Thermomechanical processing was first coined for steels in the 1950s, but it had been around since the 1850s, when Kirkaldy conducted extensive research linking processing, tensile properties, and microstructures, including fractographs [13]. Often, it was practiced without complete understanding, as for eutectoid steel in rolling and cooling and in patenting wire with transformation and wire drawing [4]. For Al, improved processing schedules were found for Al-Mg-Si alloys in press quenching after hot extrusion and in solution treating before cold impact extrusion [510]. The wide variety of TMP for Al is found in a recent book [11] that relates it to all classes of alloys and to rolling [1216], extrusion [8, 9, 16], and forging [17]. TMP has spread to many metals as noted in the adjoining papers developed to level that modeling is possible [18].

In broad definition, TMP is a sequence of temperature and strain operations to produce a shape and a microstructure with outstanding properties for that alloy [19, 20]. If a step obliterates the previous microstructures, then the whole sequence does not qualify as TMP [7, 10, 11, 16]. Time or space breaks are permitted, for example, multistage cold rolling to suitable strain, annealing to a fine grain size and finally deep drawing or preaging an Al autobody panel so that precipitation is completed in the paint baking process [21]. The processing becomes more valuable if several steps can be combined, thus saving in labor, equipment, and energy [79, 16]. Preliminary research must be conducted to understand the effects of ranges in composition, temperature, and strain rate, as exemplified in the papers that follow.

Al and Mg alloys have no allotropic transformations but can be precipitation hardened. Generally, Al can be worked over the range of 200–500°C [19, 20, 2224], whereas Mg has insufficient operating slip systems below 200°C and above that has less uniform substructures and lower ductility than comparable Al alloys [25]. Dislocation substructures vary by temperature and strain rate have significant effects on particle distributions [10, 11, 24] and in superplastic behavior [6, 26]. The paper by M. E. Kassner et al. compares quench sensitivity of two Al-Mg-Si alloys, and Fare et al. consider the effect of severe deformation on aging. The influence of temperature on an Mg alloy is reported by Yeom et al.

Steels and Ti alloys have an allotropic transformation [3, 27, 28] that develops a variety microstructures dependent on composition and cooling rate usually with different precipitation behaviors for the same alloying [1, 3, 4, 29, 30]. Structural refinement can be enhanced in the course of shaping by changing from one phase to another or by manipulating the duplex structure [3, 31, 32]. Steels have by far the widest selection of TMP, such as controlled rolling for ferrite grain refining and carry-over of substructures into bainite or martensite to name a few; each of these with many options depends on the solute or precipitation alloying [1, 18, 29, 33]. Dislocation substructures play a significant role in nucleation of the new phase or are carried through a martensitic type, as well as nucleating particles [30, 31, 34]. Fundamental aspects of these possibilities are clarified in the papers by Yeom et al. (extrusion Ti 6 Al-4V) and by Li et al. (martensite Ti-3.5Al-4.5Mo).

Hugh J. McQueen
Enrico Evangelista
Michael E. Kassner
Chongo Soo Lee