Thermomechanical powder consolidation (TPC): strategies and initiatives for costaffordable powder metallurgy Ti alloys

Ti is the most suited material for a wide range of lightweight high-strength corrosion-resistant engineering applications operating at relatively high temperatures up to approximately 600ºC; however, its industrial uptake is still affected by its high cost. The manufacturing of engineering metallic products from particulate materials through the so-called powder metallurgy (PM) methods is a well-established near-net-shape solid-state industrial technology characterised by (1) high yield of materials, (2) reduced machining, and (3) low energy consumption. These make PM ideal for priced (1 - low waste), difficult-to-machine (2 near-net-shape), high melting temperature (3 - energy), reactive (solid-state) metals like Ti (Figure 1).

Thermomechanical Powder Compaction - Dr Fei Yang

Thermomechanical Powder Compaction - Dr Leandro Bolzoni

Figure 1. Features and advantages of the thermomechanical powder consolidation (TPC) of Ti alloys.

This document presents an overview about the cost-effective manufacturing of Ti-based materials via thermomechanical powder consolidation (TPC). Three main interconnected strategies are proposed aiming to limit the production costs while maintaining high level of static and dynamic-loading properties as well as appropriate in operando performance (e.g. corrosion resistance): process efficiency enhancement, manufacturing optimisation, and development of cost-effective compositions. In particular, cost-affordable Ti alloys are achieved via individually and jointly reducing the intrinsic cost of the alloy and developing
enhanced manufacturing processes. The so-called blended elemental approach starting from cheap hydride-dehydride powder allows the design and adjustment of the composition and hot thermomechanical deformation processing can be used, if required, to enhance the performance.

Vacuum sintered in batches (temperature range of 1200-1400ºC combined with long dwell times of 1-4 h) aiming to control the amount of interstitial elements such as oxygen and nitrogen, whose excessive presence will significantly embrittle the material, is the most common method to manufacture PM Ti alloys. This lengthy, costly, and energy intensive manufacturing cycle reduces the achievable properties due to significant microstructural coarsening. Faster, more efficient and reliable alternative sintering techniques and the generation of manufacturability maps for lowering the production cost (e.g. short processing time) and to limit grain growth are therefore paramount.

 

Process efficiency enhancement

The generation of heat by eddy current directly into a conductive material is a highly efficient way of heating metals via the industrially used electromagnetic induction heating which saves time, reduces heat losses, and leads to higher mechanical performance because of the finer microstructure. Pressed Ti-based compacts can be successfully heated up to 1400ºC in matter of few minutes provided that a high level of density after compaction (i.e. relative density) is guaranteed. Through the selection of the right combination of parameters including relative density (ρr ~90%), sintering temperature (≥ 1000ºC) and dwell time at maximum temperature (≥ 0 s), the mechanical properties of induction sintered Ti are comparable to those of Ti obtained via the conventional vacuum sintering process (Figure 2). Microwave sintering has
been also proposed in recent year as an alternative technique for faster processing of PM Ti alloys. It was demonstrated that blended elemental PM Ti alloys (both lean and heavily alloyed) can be sintered via microwave radiation and industrially relevant quantity (500 g) can be manufactured. Consolidation via hybrid microwave sintering using shorter cycles derived by faster heating rates with respect to vacuum sintering (Figure 2), although not as fast as induction sintering, can successfully be performed reaching good combinations of mechanical performance.

Heating rate for Ti processed by means of induction sintering, microwave sintering, and vacuum sintering

Tensile properties for Ti processed by means of induction sintering, microwave sintering, and vacuum sintering

Manufacturing optimisation

Hot workability of metals strongly depends on the capability to plastically deform without cracking or fracturing guaranteeing the integrity of the material. A direct comparison of the hot deformation maps was done considering the Ti5553 alloy (i.e. Ti-5Al-5V-5Mo-3Cr) manufactured via PM and Ingot metallurgy. In comparison to IM Ti5553, the PM Ti5553 alloy has better hot workability, with a larger crack-free processing window, and lower deformation resistance. Specifically, the PM Ti5553 alloy has much smaller flow instability domain (including cracking, adiabatic shearing and flow localisation) and much more
pronounced microstructural changes occur in the PM Ti5553 alloy (i.e. dynamic α globularisation and coarsening, extensive dynamic full recrystallization). The latter means that a broader temperature/strain rate ranges can be industrially implemented to safely hot deform the PM Ti5553 alloy (Figure 3).

