Many reactive metals are difficult to prepare in pure form without complicated and expensive procedures Although titanium has many desirable properties (it is light, strong and corrosion-resistant), its use has been restricted because of its high processing cost In the current pyrometallurgical process--the Kroll process--the titanium minerals rutile and ilmenite are carbochlorinated to remove oxygen, iron and other impurities, producing a TiCl4 vapour This is then reduced to titanium metal by magnesium metal; the by-product MgCl2 is removed by vacuum distillation The prediction that this process would be replaced by an electrochemical route has not been fulfilled; attempts involving the electro-deposition of titanium from ionic solutions have been hampered by difficulties in eliminating the redox cycling of multivalent titanium ions and in handling very reactive dendritic products Here we report an electrochemical method for the direct reduction of solid TiO2, in which the oxygen is ionized, dissolved in a molten salt and discharged at the anode, leaving pure titanium at the cathode The simplicity and rapidity of this process compared to conventional routes should result in reduced production costs and the approach should be applicable to a wide range of metal oxides

Direct electrochemical reduction of titanium dioxide to titanium in molten calcium chloride

Cold spraying has attracted serious attention since unique coating properties can be obtained by the process that are not achievable by conventional thermal spraying This uniqueness is due to the fact that coating deposition takes place without exposing the spray or subtrate material to high temperatures and, in particular, without melting the sprayed particles Thus, oxidation and other undesired reactions can be avoided Spryy particles adhere to the substrate only because of their high kinetic energy on impact For successful bonding, powder particles have to exceed a critical velocity on impact, which is dependent on the properties of the particular spray material This requires new concepts for the description of coating formation but also indicates applications beyond the market for typical thermal spray coatings The present contribution summarizes the current “state of the art” in cold spraying and demonstrates concepts for process optimization

The cold spray process and its potential for industrial applications

The number of research papers as well as of patents and patent applications on cold spray and cold spray related technologies has grown exponentially in the current decade. This rapid growth of activity brought a tremendous amount of information on this technology in a short period of time. The main motivation for this review is to summarize the rapidly expanding common knowledge on cold spray to help researchers and engineers already or soon to be involved for their future endeavors with this new technology. Cold spray is one of the various names for describing an all-solid-state coating process that uses a high-speed gas jet to accelerate powder particles toward a substrate where they plastically deform and consolidate upon impact. Cold gas dynamic spray, cold spray, kinetic spray, supersonic particle deposition, dynamic metallization or kinetic metallization are all terminologies found in the literature that designate the above-defined coating process. This review on cold spray technology is divided into two parts. In this article, Part I, patents and patent applications related to this process are reviewed, starting from the first few mentions of the idea at the beginning of the 20th century to its practical discovery in Russia in the 1980s and its subsequent occidental development and commercialization. The patent review encompasses Russian and USA patents and patent applications. Part II will review the scientific literature giving a general perspective of the current understanding and capability of this process.

/pdf/review-on-cold-spray-process-and-technology-part-i-4yauhqb07t.pdf

Review on Cold Spray Process and Technology: Part I—Intellectual Property

The invention relates to an apparatus and process for solid-state deposition and consolidation of powder particles entrained in a subsonic or sonic gas jet onto the surface of an object. Under high velocity impact and thermal plastic deformation, the powder particles adhesively bond to the substrate and cohesively bond together to form consolidated materials with metallurgical bonds. The powder particles and optionally the surface of the object are heated to a temperature that reduces yield strength and permits plastic deformation at low flow stress levels during high velocity impact, but which is not so high as to melt the powder particles.

System and process for solid-state deposition and consolidation of high velocity powder particles using thermal plastic deformation

Processing of cermet such as WC–Co is not easy by cold (gas dynamic) spraying although cold spray process can eliminate the degradation of the phase as compared to conventional high velocity oxy-fuel (HVOF) spraying process. In this study, WC–12∼17%Co powders with nano- and microstructures were deposited by cold spray process using nitrogen and helium gases. Microstructural characterization and phase analysis of feedstock powders and as-deposited coatings were carried out by SEM and X-ray diffraction (XRD). The results show that there is no detrimental phase transformation and/or decarburization of WC by cold spray deposition as expected. It is also observed that nano-sized WC in the feedstock powder is maintained in the cold sprayed coatings. It seems that nano-sized WC is advantageous over micro-sized WC for cold spray deposition because higher particle velocity can be obtained with the same gas velocity. It is demonstrated that it is possible to fabricate the nano-structured WC–Co coating with low porosity and very high hardness (∼2050 HV) by cold spray deposition with reasonable powder preheating.

