Advancements in nonferrous metals processing are of great interest to the U.S. Department of Energy (DOE). Hydrogen sintering and phase transformations (HSPT) is a promising manufacturing technology that can be used to produce titanium-alloy components in a near-net-shape form.
Having a competitive metals industry is important for our nation’s security and economic well-being. Advancements in nonferrous metals processing could not only save energy in their manufacture, but also provide significant energy savings through the use of lightweight metals.
In particular, titanium (Ti) and its alloys have the potential for greatly expanded use in industrial, military and consumer applications. However, high costs have limited its use. After conventional wrought processing and machining, titanium parts are typically in excess of 40 times more expensive than a corresponding steel part and nearly 20 times more expensive than an aluminum part. A recently completed research project funded by the DOE Advanced Manufacturing Office (see sidebar) and led by the University of Utah evaluated a new concept for producing Ti alloys at significantly lower cost.
Project Background and Objective
Powder metallurgy (PM) approaches have been carefully investigated and are used in traditional sintering production processes due to their capacity for fabricating near-net-shape components in a range of alloy systems and ceramics. Traditional sintering of titanium powders has not been widely used, however, due to mechanical-property considerations when compared to wrought material and/or cost issues. The newly developed process technology utilizes titanium-hydride (TiH2) powder and a hydrogen sintering atmosphere, allowing increased potential for lower-cost press and sinter methods to produce significantly improved properties.
The key innovation of the novel manufacturing approach is a sintering technology for making titanium materials with ultrafine-grained microstructure in the as-sintered state with minimal or no post-sintering processes. The new sintering technology is termed hydrogen sintering and phase transformations (HSPT). It constitutes a promising manufacturing technology that can be used to produce Ti-alloy components in a near-net-shape form, thus also minimizing machining costs.
By optimizing the process parameters for the sintering of TiH2 powders blended with alloying elements in a hydrogen atmosphere and controlling the phase transformations during and after sintering, the HSPT process will reduce the energy consumption, and thus cost, of making Ti alloys and fabricating Ti components. The process was designed such that no high-temperature melting is required for producing Ti alloys, no energy-intensive sintering processes are required for full consolidation, little or no post-sintering processing is needed for producing desired microstructures (and therefore enhanced mechanical properties) and, finally, minimum machining is needed to fabricate finished Ti components.
An energy analysis calculated that the HSPT process is about half as energy-intensive as conventional wrought processing for producing 2-inch-round bar stock while producing mechanical properties that are comparable. This energy savings would be much greater for a part geometry that takes advantage of the near-net-shape capability of the process.
The primary advancement of the HSPT process over competing PM technologies is its capacity to produce ultrafine-grained microstructures in the as-sintered state. This is directly the result of phase transformations that occur during processing due to the control of the hydrogen atmosphere and temperature and dehydrogenation of the product. A depiction of manufacturing routes is shown in Figure 1.
Achieving mechanical-property targets was a key milestone for project activities. Of particular interest are yield strength, elongation, fracture toughness and fatigue limit. Tensile tests demonstrated properties exceeding the milestones for static properties. The average yield strength of 10 samples tested in the as-sintered condition was 961 MPa (milestone value of at least 900 MPa), the average tensile strength was 1,017 MPa and the average ductility in terms of elongation was 15.1% (milestone value of at least 10% elongation).
Fatigue testing occurred throughout the second and third years of the project, with emphasis on improving the microstructural features that benefit good fatigue properties. These include reducing the size of the largest pores in samples, decreasing total percent porosity and decreasing the fatigue-crack length by refining the grain size. A fatigue 107 cycle endurance limit of 500 MPa was achieved for as-sintered Ti-6Al-4V alloy samples, with several specimens enduring (surviving without fracture) for 106-107 cycles with maximum stresses in the range of 500-600 MPa.
In addition to testing the as-sintered Ti-6Al-4V samples, several samples were also heat treated to produce a range of wrought-like microstructures. It was found that fully globularized and bimodal (duplex) microstructures could be produced by utilizing subtransus heat treatments with slow cooling or quenching, respectively, followed by aging. The globularized samples showed the greatest combination of strength and ductility (876 MPa yield strength, 950 MPa tensile strength and 18.4% elongation on average). The bimodal samples had the greatest strength (981 MPa yield strength, 1,073 MPa tensile strength and 11.3 % elongation on average). Additionally, the heat treatments have been shown to increase the fatigue endurance limit by 75-100 MPa.
One of the primary reasons for the superior mechanical properties of HSPT-produced materials over other low-cost PM processes, particularly vacuum sintering, is the finer grain size. Figure 2 shows examples of the microstructures of these two production techniques, while Figure 3 shows the time versus temperature profile. Note the different scale bars in Figure 2, showing a profoundly finer grain structure in the HSPT sample.
