Deformation Processing (including Rolling, Extrusion, and Sheet Forming)
There is a need to develop improved constitutive laws (e.g., yield surface models) that take into account the effects of temperature, strain rate, and the various deformation mechanisms. These new constitutive laws are necessary for the construction of accurate models of deformation processes. Issues of anisotropy and tension/compression asymmetry and how to model them, reduce them, or take advantage of them were all discussed. In fact, there is significant overlap in the needs for Deformation Processing and those of the Alloy Development and Mechanical Performance; see these previous sections. It is emphasized that there is a difference between fundamental understanding of the deformation mechanism behavior and a constitutive model that can be employed for predicting the forming behavior or performance of an actual component. While the fundamental understanding of deformation mechanisms has been developing through the use of crystal plasticity modeling, the need remains for constitutive models which are relevant to engineering problems. Combined methods have been developed in other alloy classes, and these should be further explored for Mg alloys.
There is a need to better understand the interactions between the deformed state and that which evolves during recovery/recrystallization. Such issues have been heavily studied, in ferrous and aluminum alloys, but much less detail is available for Mg alloys. There is significant interest in developing a more quantitative understanding of dynamic recrystallization in Mg alloys, as this process may be the key to sustaining the large strains imparted during hot deformation processes, such as hot rolling and extraction. An improved general understanding of microstructure evolution during hot, and warm, deformation and its relation to recrystallization and grain growth is needed for the principal Mg alloy types. The purpose of this understanding is to apply it to the control of product microstructure, e.g., control the recrystallized grain size. The effects of Zr and rare-earth elements, particularly, must be understood. The need to understand the “rare earth effect” was noted specifically and emphatically, as well as our collective ignorance as to the specific mechanisms behind this effect. Other schemes of microstructure control based upon Hornbogen’s concept of “combined reactions,” e.g. recrystallization and phase transformation seem worthy of exploration.
Before moving on to the details individual deformation processing methods, it is mentioned that almost all metal forming processes, involve significant “friction.” The metal has a surface morphology and chemistry, which is distinct from the bulk, and interfacial mediation is applied in the form of liquids, solid particulate, films, etc. The tool/workpiece interface evolves (and hence can be described with suitable state variables) just like the bulk. It is suggested that we have put so much emphasis on modeling the bulk material, that when it comes time to actually simulate a forming process, it is absurd to appeal to an eighteenth century “Law” and use a single "friction coefficient." New research should be conducted to develop "interfacial constitutive models." This recommendation applies far beyond the present scope of Mg alloy processing, but is certainly relevant since the frictional behavior of Mg appears to be largely unknown, particularly at forming temperatures and conditions.
Extrusion offers the potential to be a high volume source of wrought magnesium products. The hydrostatic state of stress and elevated temperature present in deformation zone during extrusion allows achieving much higher strain than in many other technological processes. However, they have historically been plagued by low production rates due to limited workability and problems of hot-shortness (depending upon the alloy.) In any event the process window for Mg alloys tends to be smaller than for Al alloys. There is a need for better understanding of the required degree of homogenization of billet material. This is usually achieved by deformation processing. There is continued interest in developing alloys, which can be extruded at higher speeds, but have the desired physical properties in the finished product. Some felt that the Mg-Zn-RE alloy systems merit further investigation. Additionally, it was mentioned that the RE effect on texture was not fully understood, though there has been significant progress over the past five years. On the finished product side, there are problems associated with non-uniform grain size and texture-induced anisotropy.
Plate and Sheet Rolling
Final product should be fine grained, isotropic, and possess microstructural stability necessary for warm forming. There is also interest in developing age hardenable sheets, since the present mass-produced sheet alloy, AZ31, does not respond to heat treatment. Higher strength, heat treatable Mg alloy sheets would open up new application opportunities and are already under development by both US and Korean producers. The “Achilles heel” of many of the sheet/plate alloys applications, relative to die cast alloys, is corrosion. While the base corrosion rate of AZ31 is not significantly higher than AZ91D, thin sheets are much more sensitive to localized (particularly galvanic) corrosion than presently used die-castings, which are thick and most frequently employed in dry or oily environments.
