Performance Optimization in Machining of Aluminium Alloys for Moulds Production: HSM and EDM

Aluminium alloys offer many machining advantages such as excellent machinability and finish degree with high cutting speed, low cutting forces, outstanding tool life. Elevated thermal exchange and weight reduction compared to steels are additional characteristics that lead to increasing applications in the automotive and aerospace industry and in the field of mould production. Relatively recent Aluminium alloys derived by aeronautical uses offer very high strength and hardness: the gap with steels is thus reduced or even reversed in terms of specific properties. Two examples are Al 2219 and Al 7050. If the first examples of Aluminium moulds for plastic injection were limited to preproduction, the properties of these new alloys match the requirements of medium production volumes, which are also the main market demand.The research focuses on two distinct operations typical of the tooling phase: High Speed Milling (HSM) and Electro Discharge Machining (EDM). In both cases the machining performance is evaluated in specific tests through a multiscale approach: measurements of the macroscopic process outputs are merged with the investigation of mechanisms at a microscopic level. The methodology enhances optimization chances with respect to traditional practice.As to milling operation, several studies prove that Aluminium alloys allow the advantageous adoption of high cutting speed, ensuring time and cost savings together with excellent surface finish and low tool wear. Yet, the traditional laws between cutting parameters do not apply to the field of high-speed machining and the mechanisms of chip removal still need investigation. In order to appraise the machinability of Al 2219 and Al 7050 alloys, high speed milling tests using uncoated carbide tools are performed. The tests are carried out on Al 7075 as well, an alloy of common employment for preproduction moulds, to provide control data. Cutting speed ranges from 600 to 2200 m/min and feed per tooth between 0.075 and 0.18 mm/tooth. Correlations between cutting parameters and surface finish, tool wear and chip formation mechanisms are studied with the aid of SEM observation and EDX semi-quantitative analysis. The best surface finish is obtained for Al 7050 with high cutting speed and low feed. No tool wear is observed in the considered conditions. Microscope observation of tools and chip proves that surface finish is ruled by tool-chip adhesion and that alloys having almost the same chemical composition can provide substantially different results due to grain dimension and distribution.EDM is used for the machining of complex shapes and textures typical of plastic injection moulds. Aim of this research is to verify the EDMachinability of the Aluminum alloys previously studied with regard to HSM. Literature studies on the specific subject are very rare. EDM tests are planned and performed using electrolytic copper electrodes on Al 7050 and Al 2219, plus Al 7075 as a control. A benchmark is chosen whose geometry points out the typical problems of moulds machining and allows an easy and significant dimensional characterization, because of the presence of different geometrical features and relative orientations. Roughing, semifinishing and finishing cycles are conducted; then the effect of process parameters on dimensional tolerance, surface finish and electrode wear is studied. Moreover, the electrodes and the machined surfaces are observed through optical- and scanning electron microscope (OM, SEM) to verify the presence and composition of deposits on the electrodes surfaces, as well as the eroded surfaces morphology on the workpieces. No remarkable differences can be identified as to dimensional tolerances between the three alloys. A slight trend to give lower roughness values can be noticed for the alloy 7050. The results are coherent with the regular morphology observed on the machined surfaces of this alloy. The observation of the erosion mechanisms allows to affirm that, for the same process parameters and very similar finishing results, the morphology of the machined surface can be considerably different, for example as to cracks density.