Primary conclusions from the microstructural development investigations included:
- Ultrafine microstructures of Ti-6Al-4V can be produced by applying the HSPT process. The critical factor for forming the ultrafine microstructure is the isothermal hold temperature, which is in the range of 600-700°C (1112-1292°F) under 0.5 atm of H2.
- The mechanisms of the microstructural refinement during HSPT are the combined effects of the precipitation of fine α/α2 particles within the β grains, which refine the coarse β grains during isothermal holding, and the eutectoid transformation of remaining β into very fine (α+δ) at a lower temperature.
- During HSPT, the presence of hydrogen converts the Ti-6Al-4V (typically an α+β alloy) into a metastable β Ti alloy, hence causing the precipitation of fine α/α2 within β grains.
- The α2 phase is a temporary intermediate phase during HSPT. It forms during isothermal holds under a partial hydrogen atmosphere in the temperature range of 550-750ºC (1022-1382°F). The α2 transforms to α during dehydrogenation at 750°C.
As mentioned in the previous section, in addition to producing a unique ultrafine-grained microstructure in the as-sintered condition, this microstructure may also be transformed into traditional wrought-like globularized and bimodal microstructure via simple heat treatments. Producing such microstructures via wrought processing or traditional PM requires energy-intensive thermomechanical processing (TMP) to produce a driving force for recrystallization. However, the unique as-sintered microstructure produced via HSPT was shown to form these microstructures through unique mechanisms via simple heat treatments and without TMP. The key enabling features of this microstructure were the high degree of grain-boundary energy contained in the ultrafine-grained microstructure and the distribution of β phase as ultrafine grains at the triple points of the α colonies.
Energy Consumption Comparison
In order to generate an accurate comparison between the energy efficiency of the HSPT process and a wrought process, it is necessary to identify a specific component to be produced. While complex geometries are best suited to outline the superior efficiencies of a near-net-shape process such as HSPT, a relatively simple 2-inch-round bar stock was chosen to provide an extremely conservative comparison between HSPT and wrought processing routes.
Wrought processing of titanium is a competitive market. Consequently, actual data from the industrial processes are understandably guarded and were unavailable. Additionally, HSPT is only entering the adoption phase of commercialization, and actual production-scale data does not exist. Therefore, for this model, researchers started with first principles to model the theoretical energy requirements of each known step for each process. These theoretical calculations were then adjusted using published or calculated (when necessary) efficiencies of the equipment required for each of these processes. A summary of results for the energy model is depicted in Table 1.
The efficiency of HSPT sintering was determined by modeling mechanical work and heat flow of the entire PM process, from powder production to sintering. This was also done for wrought processing from ingot production to forging of 2-inch-round bar stock. The process efficiencies (hydraulic efficiency for mechanical work, VAR/forge furnace efficiency for heating, etc.) as produced from the literature or calculations were then applied to the theoretical minimums to calculate the total energy consumption.
It is important to realize that electricity generation and transmission losses are not included, which would further increase the energy advantage of HSPT compared to wrought processing. Furthermore, the model assumes that both processes produce round bar stock, a geometry that does not take into account the significant energy and cost savings of producing near-net-shape final parts.
Technology Outlook and Summary
Efforts have identified appropriate furnaces and process sequences for a viable production train using the HSPT process. In addition to interest from Tier-1 auto-parts suppliers, AMETEK Reading Alloys entered into discussions and negotiations with the University of Utah and renewed an option for licensing the technology in 2016. Markets and products outside this limited licensing agreement are ongoing, and the team led by Utah intends to obtain additional licensees for the technology.
In summary, this novel process technology has been successfully demonstrated. Wrought-like microstructures have been achieved, and wrought-like mechanical properties including both static and fatigue-strength targets have been achieved. DOE would like to recognize the contributions made by Professor Zak Fang to this article. Full project details are included in the final technical report by the University of Utah, which is available at: http://www.osti.gov/scitech/servlets/purl/1250865.
For more information: Contact Stephen J. Sikirica, technology manager, Advanced Manufacturing Office – U.S. Department of Energy, Washington, DC; tel: 202-586-504; e-mail: Stephen.firstname.lastname@example.org; web: www.energy.gov
Advanced Manufacturing Office (AMO)
Through research, development and demonstration efforts, the DOE Advanced Manufacturing Office (AMO) brings together manufacturers, research institutions, suppliers and institutes of higher education to develop cutting-edge manufacturing processes, information and materials technologies critical to efficient and competitive domestic manufacturing of clean-energy products to support energy productivity across the entire U.S. manufacturing sector and to reduce lifecycle energy and resource impacts of manufactured goods. This technology is one example of the research efforts recently undertaken by AMO that are of potential interest to the industrial heating community.