In industrial forming processes, the metals and alloys are subject to complex strain, strain-rate, and temperature histories. Understanding the flow behaviors of metals and alloys in hot working has a great importance for designers of metal forming processes. In order to study the workability and establish the optimum hot formation processing parameters for some metals and alloys, a number of research groups have made efforts to carry out the thermo-mechanical experiments (compressive, tensile and torsion tests) over wide forming temperatures and strain-rates, and some constitutive equations were developed to describe the hot deformation behaviors. This paper presents a critical review on some experimental results and constitutive descriptions for metals and alloys in hot working, which were reported in international publications in recent years. In this review paper, the constitutive models are divided into three categories, including the phenomenological, physical-based and artificial neural network models, to introduce their developments, prediction capabilities, and application scopes, respectively. Additionally, some limitations and objective suggestions for the further development of constitutive descriptions for metals and alloys in hot working are proposed.

Numerical simulations can be truly reliable only when a proper material flow stress relationship is built because of its effective role on metal flow pattern as well as the kinetics of metallurgical transformation. Based on the hot compressive deformation behaviors of 42CrMo steel, a comprehensive constitutive model has been developed to predict stress–strain curve up to the peak stress. A new material parameter L, which is sensitive to the forming temperature and strain rate, was proposed in the developed constitutive model. Additionally, the mathematical models to predict peak stress and peak strain were established based on experimental results. The stress-strain values predicted by the developed model well agree with experimental results, which confirmed that the developed constitutive equation gives an accurate and precise estimate for the flow stress of 42CrMo steel.

The effects of the deformation temperature and strain rate on the high-temperature flow behavior of 7075 aluminum alloy were studied by hot compressive tests. Based on the experimental data, the efficiencies of power dissipation and instability parameter were evaluated, and processing maps were constructed by superimposing the instability map over the power dissipation map. Microstructural evolution of 7075 aluminum alloy during the hot compression was analyzed to correlate with the processing maps. Results show that (1) The flow stresses increase with the increase of strain rate and the decrease of deformation temperature; (2) The high-angle boundaries and coarse precipitations distributing in the grain interior/boundaries, which may result in the deep inter-granular corrosion and large areas of denudation layer, should be avoided in the microstructures of the final products; (3) The optimum hot working domain is the temperature range of 623-723 K and strain rate range of 0.001-0.05 s-1.