May_AMP_Digital

ADVANCED MATERIALS & PROCESSES •

MAY 2014

texture. Direct casting of Ti

AlNb components has not yet

succeeded, likely due to these problems.

For the casting process, minimizing segregation is ex-

tremely important. Ingot inhomogeneities in both com-

position and microstructure are difficult to remove by

subsequent wrought processing. These inhomogeneities

can easily cause machining cracks during aerospace com-

ponent manufacturing. Figure 4a shows several hot rolled

Ti-22Al-24Nb-0.5Mo sheets after a welding test. This ma-

terial has fairly good weldability, but chemical analysis in-

dicates that the heat affected zone (HAZ) of welds is

prone to crack if Nb segregation happens in this area. As

can be seen in Figure 4b, the embedded picture illustrates

a large crack caused by Nb segregation. The yield stress,

ultimate tensile stress, and elongation of hot rolled Ti-

22Al-24Nb-0.5Mo sheets are 1130 MPa, 1233 MPa, and

7.3% at room temperature; and 845 MPa, 1002 MPa, and

12.5% at 650

C, respectively.

Hindustan Aeronautics Ltd. (Bangalore, India) man-

ufactured compressor blades from a Ti-24Al-15Nb alloy

for an experimental gas turbine GTX engine produced

by conventional close-die forging

[7]

. In addition, the Cen-

tral Iron and Steel Research Institute (Beijing, China) re-

ports component blanks made of Ti-22Al-25Nb alloy,

including rings, die-forged blades, and spin-formed com-

ponents

[18]

. However, no other public information about

testing and application of Ti

AlNb engine components

has been found.

Powder Metallurgy:

Powder metallurgy (PM) is a long-

standing method of producing Ni-base alloy components

used in turbine engines. The weakness of cast-and-

wrought processing motivated the exploration of PM tech-

nology to produce Ti

AlNb products. The advantage of the

PM technique is that high quality, near-net-shape, and ho-

mogeneous parts can be made with substantially reduced

machining and scrap. For high-alloyed materials such as

AlNb, PM methods are far superior to cast-and-

wrought processing for various reasons. These include el-

ement segregation, texture, and residual stress upon

cooling from processing conditions.

AlNb powders are generally too strong to be cold

compacted. They are therefore usually processed by hot

isostatic pressing (HIP) followed by heat treatment. IMR

developed a net-shape PM method for making various Ti

components with complex shapes using pre-alloyed, gas-

atomized powders. Dimensions of the final components

are calculated based on finite element modeling of powder

shrinkage during HIP. This technique has matured to a

state of designing and making aerospace components with

high precision. Figure 5 shows a PM net-shape Ti-22Al-

24Nb-0.5Mo aerospace engine component.

However, PM is no panacea either. Small volume frac-

tions of residual pores (less than several microns in diam-

eter) do not seem to affect tensile properties because

AlNb alloys have adequate ductility to withstand small

internal stress risers, but these same pores can act as fa-

tigue crack initiation sites during cyclic loading. In addi-

tion, weldability is also expected to be degraded by residual

porosity. Investigation of this issue is currently underway.

Opportunities and challenges

The potential for Ti

AlNb alloys to reach the engine

components market critically depends on understanding

and tailoring composition, and on subsequent processing

to achieve the required microstructure and mechanical

properties. A few challenges related to Ti

AlNb process-

ing were already described. The following discussion iden-

Fig. 4 —

(a) Hot rolled Ti-22Al-24Nb-0.5Mo sheets after welding test, and (b) a crack in the weld HAZ caused by Nb segregation.

(a)

(b)

Fig. 5 —

A PM net-shape Ti-22Al-24Nb-0.5Mo aerospace engine

component.

100 µm