DOI : 10.2240/azojomo0307

HIP of Complex Shape Parts from Various Ti Alloys

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Dec 19 2011

V. Samarov, E. Khomyakov, A Bisikalov and D. Seliverstov

Presented at the 2011 International Conference on Hot Isostatic Pressing Kobe, Japan, 12-14 April 2011.
Submitted: 12 April 2011, Accepted: 24 May 2011

Topics Covered

Abstract
Keywords
Introduction
Problems and Their Solutions
Analysis and Conclusions
References
Contact Details

Abstract

Innovative developments in HIP technology provide novel answers to the old problems of traditional technology of processing complex shape parts from Ti alloys such as:

very low material yield during processing of complex shape parts (“buy-to-fly” ratio);
dominating costs of machining to the final geometry;
high cost on initial materials, including alloyed powders;
increasing technical difficulties and cost with larger parts, especially when low processing temperatures are required;
inherent problems in achieving appropriate micro-structure and properties in large forgings

These new developments are based on the selectively net shape computer controlled HIP processing of pre-alloyed powders atomized with high solidification rates and enable

to cut substantially the material losses and reduce the “buy-to-fly” ratio several times;
to improve machinability of Ti parts;
to provide uniformity and homogeneity of complex shape parts
to build the material properties above the level of wrought

Various examples of developed parts and processes illustrate these novel solutions solutions for several Ti alloys such as Ti 6-4, Ti 5-2.5, Ti 6-2-4-2.

Keywords

Hot Isostatic Pressing, Powder, Ti Alloys, Complex Shape Parts

Introduction

Titanium is a very attractive material for many applications because of it’s low density, high strength and excellent corrosion resistance, however, it’s use is rather limited because of it’s high cost, being a result of high extraction cost and high processing cost. This has resulted in use of titanium mainly in the aerospace industry (engines and airframes) where it’s high cost can be justified by the increased systems performance resulting from it’s use. However, even in this industry cost reduction is always desirable. Broadly speaking, cost reduction can come from either a reduction in the cost of production of the metal itself or from creative techniques for the fabrication of the final com¬ponents. Over the past few years there has been much activity in the area of reduced-cost titanium extraction and fabrication processes , with some success, in part resulting in new and developing applications such as armor, medical and auto use [1,2].

However the P/M HIP approach offers for Ti alloys both the potential for cost reduction (because of it’s capability to produce very close to net shapes) and an enhancement in mechanical properties compared to conventional product (because of the inherent lack of directionality in the P/M component) This approach has the following significant advantages over other techniques:

the shape can be as precise as that of investment casting;
the mechanical properties including fatigue can be the same or higher than those of a wrought material;
the size and the weight of the parts can be the same or larger than that of the largest forgings with the ability of maintaining a complex shape and a superior microstructure
the surface of the parts can be free of “alpha “case (oxygen enriched surface)
all issues associated with the low processing temperatures can be eliminated

In addition, this technology can also provide the design enhancement of the critically loaded parts due to elimination of the design restrictions caused by traditional machining from aforging

Problems and Their Solutions

Though the concept of the near net shape approach sounds very attractive, there were solid technical reasons, why HIP of net shape Ti PM parts was not extensively used before:

Coarse powders caused potential inclusions in as HIPed condition and fatigue issues;
Oxygen pick up was associated with finer powders ;
Difficulties existed in non-destructive inspection for complex shape parts;
Difficulties existed in shape control during HIP to reach the necessary dimensional precision;

To achieve the goals of the net shape approach and to realize through the shape control the potential advantages of P/M HIP, the following technical problems have to be addressed and solved:

development of the data base of rheological properties for pre-alloyed Ti powders for the more effective mathematical modeling of shape effective mathematical modeling of shape formation during HIP;
advancement of these numerical models to fully account for the mass and heat transfer during HIP of large size Ti components;
modeling and study of the interaction of Ti powders with the HIP capsule material to enhance the surface finish and quality of the non-machined surfaces;
modeling and study of the mechanical properties as a function of the powder particle size and powder processing parameters

Data bases of rheological properties for every new Ti powder alloy are sistematically built through special experiments in generating porous sample and testing them at the temperatures of generation (interruption of the HIP cycle) [3]

Numerical models fully accounting heat and related mass transfer during densification of powder material through HIP, reveal densification waves caused by very low thermal conductivity of powder mterial at the intial stages of HIP and new geometrical features casued by these effects, especially for the large parts[4].

Design of the HIP capsules based on these models allows to develop and manufacture copmplex near net shape parts through one step and selectively net shape parts- by using one intermediate iteration.

