Mechanical Properties of Powder Metallugry (Ti-6Al- 4V) with Hot Isostatic Pressing

Titanium alloys are widely used due to their high performance and low density in comparison with iron-based alloys. Their applications extend to aerospace and military in order to utilize their high resistance for corrosion. Understanding the mechanical properties and microstructure of titanium alloys is critical for performance optimization, as well as their implications on strength, plasticity, and fatigue. Ti-6Al-4V is an α+β two-phase alloy and is considered one of the most commonly used titanium alloys for weight reduction and high-performance. To avoid manufacturing defects, such as porosity and composition segregation, Hot Isostatic Pressing (HIP) is used to consolidate alloy powder. The HIP method is also used to facilitate the manufacturing of complex structures that cannot be made with forging and casting. In the current research, Ti-6Al4V alloys were manufactured with HIP and the impact on heat treatment under different temperatures and sintering durations on the performance and microstructure of the alloy was studied. The results show changes in mechanical properties and microstructure with the increase of temperature and duration. Keywords-titanium alloy; Ti-6-4; hot isostatic pressing (HIP)


INTRODUCTION
The utilization of titanium for industrial purposes became popular during the last fifty years due to its competitive properties in comparison with traditionally used metals, such as iron, steel, etc. [1]. Titanium alloys are commonly used in aerospace and military applications due to their low density and high resistance to corrosion, high performance, and strength. Despite being expensive and hard to manufacture, these alloys became the engineering choice for experts due to their light weight and high performance under high temperatures [2]. The applications of titanium alloys are correlated to their mechanical properties. Therefore, it is important to understand the factors that affect these properties in order to optimize their performance. The mechanical properties of titanium alloys, such as creep, fatigue, plasticity, and strength, are highly affected by their microstructure [3]. The small size of α grains are correlated to the increase in ultimate tensile strength [4]. One of the most common titanium alloys is Grade 5 (Ti-6Al-4V or Ti-6-4), which is used extensively for engineering and industrial purposes. Due to its high mechanical performance and facilitation of weight reduction, applications of this alloy include marine equipment, automotive, and aerospace. Medical and pharmaceutical applications can also be found for Ti-6Al-4V due to its exceptional biocompatibility [5]. However, the alloy has a difficult machinability because of its low thermal conductivity and high strength to density ratio [6]. Ti-6-4 is classified as an α+β two phase alloy with wide usage range in mechanical applications. Due to the structure complexity of the manufactured components and defects such as composition segregation and porosity, forging and casting is not the most preferred manufacturing method [7]. Thus, Hot Isostatic Pressing (HIP) is used for the consolidation of alloy powder in order to facilitate manufacturing complex components and avoid those defects [8]. The HIP method has also economic advantages as it maximizes the utilization of material and reduces cost [9]. In [10], further advantages were found for the HIP on Ti-6-4 on the microstructure level. Several studies attempted to optimize the heat and pressure conditions of the alloy in order to reach optimum performance. Some studies solely tested the heat or pressure change implications on titanium alloys, while a few investigated the change in both parameters [11]. Authors in [12] investigated the heat and pressure conditions of the HIP and the cooling rate of the Ti-6-4 samples on their microstructure and mechanical properties. They found that temperatures ranging between 900 and 940ºC and a pressure of 100MPa yield the most optimized tensile strength, with sample holding for 3 hours in room temperature. Authors in [13] used HIP with high temperature and pressure conditions: 1200ºC and 120MPa, while samples cooled in the furnace for 3 hours. Microstructure analysis showed an increase on the needle structures of Ti-6-4 and an increase in the nano hardness and the wear resistance of the samples. In the current study, Ti-6-4 alloys were manufactured under HIP and the samples were put under very high temperatures with different exposure timings. The impacts on wear resistance, microstructure, and mechanical properties were investigated and compared to the results of previous research for verification and possible enhancements on the properties of the alloy.

A. Research Aim
The aim of this research was to investigate the effect of very high heat exposure on Ti-6Al-4V alloys manufactured by HIP by comparing samples with different exposure durations.

B. Material and Sample Preparation
The Ti-6-4 alloy used in the experiment was prepared through the cost-effective Blended Elemental (PM) technique. The hydrogenated titanium powder (3.5% H wt, Ti particle size 100µm) was mixed with 60Al-40V master alloy powder with particle size ranging between 40 to 63µm in order to obtain the required alloy composition [15], as shown in Table I. The powder was blended for 1200s (20m) and die pressed under 300MPa at a temperature of 500ºC for 4h to form green compacts of 60×60×3mm, as shown in Figure 1. The approximate density of the sintered alloy was determined through hydrostatic weighing as 4.42g/cm 3 , corresponding to 97.73% of its theoretical value. The compacted specimens are shown in Figure 2. For each produced sample, 50g of alloy powder was used, while the final average weight of each of the nine produced sample was 47.844g. The sintering of the specimens was performed in a vacuum furnace at a vacuum pressure of 10 -3 Pa. The nine specimens were divided into three groups as shown in Table 2. Three temperatures were applied: 1200ºC, 1300ºC and 1400ºC, at 2, 4 and 8h for each group. The described properties of powder metallurgy compacts are not comparable to the obtained wrought metal. It is expected that the sintered metal would have lower ductility, for both alloyed and unalloyed titanium, than the wrought metal [16]. Five tests were performed on each sample: wear resistance test, microhardness test, XRF analysis, XRD analysis, and SEM analysis. The results are presented and discussed along with the relevant literature below.

