Screw Extrusion of Light Metals: Development of materials, characterization and process analysis
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The presented work highlights the possibilities inherent in a new method for continuous extrusion, i.e. hereafter named metal continuous screw extrusion (MCSE). This was done by expanding the knowledge of the process itself, as well as exemplifying a wide range of possible novel applications, i.e. formation of composites, processing of rapidly solidified aluminium (RS-Al) and the combination of MCSE with severe plastic deformation (SPD). In addition, hard-to-deform binary Al-Mg alloys processed through MCSE and conventional ram extrusion (RE) have been compared. The thermomechanical history of the two differently processed materials was depicted through the difference in microstructures and mechanical properties. The material processed through MCSE has a more complex deformation history compared to the material processed by RE. It was also shown that the material processed through MCSE contained finely dispersed oxide particles, aligned in patterns related to the material flow. Moreover, in the work related to formation of composites, on found that MCSE could be used as a one-step method to form composite profiles by mixing metals or alloys before extrusion. By combining Al– and Mg alloys as feedstock, it was possible to obtain fully dense profiles, having domains of the input constituents as well as intermetallic phases formed in the domain boundary regions. These intermetallics are known to be brittle, and a pronounced formation of such phases dominated the mechanical properties, i.e. reducing the overall ductility. However, the composites exhibited an increased strength by increasing Mg to Al ratio of the feedstock. Thus, the bi-metal composite trials were a promising starting point, motivating for future exploration of applying MCSE to generate composites, e.g. metal-metal or metal-ceramic combinations. In the part related to processing of RS-Al, MCSE was applied to both commercial RS-Al flakes, as well as novel melt spun alloys. The latter consisted of two series (binary Al-Mg alloys and Al-Mg-Si alloys), which were produced in an in-house rebuilt melt spinner. These alloys were produced after promising results obtained using MCSE on a commercial RSP6061 and a melt spun 6060.35 alloy. Analysis of the Al-Mg-Si alloy having a Mg and Si content well above the solubility limit (4wt.%Mg and 5.4wt.%Si), showed that the melt spun ribbons consisted of a structure originating from a mix of columnar and dendritic growth, having an intercolumnar phase of Mg and Si. Moreover, even though these growth domains were discovered in TEM and were very fine, they were likely predecessors to coarser particles precipitated from solid solution. The latter particles were observed after MCSE processing, giving medium strength and a low ductility to the resulting profiles. However, the TEM investigation indicated that the α-phase was super- saturated, motivating for future trials on slightly leaner alloys and more optimized processing. This might result in materials having beneficial properties. The binary Al-Mg alloys processed by melt spinning and MCSE were the same alloys as also processed by MCSE from machined chips or RE from cylindrical billets. Here, the profiles from screw extruded melt spun material had similar properties as the screw extruded profiles from machined chips. In addition, these binary Al-Mg alloys were used when combining MCSE- with SPD processing. Screw extrusion and subsequent high-pressure torsion (HPT) were engaged to show enhanced properties of Al-Mg alloys having a large amount of Mg in solid solution. Advanced TEM methods (precession STEM imaging and nano-EDS) were used to analyze the materials, and the HPT processed alloys had a grain size in the nanometer range. These materials had a high degree of Mg in solid solution (i.e. no domains were found, indicating atomic clustering as the largest possible scale), resulting in a very high hardness and strength. For an Al-10wt%Mg alloy, a Vickers hardness of approx. 100 HV0.3 and a tensile strength of 430 MPa was measured in the screw extruded profile. After further HPT processing this alloy exhibited a hardness of 220 HV0.3, and a tensile strength of 760MPa. The actual strength of this alloy could be even higher, but premature failure of small samples sensitive to defects at such high stresses limited the measurable strength. Further, a physical empirical model was developed for the MCSE process. This model is based on experience gained in the above described work, combined with observations of profile microstructures and of butt ends available after screw extrusion trials, but also prior work on the screw extrusion process. The model predict the effects of major processing parameters. In fact, it predicted an accumulated strain of ε ≈ 15 when applying the most common processing parameters. However, further verification and fine tuning of this model is needed. This work will hopefully motivate for further explorations in regard to MCSE processing and related development of novel materials.