Fabrication of WC–Co coatings by cold spray deposition

An environmentally compliant triboelectric applicator and process for coating or ablating a substrate and for retrieving excess or ejected material from the substrate. The applicator comprises an inner supersonic nozzle for accelerating triboelectrically charged projectile particles entrained in a supersonic gas to speeds sufficiently high to coat or ablate a substrate. The applicator further comprises an outer evacuator nozzle coaxially surrounding the inner supersonic nozzle for retrieving excess projectile particles, ablated substrate powders, or other environmentally hazardous materials. A fluid dynamic coupling uses the efficacy of the Mach turning angle associated with a supersonic boundary layer of carrier gas to aspirate the central core of the supersonic two-phase jet. This fluid coupling and spacing between the outlet of the supersonic nozzle and the substrate also permits the substrate to triboelectrically charge to levels which induce electrostatic discharges at the substrate simultaneous to the impacts. The aspiration feature reduces the outlet pressure in the central core of the nozzle below ambient pressure which allows the projectiles to travel unimpeded to the substrate, and reduces the inlet pressure required to achieve parallel and shock-free flow with the inner supersonic nozzle. The powders are injected into the carrier gas using powder feeders modified for high pressure and the ablated debris with excess projectile particles are collected in a particle precipitator and filter unit using a suction blower. A special nozzle applicator embodiment and process for coating or ablating the internal surface of a cylinder bore is also disclosed. This nozzle applicator comprises an axisymmetric cylindrical nozzle for conveying, accelerating, and triboelectrically charging projectile particles entrained in a carrier gas to speeds sufficiently high to coat or ablate the internal surface of the cylinder bore substrate when impacted by a triboelectrically charged jet comprising a central core.

Coating or ablation applicator with a debris recovery attachment

The invention relates to a powder-fluidizing and feeding device for use with coating and spray forming nozzles and guns. The device includes novel provisions for controlling and feeding powder from a hopper to a vibrating bowl, for heating and vibrating powders in the hopper to dissipate agglomeration and clumping of the powder, and for metering powders from a vibrating bowl to the spray nozzles and guns using a feedback control derived from powder mass loss rate measurements. The device is particularly useful for feeding ultra-fine and nanoscale particles, which are difficult to feed uniformly with the prior art of conventional powder feeders.

Powder fluidizing devices and portable powder-deposition apparatus for coating and spray forming

Figure 2. The SSF system for fabricating near-net-shape OFTi components. Editor’s Note: This paper was presented at the 1998 TMS Annual Meeting as part of the Innovations in Titanium symposium, which was sponsored by the Office of Industrial Technologies, Energy Efficiency and Renewable Energy, U.S. Department of Energy, Innovative Concepts program. This issue of JOM represents the proceedings of that symposium.

/pdf/the-solid-state-spray-forming-of-low-oxide-titanium-1ikav8srt7.pdf

The Solid-State Spray Forming of Low-Oxide Titanium Components

The next steps in developing SSF will require sufficient funding to produce samples with the suggested refinements and evaluate these samples. The steps include developing a proprietary auxiliary process that renders the annealing step unnecessary and allows for the direct use of the near-net shape in the assprayed condition, investigating specially prepared strengthening phase powders that better match the mechanical impedance of aluminum for producting near-net-shape aluminum MMC, using heated air as an accelerating gas, and conducting additional mechanical tests of aluminum and aluminum MMC to determine material properties.

Ralph Tapphorn

Papers

System and process for solid-state deposition and consolidation of high velocity powder particles using thermal plastic deformation

Coating or ablation applicator with a debris recovery attachment

Powder fluidizing devices and portable powder-deposition apparatus for coating and spray forming

The Solid-State Spray Forming of Low-Oxide Titanium Components

Solid-state spray forming of aluminum near-net shapes