2016 Volume 63 Issue 7 Pages 451-456
Ti-6Al-4V alloy has attracted a lot of attention from the automotive and aerospace industries because of their outstanding specific strength and corrosion resistance. On the other hand, Ti-6Al-4V alloy has normally poor workability because of the low thermal conductivity and the low elastic modulus. Metal injection molding (MIM) process is expected to manufacture a complex part with near net shape and reduce the manufacturing cost. However, Ti-6Al-4V alloy compacts by MIM process had a problem of low fatigue strength compared to wrought material.
In this study, we tried to add a minor amount of boron using TiB2 powder for improving the fatigue strength. Addition of boron resulted refinement of the grain size of lamellar structure, which lead to increase the high cycle fatigue strength and fatigue limit. In addition, tensile properties at high temperature was investigated.
Improving the fuel efficiency in transportation has become a large priority today, and also the weight reduction of vehicles especially in the automotive and aerospace fields has been in progress. Ti-6Al-4V alloy which has a high specific strength could be a next major material for applications such as fan blades of turbine engines for aircrafts and connecting rod for automobile. On the other hand, Ti-6Al-4V alloy has poor workability and the fabrication cost of complex components by machining is significantly high1–6). Therefore, metal injection molding (MIM) process is hoped to reduce the productive cost of such complex parts. There are large number of research on the injection molded of Ti-6Al-4V alloy compacts. These MIM Ti-6Al-4V alloy compacts have nearly the same tensile properties as wrought materials. However, MIM components have low fatigue strength as compared to wrought materials2,4). One of the major reasons for the low fatigue strength is the grain growth during the sintering process. It has been reported that the addition of a small quantity of boron to this alloy has resulted refining the grain size of the lamellar microstructure5–8).
In this study, we added boron by mixing TiB2 powder to the Ti-6Al-4V alloy powder and investigated mainly the effect on the fatigue properties. Furthermore, Ti-6Al-4V alloy could be used at high temperature up to 300 °C. However, mechanical properties at high temperature of boron added Ti-6Al-4V alloy compacts by MIM process was not issued yet. Therefore, tensile properties at temperature 100 °C and 260 °C of boron added MIM Ti-6Al-4V alloy compacts by MIM were investigated.
The base powder was pre-alloyed Ti-6Al-4V alloy powder (TILOP64-45, Osaka Titanium Technologies, Co., Ltd) which was gas-atomized. TiB2 powder (TiB2-NF, Japan New Metal Co., Ltd) was also prepared as boron addition. Fig. 1 shows the SEM images of the powders. The Ti-6Al-4V alloy powder was spherical and was classified to be less than 45 μm diameter with a mean particle size of 31.43 μm. The TiB2 powder was a relatively small with a mean particle size of 1.36 μm. The binder consisted of 69 mass% paraffin wax (PW), 10 mass% carnauba wax (CW), 10 mass% atactic polypropylene (APP), 10 mass% ethylene vinyl acetate polymer (EVA), and 1 mass% di-n-butyl phthalate (DBP), and they were mixed and kneaded with the powders as a feedstock. Powder loading of the feedstock was 65 vol%. TiB2 powder was added from 0.1 to 2.6 mass% during the mixing process which correspond to 0.03 to 0.80 mass% of boron. In this paper, each alloy was designated as 0.03 B to 0.80 B. Fig. 2 shows the dimensions of the tensile and fatigue test specimens molded for the experiments. After injection molding, obtained green parts were solvent debound in heptane vapor at 58 °C for 5 hours. Subsequently, the samples were thermally debound at 600 °C for 1 hour in an argon atmosphere, and sintered at 1350 °C for 4 hours under vacuum.
SEM images of powders (a) Ti-6Al-4V, and (b) TiB2.
Dimension of molds’ cavity: (a) Tensile test specimen, and (b) Round bar specimen for rotary bending fatigue test.
Archimedes’ principle was used to measure the density of sintered parts. Oxygen content was also measured by oxygen and nitrogen analyzer (ON736, LECO Corp.). Grain size was measured by processing the image of the electron back scatter diffraction pattern (EBSD). Distribution of boron in the microstructure was analyzed and mapped by an electron probe micro analyzer (EPMA). The tensile tests were performed at room temperature, 100 °C and 260 °C using a cross head traverse speed of 1 mm/min in an universal testing machine. The fatigue tests were performed at room temperature by an Ono-type rotary bending fatigue tester. The stress ratio was R = −1.
The relative density of sintered compacts was shown in Fig. 3. The relative density of 0 B to 0.40 B compacts was within a range of 96 % to 97 %, while that of 0.80 B compacts was under 95 %. Ferri et al.7) also reported that the relative density of compacts sintered at 1250 °C decreased to 91 % by the addition of 0.5 mass% boron. However, the relative density of compacts sintered at 1400 °C did not decreased, and thier relative densities was 98 %. This 0.5 mass% boron might be the critical content of boron which affects as sintering inhibitor at 1250 °C. From his result, the critical content of boron at 1350 °C might be existed within 0.4 % to 0.8 % boron content.
Relative density of sintered compacts.
Fig. 4 shows the microstructure of each sample after etching. All compacts showed α-β full lamellar microstructure. Fig. 4 (a) and (b) show the grain refining definitely. However, grain size of higher boron content added compacts was too small to calculate from this microscopy. The prior β grain size measured by useing EBSD are shown in Figs. 5 and 6. The prior β grain size of boron added compacts were smaller than that of 0 B compact. Furthermore, as increasing of boron content, the prior β grain size decreased.