Figure 3. Underpinning scientific metallurgical knowledge for industrial thermomechanical powder processing of Ti alloys (processing map, activation energy map, and example of metallurgical mechanisms operating when selecting a variety of combinations of manufacturing parameters).

Development of cost-effective compositions

Another strategy that can be easily implemented to achieve cost-affordable Ti alloys is to reduce the intrinsic material cost making use of cheaper alloying elements than the ones commonly used. Market price/availability and their effect on the microstructure and performance are the two main factor guiding the selection of new alloying elements. For historic reasons, Fe is extremely abundant and significantly cheaper than Ti, therefore the development of low-cost Fe-bearing PM Ti alloys is of interest. Moreover, ferrous powders already optimised for their processing via PM are readily available in the market. Low-cost Fe-bearing PM Ti alloys can be successfully processed using a variety of PM methods including: (1) conventional vacuum sintering, (2) alternative sintering techniques (i.e. induction and microwave), (3) thermomechanical powder consolidation to enhance the mechanical behaviour and impart the desired geometry, and (4) tailoring of the mechanical properties of this new class of alloys via expressly-developed heat treatments. Thermomechanical processing by means of β hot forging and β hot extrusion of low-cost Febearing PM Ti alloys leads to fully dense materials with better overall compromise of tensile properties (UTS vs. elongation) in comparison to the Ti workhorse Ti-6Al-4V alloys processed under the same manufacturing conditions. The higher malleability of the Fe-bearing alloys means, for the same processing temperature, that the material deforms more resulting for example in longer extruded bars. Conversely, this also means lower processing temperatures can be used for the hot deformation of low-cost Fe-bearing PM Ti alloys (Figure 4).

Figure 4. Representative results of (a) the tensile behaviour (ultimate tensile strength vs. elongation at fracture) and (b) the deformability of low-cost Fe-bearing PM Ti alloys.

Industrial case study

The advantages derived by using near-net-shape processing (principally material and manufacturing cost reduction) still ensuring the required level of mechanical performance is easily demonstrated with a asymmetric mechanical fitting flange made out of Ti requiring to be lightweight, highly corrosion resistant and mechanically strong (Table 1). The conventional route entails purchasing commercially available Ti plates to be cut to the approximate required size for subsequent drilling and milling machining operations where approximately 83% of the starting material ends up as waste and a significant number of
operations are needed to obtain the final shape with the desired features. Conversely, the standard powder metallurgy route comprises acquiring commercially available powder to be pressed and sintered into a near-net-shape preform for its finishing by profile milling resulting in much lower number of operations required, higher yield of material, and significantly lower wastage material (equivalent to saving 77% of the original 450g).

Concluding remarks

Strategies and initiatives to create cost-affordable powder metallurgy Ti alloys include designing cost-effective chemical compositions and develop high-efficient optimised purposely-engineered manufacturing processes. In particular, the flexibility of manufacturing lean or highly alloyed wrought-equivalent as well as novel low-cost compositions is granted by the adoption of the blending elemental approach. Shortening of the production cycle and reduction of the overall cost while guaranteeing the production of high performance materials is achieved via fast and reliable alternative sintering techniques and manufacturing optimisation through understanding the underpinning metallurgical mechanisms controlling the response of the material during its processing. A broad variety of Ti alloy compositions
can successfully be manufactured via simple powder metallurgy approaches and thermomechanical powder consolidation is used for simultaneously shaping and enhancing  the properties of alloys where dynamic-loading properties such as fatigue and fracture toughness are critical.