The TMT process combines heat treatment (thermo) and plastic deformation (mechanical) to effectively influence grain refinement and help create a material with improved characteristics.
Specific effects of the TMT process include increased hardness, decrease in electrical conductivity and much improved distribution of alloying elements.
Thermo-mechanical treatment (TMT) is a kind of strengthening method which combines plastic deformation with heat treatment. It can refine the grains effectively, change the aging decomposition characteristics make the dispersion strengthening exceed the phase hardening strengthening. It can also restrain the recrystallization and increases the dislocation density for the alloy so that the titanium alloy obtains high strength and satisfactory ductility-toughness thus improving the fatigue strength, creeps rupture strength, thermal strength and corrosion resistance. However, compared with the traditional deformation process, the microstructure mechanism during TMT may be much more complex, which mainly in the restoration mechanism will change according to the base metal (BM) characteristics such as the initial microstructure (rolling, annealing state) and physical characteristics (mainly stacking fault energy). The stacking fault energy (SFE) is known to strongly influence the hot deformation behavior of metallic materials. These are the primary restoration mechanisms in deformation and heat treatment, including dynamic recovery, discontinuous dynamic recrystallization and continuous dynamic recrystallization and so forth.
One research investigated the influence of deformation on the properties of aged EN AW-6060 and EN AW-6082 alloys (T9 temper). Deformation was conducted using cold rolling at relatively small deformation degrees compared with the other studies. According to the results obtained, the following conclusions can be drawn:
• The applied TMT has a significant impact on the mechanical and structural properties of the investigated alloys. A greater influence of TMT is achieved in the EN AW-6082 alloy that contains more alloying elements.
• The hardness and microhardness gradually increase with the deformation degree after aging (T9 temper). The hardness slightly increases from 95 HV10 for the aged EN AW-6060 samples to 100 HV10 after the aging and 40-% deformation. This increase is more evident for the EN AW-6082 alloy where the hardness increases from 124 HV10 in the aged state to 150 HV10 after the 50-% deformation. The relative increase in the microhardness of the deformed samples relative to the aged ones was 17.3 % (from 110 HV0.1 to 129 HV0.1) for the EN AW-6060 alloy and 29.4 % (from 146 HV0.1 to 189 HV0.1) for the EN AW-6082 alloy.
• The electrical conductivity of the aged samples gradually decreases with an increase in the applied deformation. The relative decrease in the electrical conductivity of the deformed samples relative to the aged ones is 5.42 % for EN AW-6060 and 8.39 % for EN AW-6082.
• The matrix of the investigated alloys is covered with finely dispersed particles of the metastable β" phase. The ratio of Mg:Si in the metastable β" phase in deformed samples is closer to the ideal, indicating a positive effect of the deformation on the distribution of alloying elements.
• Maps of the element distribution in the post-deformed state show excellent homogeneity and distribution of the elements. The clustering of iron, nickel and silicon atoms is noticed while magnesium and silicon are uniformly distributed within the matrix.
References
1. Yan Han, Fei Zhao Yuan Liu, Chaowen Huang: Quantitative Relationships between Mechanical Properties and Microstructure of Ti17 Alloy after Thermomechanical Treatment, Metals 2020, 10, 67; doi:10.3390/met10010067;
2. U. Stamenković et al.: Influence of thermomechanical treatment on the properties of commercial aluminium alloys from the 6000 series, ISSN 1580-2949, MTAEC9, 54(4), 2020, p.489-494;
3. H. Jafari, M.H. Idris, A. Ourdjini, G. Payganeh: Effect of thermomechanical treatment on microstructure and hardness behavior of AZ63 magnesium alloy, Acta Metall. Sin. (Engl. Lett.) Vol.22 No.6, December 2009, p. 401-407.