A. Wear Resistance
The specimens were subjected to a wear resistance test, where machine parameters were set as shown in Table III. As shown in Figure 3, the friction coefficient for specimens in group A differed with the sintering duration. While the maximum friction coefficient increased from 3.054 in A1 to 3.882 in A2, it decreased to 1.929 in A3, which corresponds to the longest sintering duration. Wear resistance friction coefficient change for Group A In A1, fluctuations with an increase in friction appear between zero to 40m. Smooth sliding gaps started appearing afterwards with the maximum friction towards the end of the distance. Specimen A2 showed high fluctuation rate with a steady overall increase in friction, while A3 showed a steady friction coefficient with a wave of higher friction between 5 to 35m. The results for group B specimens are presented in Figure  4. A reduction in the overall friction coefficient was found in B2. Specimen B1 showed an increase of friction between 0 to 30m and had a maximum coefficient of 3  The average hardness values are presented in Figure 5. The hardness value of group A increased with the increase of sintering time between 2h and 4h, while it dropped at 8h. For group B, the hardness value increased at both 2 and 4h, while it decreased for both sintering times for group C. Generally, microhardness values increased with the increase of heat treatment temperatures, while increased sintering times decreased hardness values. Authors in [17,18] showed the increase in microhardness values between heat treated and untreated Ti-6-4 samples and authors in [19] reported a decrease in the hardness values with the increase of heat treatment temperatures without specifying changes in sintering durations. Average hardness values for groups A, B and C C. XRF Analysis X-ray fluorescence (XRF) is a microscopy method used to determine the concentration of metals in an alloy in a specified area. The method is based on the interaction between the x-ray arrays with the matter through dislodging electrons from atoms and measuring the energy resulting to correlate it with a specific element [20]. Table V shows the results of the XRF analysis in the tested alloy. The analysis depth ranged between 0.0030 to 0.8550mm. The titanium percentage reduced from 58.85% to 47.79%, while the percentages of Al and V reduced from 5.12% to 4.16% and from 8.15% to 6.62% respectively. The 90:6:4 ratio has been confirmed for the Ti-6-4 in various studies. Authors in [21]  error margins ranging between 0.13% and 10%, while authors in [22] identified 5.5% aluminium, 3.87% vanadium, and 0.22% iron by weight.

D. XRD Analysis
Identification of crystalline material in the samples that were subject to heat treatment was carried out through an X-Ray powder Diffraction (XRD) analysis. Figure 6 shows the peaks of 8 samples, which are identical in phase pattern and position, while differing in intensity. The higher the temperature and duration of the heat treatment, the higher is the intensity. The shift in the peaks with the increase of the heat treatment conditions is caused by the creation of equilibrium phases of the lamellar structure α+β. The decrease in the width accompanied with the increase in intensity of the peaks is caused by the non-equilibrium α phase of the non-treated samples and the transformation to the equilibrium phase with heat treatment [23].

E. Scanning Electron Microscopy (SEM)
The specimens A1 to C2 were analyzed through scanning electron microscopy (Figures 7-14). The behavior of aluminum and vanadium vary under heat treatment. While Al tends to enter the α phase, the V tends to enter the β phase [24]. In the images taken for the 8 specimens at the same magnification, the dark regions show the α phase, while the light regions present the β phase [25]. The presence of delamination wear, abrasion and debris in the SEM pictures of Ti-6Al-4V is in accordance with the test results of other titanium alloy testing [26]. SEM image of heat-treated specimens at ×500 for A1 (layer thickness: A1=8.8µm) Fig. 8.
SEM image of heat-treated specimens at ×500 for A3 (layer thickness: A3=11.5µm) Fig. 10 IV. CONCLUSION In the current research, Ti-6Al-4V alloys were manufactured with the HIP method and the impact of heat treatment, under different temperatures and sintering durations on the performance and microstructure of the alloy, was studied. Three temperatures were tested (1200ºC, 1300ºC and 1400ºC) with exposure durations of 2, 4 and 8 hours. The results show that sintering temperatures and durations had significant effects on wear resistance and hardness, namely higher temperatures and longer durations reduced wear resistance and hardness. XRD analysis showed a shift in the peaks with the increase of temperature and sintering duration, while an equilibrium in lamellar structure α+β was achieved. The peaks were increased in intensity and reduced in width. Moreover, SEM analysis showed that the α and β phases were balanced with the heat treatment.