Influence of heating rate and atmosphere in the sintering of Ti-6Al-4V alloy with SPS

Titanium is one of the most commonly found metals in the earth's crust, and has a wide range of applications due to its superior corrosion resistance and high strength compared to its density. Being a biocompatible material, it is also possible to use it as biomaterial. Thanks to its surface appearance, it can also be used in areas where aesthetic is important. Titanium is an element that has allotropic properties with its multiple crystallographic structures. At room temperature, it has a hexagonal close packed (HCP) structure, also called α titanium. The structure turns into body centered cubic (BCC) form called β-titanium at 882 ± 2 ° C. The most commonly used technique for producing crude titanium is the Kroll process. Other than the Kroll process; the Hunter process, the Cambridge process, and the Armstrong process are other known techniques. Ti-6Al-4V alloy is the most widely used alloy among titanium alloys. Ti-6Al-4V alloy is used in aircraft bodies, aircraft engines, automobile engines, steam turbines, heat exchangers, drilling pipes, implants and sporting equipment. The Ti-6Al-4V alloy is an α + β alloy containing, by weight, 6% Al as an α-phase stabilizer and 4% V as a β-phase stabilizer. Ti-6Al-4V alloy can be obtained by casting, forging and powder metallurgy methods. Depending on the mechanical and thermal processes performed; equiaxed, bimodal or lamellar microstructure can be obtained. In Spark Plasma Sintering (SPS) method, which is one of Electric Current Activated/Assisted Sintering (ECAS) techniques, the pulsed direct current is applied under coaxial pressure with a low voltage. It is also known as Pulsed electric current sintering (PECS) and field assisted sintering technique (FAST). In spark plasma sintering technique,in addition to applied pulsed current, pressure and temperature; the Joule heating effect allows the temperature of the conductive material to increase. With the mechanical, thermal and electrical effects applied in the SPS technique, the heat doesnot only become volumetrically dispersed homogeneously at the macroscopic level in the compact state; it also disperses at the contact points of the particles at the microscopic level. Therefore, the sintering takes place faster. In this thesis, pre-alloyed Ti-6Al-4V powders with an average grain size of about 25 μm were sintered by spark plasma sintering method. Dr Sinter 7.40MK-VII model SPS device in Istanbul Technical University Metallurgical and Materials Engineering Department was used for sintering. Sintering was carried out by allowing the molded powder samples to stand at 1000 °C for 5 minutes under a pressure of 50 MPa. Experiments were carried out at heating rates of 50 °C/min, 100 °C/min, 200 °C/min and 250 °C/min in order to investigate the effect of heating rate. Experiments werecarried out in a vacuum environment and also in an argon gas environment. Thus, the effect of the use of vacuum and argon gas on the process has also been examined. Samples obtained after sintering were sandblasted to clean the surfaces of the samples. The samples were cut after the sand blasting operation. Densities were measured according to the Archimedes principle in order to calculate the relative densities of the cut samples. The relative density was calculated by proportioning the measured true density to the theoretical density of Ti-6Al-4V alloy, 4,43 g/cm3. Hardness measurements of the cut samples were performed. For microstructure analysis, specimens were examined under an optical microscope after polishing and etching with Kroll solution. Tensile strength and% elongation values were examined by tensile test. It is expected that Ti-6Al-4V alloys, especially used in aviation and automotive sectors, will have lamellar microstructure. According to the standards, the Ti-6Al-4V alloy is required to have a tensile strength of at least 900 MPa and a minimum elongation of 10%. In this present study, it has been examined at what conditions these values specified by the standards can be reached. It was observed that the lamellar α + β phase structure was obtained with all the heating rates (50 °C/min, 100 °C/min, 200 °C/min and 250 °C/min) in vacuum environment and also in argon gas environment. The relative density of 99,15% was achieved with sintering performed in a vacuum environment at a heating rate of 50°C/min. The relative densities obtained by sintering at heating rates of 100°C/min, 200 °C/min and 250°C/min were 98,56%, 96,23% and 96,07%, respectively. In vacuum experiments; at the heating rates of 50°C/min, 100°C/min, 200°C/min and 250°C/min,; 335,4 ± 4.8 HV, 333,2 ± 3.2 HV; 325,2 ± 4,7 HV and 323,2 ± 4,3 HV hardness values were obtained, respectively. The tensile strengths of the samples were obtained as 903,54 and 1077,13 MPa, in the sintering process with heating rates of 50°C/min and 100°C/min, respectively. At 200°C/min and 250°C/min heating rates, tensile strengths were 860,69 and 815,79 MPa, respectively. At all four different heating rates, % elongation is above 10% according to the results of the tensile test of the samples. In the sintering process with argon, relative density of 99,64% at a heating rate of 50°C/min; 99,36% at a heating rate of 100°C/min; 99,25% at a heating rate of 200°C/min were obtained. At a heating rate of 200°C/min, the relative density was 96,31%. In the experiments under argon gas, at the heating rates of 50°C/min, 100°C/min, 200°C/min and 250°C/min, 340,8 ± 3,7 HV; 337,6 ± 2,9 HV; 332,4 ± 2,4 HV and 330,2 ± 3,7 HV hardness values were obtained. The tensile strengths of the samples sintered under argon gas at heating rates of 50°C/min and 100°C/min were 914,68 and 913,41 MPa, respectively, above 900 MPa. At heating rates of 200°C/min and 250°C/min, the tensile strengths are less than 900 MPa with 897,96 and 850,33 MPa, respectively. At all heating rates, the% elongation at the end of the tensile test of the samples is above 10%. As the heating rate increases, the relative density values decrease for both under vacuum and under argon gas experiments. In line with the density values, the hardness values decrease with the increase of the heating rate. In the same way, the tensile strength decreases with increasing heating rate. However, as an exception, it has been found that the highest tensile strength is obtained as 1077,13 MPa at a heating rate of 100°C/min under vacuum. When sintering performed under argon gas, samples with higher density values were obtained as the result of higher shrinkage compared to the experiments performed under vacuum. Hardness values and tensile strengths are higher in experiments carried out in the argon atmosphere, due to the higher density. However, as mentioned above, the tensile strength of 1077,13 MPa obtained at a heating rate of 100°C/min in a vacuum environment is an exception. It is predicted that when sintering is carried out under an argon atmosphere instead of vacuum, plasma and spark effects are higher during SPS, resulting in samples with higher density and better mechanical properties. Especially in the automotive and aviation sectors, it is expected for Ti-6Al-4V alloys to have lamellar α + β microstructure, at least 900 MPa and 10% elongation. It was observed that Ti-6Al-4V alloy having these properties and relative density of more than 99% can be obtained with a heating rate of 50°C/min under vacuum; 50°C/min and 100°C/min under argon atmosphere.