Fatigue property enhacenebt of a b titanium alloy by blended

ISIJ International.

Vol. 31 (1991

),

No.

8, pp.

922--930

o Fatigue Property Enhancement f Elemental P/M Approach

oe-

Titanium Alloys by Blended

Yoshinari KAIEDA.1) Yoshikuni KAWABE Shin MIURA2) and MasuoHAGIWARA.
Tsukuba Laboratories,
1) National National Research Institute for Metais. Sengen, Tsukuba, Ibaraki-ken, 305 Japan 2) Chichibu Research Laboratory. ShowaResearch Institute for Metals, Nakameguro,Meguro-ku. Tokyo, 153 Japan. Denko K. K., Shimokagemori. Chichibu. Saitama-ken, 369-1 8 Japan.

(Received on January 25. 199l.' accepted in

final

form on March 22. 1991)

Attempts were made produce fatigue tolerant c(-p titanium alloys by the blended elemental (BE) P/M to method using extra low chlorine (ELCL) titanium powder. The study was conductedin two stages. In the first stage, the effect of microstructural modification on the high cycle fatigue strength of ELCL Ti-6A1 -4V BE investigated. Several different microstructural conditions were generated through combinations of was processing and heat treatment and these were fatigue tested to find out the optimum microstructure. In a later stage, various kinds of ELCL BE ce-p titaniumalloys were produced by both conventional and microstructure-controllable new BE methods with emphasis on relating composition/processing/microstructure to fatigue properties. The precision sectioning method which enables the concurrent observation of the initiation facets and the underlying microstructure was utilized to analyze fatigue crack initiation

mechanisms.
modification;

a:-fi KEYWORDS:alloy; titanium powder metallurgy; precision sectioning method; fatigue.

blended elemental

method mlcrostructure

l

.

Introduction

by using extra low chlorine (ELCL) titanium powder. The study wasconducted in two stages. In the first stage,
ideally

Titanium alloy parts are

suited for

advanced

aerospace systems because of their excellent combination of mechanicalproperties. However, the use of titanium alloys in engine and aircraft componentsis still limited by a high manufacturing cost associated with forging

the effect of microstructural cycle fatigue strength of investigated. Several different

modification

ELCLBE Ti6Al~;V was

on the high

and machining difficulties. To overcome this cost problem, several processes such as isothermalforging, precision casting, diffusion bonding/superplastic forming and others are being developed for producing near net shape componentsdirectly.1) Blended elemental (BE) titanium powder metallurgy, which uses both press and sinter technique and hot is isostatic pressing (HIP) for further densification, considered to be one of the most attractive titanium net shape technologies. I ~ 4) However,mechanical properties, particularly fatigue strength, are inferior to those of materials produced by prealloyed powdermetallurgy and ingot metallurgy (IM) due to microstructural formation of alpha-platelet colonies and a massive grain boundary alpha phase.5~8) In addition, the presence of residual
porosity attributed to the chloride salts inherited in the titanium powder leads to a degradation infatigue strength even with chloride levels as low as 0.016 wt~/* 9) The present investigation was aimed at producing highly fatigue tolerant BE oc-p titanium alloys by eliminating the porosity and modifying the microstructure. The elimination of the porosity was achieved

microstructural conditions generated through combinations of processing and were heat treatment and these were fatigue tested tofind out the optimum microstructure. In a later stage, various kinds of ELCL BE o(-p titanium alloys were produced by the microstructure-controllable newBEmethodwith composition/processing/microemphasis on relating structure to high cycle fatigue strength.

2.

Experimental Procedure

The BE o(-p titanium alloys were produced using a titanium powderfabricated mixture of -lOO meshELCL by...