Additive manufacturing (AM), the layer-by-layer building-up of parts from metals and alloys represents an option for small scale production due to advantages compared to conventional manufacturing such as reduction in lead-time, reduced material wastage, freedom in the design of the parts [1–8]. The opportunity of manufacturing of metal parts of any complexity in a short term is especially important in aerospace and medical industries, since their manufacturing via conventional technologies including casting, metal forming and subsequent machining is expensive and time-consuming process. In these industry fields, Ti-6Al-4V titanium alloy is of great interest due to its high strength, low density, excellent corrosion resistance and biocompatibility [8–10]. Approximately 50 % of all titanium alloys used in aerospace fall to the lot of Ti-6Al-4V alloy. Among different methods of AM of parts from Ti-6Al-4V alloy, selective laser melting (SLM) is widely used [9–19].
In view of this, the study of strength, plastic and fatigue properties of SLM-produced Ti-6Al-4V alloy and its applicability for responsible parts is of special interest [8, 9, 11, 12, 13, 15, 16, 20–28]. Table 1 presents a summary of the mechanical properties of Ti-6Al-4V alloy produced by SLM of different conditions – initial condition (as-built) not machined, machined, stress-relieved, annealed, after hot isostatic pressing (HIP) in comparison with material produced by conventional manufacturing methods – cast, wrought, wrought and mill annealed. Spread in values in Table 1 is explained by choice of different process parameters.
As can be seen from Table 1, strength properties of as-built SLM-produced Ti-6Al-4V alloy without any post-treatment are comparable with those for wrought material, but ductility and fatigue life are significantly lower. Heat treatment allow to slightly improve ductility and fatigue life of SLM-produced Ti-6Al-4V, however their values still less than those for wrought Ti-6Al-4V. Besides, it must be noted greater spread in values of mechanical properties of SLM-produced Ti-6Al-4V. To make both ductility and fatigue life of SLM-produced Ti-6Al-4V comparable to wrought, special treatment by HIP is necessary. HIP allows removing bulk defects, undesirable martensitic microstructure, porosity, residual stresses which are typical for as-built SLM-produced Ti-6Al-4V [7, 8, 29]. However, HIP is very expensive process due to high equipment cost, besides it’s necessary to note limitations related to treatment of large-scale parts [30].
As shown above, in order to improve the mechanical properties and reliability of parts produced by SLM, a thermo-mechanical post-treatment is necessary. An alternative to SLM with subsequent HIP can be a combination SLM with deformation post-treatment. One strategy is presented in [31–36]. In [31], the technology of AM including plastic deformation each built-up layer by the roller is suggested. This method provides better mechanical properties for Ti-6Al-4V in comparison with wrought alloy. In [32, 33], the method of ultrasonic AM which includes rolling each built-up layer by the roller with oscillation along its axis is presented. In [36], deformation is suggested to be applied by a hammer immediately after deposition. Described methods of deformation post-treatment allow decreasing grain size and porosity in AM-parts.
Another strategy is described in [37–39]. Semiatin S.L. [37], Sizova I., Bambach M. [38, 39] suggested novel processing route “AM + hot forming” consisting of two stage. At the first stage, an appropriate pre-form from Ti-6Al-4V is produced by SLM and at the second one, built pre-form undergoes die forging with the aim to obtain final dimensions and mechanical properties of the part. This method is of special interest because it combines advantages of SLM and die forging. On the one hand the processing route “SLM + hot forming” compared to conventional route “cast + forging + machining” provides significant reduction of processing route steps, reduction in lead-time and in material wastage at the same level of mechanical properties of parts, on the other hand suggested processing route compared to SLM provides high performance characteristics without expensive downstream post-processing by HIP. Processing route “SLM + hot forming” is especially efficient for producing parts from Ti-6Al-4V titanium alloy but applicable for large-scale and mass production.
To develop this approach, it is necessary to know stress-strain curves, flow behaviour and microstructure evolution of Ti-6Al-4V alloy. There are several studies related to rheological properties of AM Ti-6Al-4V alloy [8, 11, 13, 15, 37, 38, 39]. In [8, 11, 13, 15], tensile curves at room temperature of SLM-produced Ti-6Al-4V are presented only. Such tests are carried out at room temperature up to strains e equal to 0.12–0.15 and don’t correspond to hot working conditions. In [37], stress-strain curves of laser-deposited Ti-6Al-4V obtained via hot compression method at temperatures from 815 to 1010 °C, at strain rates ξ of 0.1 and 5.0 s− 1 up to a true strain e of 0.9 are given. In [38, 39], stress-strain curves of SLM-produced Ti-6Al-4V obtained via hot compression method at temperatures from 850 to 1000 °C, at strain rates ξ from 0.001 to 1.0 s− 1 up to a true strain e of 0.9 are presented. Besides, it is worth to mention the stress-strain curves of conventional manufactured Ti-6Al-4V in [40–50]. Bambach and Sizova [38, 39] reported that flow stresses of SLM-produced Ti-6Al-4V show lower values than that of conventional wrought in the temperature range of 850–950 °C. This can relate to a different initial microstructure of SLM-produced and conventional wrought materials.
Published studies on stress-strain curves of SLM-produced Ti-6Al-4V was made under isothermal condition in the temperature range 850–1000 °C, however the difference in temperatures in individual regions of the parts during die forging (which is typically non-isothermal process) may be large and reach values of 100–160 °C [45, 51]. In turn, a large temperature gradient leads to an inhomogeneous deformation of forgings. For this reason, the aim of this study is to investigate the flow curves and flow behaviour of SLM-produced Ti-6Al-4V alloy in the wide temperature range in comparison with conventional wrought Ti-6Al-4V alloy. In addition, microstructure evolution, softening rate and temperature sensitivity of SLM-produced Ti-6Al-4V also were assessed. This data can be used in the design of processing route “SLM + hot forming” of parts from Ti-6Al-4V alloy.