PSI - Issue 42

Tsanka Dikova et al. / Procedia Structural Integrity 42 (2022) 1520–1528 Tsanka Dikova, Angel Anchev, Vladimir Dunchev, Dzhendo Dzhendov, Yavor Gagov / Structural Integrity Procedia 00 (2019) 000 – 000 5

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• Determination of the modulus of elasticity A combined approach, including three-point bending test and subsequent finite element (FE) analysis, is applied to define the Young's modulus. Physically, the experiment represents bending of a beam on two supports loaded with punch. The supports and punch are rounded with 1 mm radius. The beam is bent in the middle up to 0.3 mm, in order to preserve the linear relationship between force and displacement, thus ensuring loading only in the elastic region. Three samples of the milled and SLM Ti6Al4V alloy are subjected to bending with a ZWICK Roell Vibrophore 100 machine (ZWICK Roell, Germany). The specimens are preliminary heat treated according to the modes from Table 2, imitating firing of the porcelain coating. In order to minimize the computational time and due to the test symmetry, a FE model was used only for half of the calculation scheme. The 3D FE stress analysis was carried out via ABAQUS software. The model consists of

Fig. 2 Experimental set-up for adhesion strength test.

three bodies - beam, punch and support. The beam was modeled as a linearly deformable body with the absolute dimensions of the tested samples (the length of the beam is half of the real one). The punch and the support were modeled as cylindrical analytical rigid bodies with a base radius of 1 mm. In order to read the force accurately for the entire load duration, the punch displacement and time were adopted to be 0.3 mm and 0.3 s, respectively. The Poisson's ratio for the titanium alloy was assumed to be 0.33. For fitting of the Young’s modulus, an iterative approach was developed, in which the magnitude of the modulus varied to match the force-displacement dependence obtained by the experiment with this in the FE model. 3. Results obtained The values of the modulus of elasticity of heat treated Ti6Al4V alloy, determined by the combined methodology, are 179.3 GPa for the milled samples and 125 GPa for the SLM ones. They are higher than the data given in the literature (110-136 GPa for the milling alloy) (Antanasova et al. (2018), Antanasova et al. (2020)) or from the manufacturers (110-115 GPa for the SLM alloy used) (Technical data sheet (2019)). The force, at which the porcelain coating is debonded from the metal substrate, is greater in the milled samples compared to the SLM ones (Fig. 3). In the group of milled samples, the fracture force of the ceramic of the sandblasted plates is highest (9.79 N), followed by those of samples with combined treatment (8.86 N) and the control subgroup (7.94 N). The porcelain coating of the samples from subgroup 3, with surfaces treated only with bonding agent, is destroyed in the lowest load (5.59 N). In the group of SLM specimens, the porcelain of the control subgroup is debonded at the highest force (7.02 N), followed by the sandblasted and the specimens treated with bonding agent (6.54 N). The coating in the subgroup with combined treatment is fractured in the lowest load (5.33 N). The deformation of the metal substrate, at which the porcelain coating of the specimens debonds, follows the trend of the loading change (Fig. 4). In contrast to the previous parameter, the ceramic of the samples made by SLM destroys at larger deformations compared to the milled ones: 0.032-0.071 mm and 0.018-0.050 mm, respectively. The adhesion strength of the porcelain to milled Ti6Al4V alloy is highest in the specimens with sandblasted surfaces (30.89 MPa), followed by those with combined treatment (27.96 MPa) and the control group (25.04 MPa) (Fig. 5). These values are equal to or higher than the required by the standard 25 MPa. The porcelain coating on the surface treated with bonding agent is characterized by the lowest adhesion of 17.63 MPa. In the SLM alloy, the adhesion strength of the porcelain coating is in almost the same range. It is highest in the control subgroup (31.04 MPa), followed by the sandblasted samples (27.91 MPa) and those treated with bonding agent (25.04 MPa). The