Ti6Al4V alloys modified with minor amounts of boron (B) were prepared, and two types of microstructures, a full lamellar microstructure and an equiaxed microstructure, were generated through combinations of hot-deformation and heat treatments. The beneficial effect of adding a minor amount of B in refining microstructures was confirmed in as-cast ingots and a full lamellar microstructure. For example, a refined prior ¢ grain size of about 100 µm in diameter was obtained for the 0.1 mass percent B-modified alloy with a full lamellar microstructure: accordingly, the size of each colony within the grains was reduced. Contrary to this, equiaxed microstructures with ¡ grain sizes of about 8 µm were obtained for both B-free and B-modified alloys. The room temperature high cycle fatigue (HCF) strength of the B-modified alloys increased compared to the B-free alloy for both microstructures. For example, HCF strength at 10 7 cycles for the alloy with an equiaxed microstructure increased to 750 MPa by the addition of 0.1% B from 650 MPa for B-free alloy. The fatigue crack was found to originate neither from the TiB/matrix interface nor from the TiB itself but rather from the shear fractures across microstructural units such as colonies or spherical ¡ phases. The reduced colony size and the retarding effect of TiB against the movement of the fatigue initiation area were thought to be responsible for the improved HCF properties of Ti6Al4V with lamellar and equiaxed microstructures, respectively.

The effect of Fe addition on the mechanical properties of Ti-6Al-4V alloys was investigated. Ti-6Al-4V prealloyed powders were prepared via a hydrogenation and dehydrogenation process using turning chips of Ti-6Al-4V alloys as the starting material. Mixed powders of Ti-6Al-4V powders and 3-4 mass% Fe powders were consolidated using a hot extrusion process and subsequently hot rolled. With increasing Fe content, the tensile strength and 0.2% proof stress of the Fe-containing alloys increased by 2%-30% compared to those of the Fe-free alloy in both as-hot-rolled and air-cooled specimens. The results also showed that the elongation in the 3% Fe-containing alloys was at least 10%, regardless of the treatment procedure. The balance between the generated martensitic phase and the β-phase ratio in (α+β)-dual-phase alloys appears to determine both the strength and elongation of the alloys. The Ti alloys obtained in this study have strong potential for application in automobiles and aircraft.

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Spherical powder with excellent fluidity is generally considered to be a suitable raw material powder for additive manufacturing (AM), as opposed to irregularly shaped powder produced by pulverization. The authors have investigated the possibility of obtaining a mixed powder comprising irregularly shaped Ti-6Al-4V alloy powder and spherical Ti-6Al-4V alloy powder. The powders were produced by the hydride-dehydride process and the plasma atomization process, respectively. The fluidity of the mixed powder was confirmed to be an acceptable level for AM and the chemical composition and tensile properties of the AM product using the mixed powder were same as those of the AM product using 100% spherical powder.

In order to develop low-cost and high-strength titanium alloy, a production method using pre-alloyed titanium powder has been investigated. Ti-6Al-4V pre-alloyed powder was prepared by HDH process using Ti-6Al-4V alloy turning chip as a starting material. Mixed powder of Ti-6Al-4V pre-alloyed powder and 3 to 4 % iron powder was consolidated by hot extrusion process. In this study, the following results were obtained. Almost full densification of pre-alloyed powder has been confirmed by hot extrusion process. Titanium alloy containing high amount of iron up to 4 % can be produced without iron segregation by powder metallurgical process. This titanium alloy possibly shows higher tensile strength by 2 to 30 % than Ti-6Al-4V alloy maintaining elongation of 10 %. These data suggest that highly iron containing titanium alloy produced by pre-alloyed powder method possesses high potential as a material for aerospace and automobile.