There is interest in developing alternative rolling processes, such as asymmetric and high speed rolling, both of which have demonstrated potential in preliminary trials. In a more conventional sense, there is interest in comparisons of microstructures and sheet properties developed on reversing coil mills vs. unidirectional (e.g. tandem) rolling mills. As has been mentioned in many of the discussions above, there is a sense that fine grained materials may be part of the answer. In this regard, Mg grain refinement in casting, such as that provided by twin-roll casting (TRC) possibly combined with severe shear is of interest. An extreme angle endorsed by some participants was the exploration of rapidly solidified, powder metallurgical routes.
Because of very limited cold formability, Mg alloys have not historically been used in sheet form to produce components with complex shapes. The hot formability of Mg alloys, however, can be outstanding, e.g., superplastic. This excellent hot formability is often attributed to the fine grain structure (relative to many Al alloys and steels) that can be readily developed during conventional wrought magnesium production. Plate applications are, for all practical purposes, presently limited to photo-engraving plate, which benefits from ease of machining and etching and tooling applications which benefit from stiffness combined with low inertia. There is current military interest in broadening the applications of plate, but concerns do exist regarding the mechanical properties at high strain rates.
There was a sense that novel hot- and warm-forming processes (non-isothermal, press quench, etc.), which overcome the difficulties of current technologies or take advantage of particular opportunities specific to Mg alloys, should be explored. There is interest in leveraging the significant current research into alloy and process development, as well as constitutive modeling relevant to warm and hot forming. A specific need to develop new and innovative “joining” processes, such as warm hemming, that take advantage of local heating was recognized. Friction and tribological issues influence all sorts of sheet forming operations, but there is very little knowledge of these phenomena with respect to Mg alloy sheet materials. The success of future forming technologies for Mg alloys will rely upon improved understanding of tribological interactions as a function of alloy, surface condition, temperature and surface deformation (strain rate of surface deformation, surface morphology changes, oxide breakage/formation, etc.)
A clearer answer regarding the optimal grain size for forming of various alloys needs to be provided. Many researchers are suggesting that a finer grain size may produce better forming behaviors (e.g., down to 1 or 2 um), but others suggest that a larger grain size can promote higher greater strain hardening to delay plastic instability. Additionally, questions concerning failure mechanisms abound. What are the failure initiation sites (are they inclusions, twins, shear bands, etc.?) Does void nucleation and growth or necking control ultimate failure during sheet forming? Is there a relationship between alloy content and the role of grain boundary sliding? As mentioned in the deformation processing section, there is a great need to develop our understanding of recrystallization, both static and dynamic. There is a need to better understand how to appropriately suppress or enhance recrystallization for different applications. Finally, an alloy which can be formed and subsequently aged to increase strength is viewed as desirable; perhaps a press-quenching process and an applicable alloy should be developed in parallel. The group feels that answers to these and many other questions about optimal microstructure and alloying may be specific to the target forming temperature and strain rate conditions. Hence, this section is divided into three sections: high temperature (frequently superplastic), warm, and room temperature.
High-temperature forming: Microstructure evolution during hot forming is still poorly understood. It has not been adequately quantified or modeled, for the purpose of prediction. The optimal alloy composition for hot forming is not known. It is known that texture plays a primary role in determining the behavior at low homologous temperatures. However, it is not known what role texture plays during hot deformation.
Warm Forming (< 250ºC): The basics physics of warm deformation in Mg alloys is not well understood. For example, solute-drag creep in Mg is not understood, but solute-drag creep is recognized as critical for developing warm formability in Al alloys. The idea of press quenching merits further exploration. Multi-physics modeling to enable exploration of novel non-isothermal conditions is viewed as essential. For example, if one were to develop a press-quench process, questions arise concerning the relevant constitutive model to use when the temperature range in the material could span hundreds of degrees Kelvin. Understanding tribological effects and determination of the optimal lubricants for forming in this temperature regime are needed.
Room-temperature forming: All of the aforementioned issues relating to grain size, texture, etc. are pertinent here. A form-anneal-form, multi-step process may be a viable solution to forming somewhat complex shapes. However, limitations of such an approach, such as production speed, must be explored. It is emphasized that determining the stress exponent describing the hardening rate during uniaxial tension testing is not a standalone answer to the question of multiaxial formability. Multiaxial experiments and modeling are required, and this is also true for warm- and hot-forming applications. There is strong evidence that the “RE effect” can improve low-temperature formability. A better understanding of this effect is required. Can it be induced by alloying with other elements? Spring back is anticipated to be complicated in a magnesium alloy which can be formed at low temperatures, due to the low modulus and relatively high strength these alloys exhibit, in combination with the effects of twinning.