Optical microcopies after etching, (a) 0 B, (b) 0.03 B, (c) 0.06 B, (d) 0.12 B, (e) 0.24 B, (f) 0.40 B, and (g) 0.80 B compacts.
Grains of (a) 0 B, (b) 0.03 B, (c) 0.06 B, (d) 0.12 B, (e) 0.24 B, (f) 0.40 B, and (g) 0.80 B compacts analyzed by EBSD.
Relationship between average grain size and boron content.
Fig. 7 shows the boron distribution by EPMA in the 0.03 B, 0.12 B and 0.24 B compacts. Boron formed titanium boride (TiB) in the sintered parts since the boron in this form had little solubility in the Ti-6Al-4V alloy matrix6,9). The TiB particles precipitated on the grain boundaries and prevented the grain growth. In case of 0.03 B compact, TiB particles were seen as a tiny spot. On the other hand, the larger TiB particles were found in the 0.12 B and 0.24 B compacts. In Fig. 8, a white arrow points out TiB particle as an example. The TiB particles have a sharp needle like shape with less than 40 μm length.
Boron mapping of 0.03 B compact by EPMA analyzer (a) boron analytical result (b) combined boron analytical result with SEM image.
Optical microphoto of 0.06 B compact. White arrow shows TiB inclusion.
Fig. 9 shows the oxygen content of the sintered compacts and base powder. The oxygen content of sintered compacts increased from the base powder. Fig. 10 shows the results of tensile test at room temperature. In all cases, tensile strength was over 850 MPa. The tensile strength seems to be not affected by the grain size or the boron content. Since the oxygen contents of all compacts were below the critical oxygen content level of 0.33 mass% as reported10), the elongation did not decrease with the analyzed oxygen content. However, the elongation of 0.80 B compacts decreased compared to the others. Godfrey et al.8) also showed the addition of excessive amounts of boron lead to lower ductility. The lower relative density by increasing of TiB would be the reason why the elongation of 0.80 B compacts decreased. According to ASTM B348 Gr5, wrought Ti-6Al-4V alloy showed tensile strength of 895 MPa and elongation of 10 %. Except the 0.80 B compacts, MIM compacts in this work satisfied the standard requirement. Figs. 11 and 12 show the tensile properties at 100 °C and 260 °C, respectively. Tensile properties at high temperature showed the same tendency with those at room temperature. The elongation of 0.80 B compact was improved at higher temperatures.
Oxygen content of the sintered compacts and original powder.
Tensile test result at room temperature.
Tensile test result at 100 °C.
Tensile test result at 260 °C.
Fig. 13 shows the results of the rotary bending fatigue tests. The 0.80 B compact was not tested, since they did not meet the standard of elongation. Fatigue limit was defined as the maximum load at 107 cycles without fracture. The fatigue limit of each compact is clearly reflected the effect of grain size refining. Fatigue limit of the 0 B compacts was 291 MPa, and it increased with increasing of boron content. Fatigue limit of the 0.40 B compacts was 435 MPa, which was maximum value in this work. As shown in Figs. 10 and 13, 0.40 B compacts had the highest fatigue limit but not the highest tensile strength. The length of dislocation mean free path which was determined by prior β grain size had a role as critical factor for fatigue limit4). At the high stress amplitude range around 600 MPa, the differential in the number of cycles to fracture with different boron content was almost negligible. The effect of boron addition was found only in the low stress and high cycle region. A data of wrought Ti-6Al-4V alloy which was mill annealed and equiaxed microstructure is also included in Fig. 13 for comparison11). The fatigue test results showed that the boron addition increased the fatigue limit of MIM Ti-6Al-4V alloy compacts, which was reached to 81 % of wrought material for 0.40 B compacts. Fig. 14 shows the relationship of grain size and fatigue limit. Hall-Petch equation which could be usually applied to the tensile strength was used by drawing in Fig. 14. The experimental result of fatigue limit corresponded with the Hall-Petch equation.
Rotary bending fatigue test result.
Relationship between fatigue limit and grain size.
Fig. 15 show the SEM images of the fracture surfaces of the fatigue compacts for the each boron content. The location of crack initiation are marked with white circles in Fig. 15. Fig. 16 shows higher magnification SEM images of (a) 0 B and (b) 0.40 B compacts. The crack initiation locations had a smoother surface, hence the fatigue crack of Ti-6Al-4V alloy with α-β full lamellar microstructure were initiated by shear fracture across a colony6,12). As shown in the Fig. 16, the fracture surface unit of 0.40 B compact was smaller than that of 0 B compact, so that the former might show high fatigue limit.
Fracture surface SEM image of (a) 0 B, (b) 0.03 B, (c) 0.06 B, (d) 0.12 B, (e) 0.24 B, and (f) 0.40 B compacts. White circle shows crack initiation location.
Higher magnification SEM images of crack initiation location (a) 0 B, and (b) 0.40 B compacts.
In this study, the addition of TiB2 powder to MIM Ti-6Al-4V alloy compact was investigated to improve the fatigue strength and tensile properties at high temperature. The obtained results are as follows;
This work was supported by Council for Science, Technology and Innovation (CSTI), Cross-ministerial Strategic Innovation Promotion Program (SIP), “Structural Materials for Innovation” (Funding agency: JST).