Figure 1 shows complex ner net shape casings from Ti 6Al-4V made in one development step. The geometry of these parts is very close to the print (Figure 2), leaving only 2-3 mm for the final machining and enabling reduction of material 3-4 times compared to forgings. With the cost efficient powder manufacturing processes such as VIGA and EIGA and accounting very high cost of machining for Ti alloys this makes the near net shape HIP process beneficial.

For the selectively net shape parts such as impellers with the non-machinable precise internal channels, usually an experimental iteration is needed to provide th necessary tolerances.

Figure 3 illustrates modeling, HIP tooling desing and acheived dimensional accuracy for a rocket engine impeller from Ti 5 Al-2.5 Sn alloy.

Figure 1. Complex PM HIPed near net shape casings from Ti 6Al-4V

Figure 2. Dimensional precision for the near net shape casings

Figure 3. HIP modeling and dimensional preciswion for rocket engine impeller

Non-destructuve (ultra-sonic) inspection for complex shape parts is one of the key elements for the success of the near rnet shape HIP technology. Usually the requirements of this technology for simplified external shape and surface quality build barriers for enhancing the shape of the forgings and for radical reduction of the "buy to fly" ratio. HIP allows to resolve this contradiction and provide inspection for very complex shae parts and even thoose with internal cavities and channels formed by steel inserts. This is done by performing such inspection by keeping some of the HIP tooling outside and inside the very complex shae parts, i.e. machining them to the "sonic" shape and inspecting "bimetallic" structures.

This approach to the UT inspection has been very successfully demonstrated while manufacturing such complex shape shrouded impellers for the upper stage rocket engines and gas compressors.

Figure 4 shows an impeller after pre-machining of the HIP capsule and prior to ultra-sonic inspection. The Figure 4 clearly shows the ring material (Ti alloy) of the hub, shroud and vanes and low carbon steel surrounding it. Stell will be removed after UT inspection thrugh acid leaching.

Figure 4. Rocket engine Impeller Pre-machined for UT Inspection and acid leaching

Special software has been developed to conduct ultrasonic inspection of the impellers with the low carbon steel HIP tooling being still on the on the part. This software can distinguish the signals from the Ti alloy from the response from the steel and indicate the defects (if any) in each of them (Figure 5). The advantage of this approach is that the net shape geometries can be inspected, while the inaccessible “dead areas” typical for UT inspection can be “hidden” inside the steel tooling.

Figure 5. The results of UT scanning for a a Ti 6-4 impeller

The dark blue and black areas on Figure 5 are the scans of the Ti alloy while yellow, red and green areas present the signals from low carbon steel tooling different in its acoustic characteristics. The presented scans demonstrate the excellent UT quality of the PM impellers with no detectable indications of the size more than 0.015” in the parts.

In the case of critically loaded rotating parts fine Ti powders (less than 100 microns) were used. Figures 6, 7, and 8 show gradual enhancement of the micro-structure and mechanical properties while the powder particle size is reduced.

Figure 6. Microstructure of as HIPed Ti 6-4 powder of -35 mesh (-500 microns)

Figure 7. Microstructure and properties of as HIPed Ti 6-4 powder of -100 mesh (-150 microns)

Figure 8. Microstructure and properties of as HIPed Ti 6-4 powder of -325 mesh (-45 microns)

Analysis and Conclusions

HIP of complex shape parts is a synergetic technology that requires and involves:

Exceptional engineering in mathematical modeling and design of the entire process;

High quality powders and elaborate handling technology;
Well established and diversified canning technology;
Continuous control of the bulk properties, surface and shape
Well controlled, reproducible and reliable HIP cycles
Novel non-destructive inspection techniques

With all these requirements answered, HIP is becoming a cost efficient alternative to machining of the forgings to the final complex shape.

References

1. F. H. Froes, M. A. Imam and D. Fray Eds., “Cost Affordable Titanium”, Symposium Organized by TMS, Warrendale, Pa, (2004).
2. M. A. Imam and F. H. Froes , “Cost Effective Titanium”, Symposium Organized by TMS, Warrendale, Pa, (2010).
3. Khazami Zadeh “Finite Element Simulation of Near Net Shape Parts Produced by Hot Isostatic Pressing”. Proceedings of International Conference on Hot Isostatic Pressing HIP 96, ASM (1996).
4. V. Goloveshkin, G. Raisson, A. Ponomarev and A. Bochkov “Accounting the non-stationary temperature field while modelling of HIP for large size components ” Proceedings of International Conference on Hot Isostatic Pressing HIP (2011).