of these properties commonly results in rapid and unpredictable wear of the cutting tool. Plastic deformation of the cutting tool is always a concern when machining these materials and a first step towards establishing a method for measuring the initiation of plastic deformation by using the measured cutting force has been proposed. Also, through using a proposed method for determining the potential machinability of a specific workpiece material these effects could be reduced through the use of reasonable process parameters before commencing production. Methods for improving the machining process in terms of for example part cost or sustainability has been developed as part of this research. Even though each of these methods only improves a small part of the whole production process these improvements should not be neglected as all parts of the process should be optimized in order to achieve a truly sustainable and cost efficient machining process.