Stefan Gulizia

Effect of processing conditions on porosity formation in cold gas dynamic spraying of copper

The cold gas dynamics process is a promising low-temperature spray process in which particles are accelerated in a supersonic flow before impacting with substrate to be coated. In this study the effect of spray temperature, spray pressure, and particle size on porosity formation in cold spray coatings are investigated. Results show that an increase in spray temperature and a decrease in particle size lead to a decline in volume fraction of porosity. Furthermore, particle velocity and particle temperature are determined to be the significant parameters for elimination of porosity. A model is proposed for estimation of the volume fraction of porosity for alloy of this study.

Direct Manufacturing of Titanium Parts by Cold Spray

Titanium has excellent properties as an engineering material such as light weight, high strength and high resistance to corrosion and fracture. However, the high cost associated with the materials and current process technologies is not conducive to higher-volume production for consumer industry. It appears near net shape manufacturing has to be used to manufacture titanium and titanium alloys parts. Investigators are exploring several near net shape technologies. However, most of these technologies involve melting and solidification. Each new layer starts out molten, solidifies, and must eventually cool to room temperature. Oxygen sensitive material such as titanium needs to be processed under vacuum. There is a great need for revolutionary coating and direct Manufacturing technology to extend the application of titanium and titanium alloys from top end, aerospace and biomedical to lower end consumer use. It appears Cold Spray Technology can deliver a suitable and cost effective coating and direct manufacturing solution for titanium industry. CSIRO Light Metals Flagship has pioneered in developing direct manufacturing technologies to fabricate titanium parts using Cold Spray. Mechanical properties of Cold Spray titanium in as sprayed and heat treated conditions are presented and compared with wrought titanium. Some of technologies such as Cold Spray for direct manufacturing of seamless titanium pipes are discussed.

Characterisation of Cold Spray Titanium Coatings

Cold spray is a solid state spray deposition process utilizing a supersonic De Laval nozzle to accelerate fine particles to high velocities. Particles plastically deform on impact to the substrate and to each other to create dense well adhered structures. In this study, the microstructure and mechanical properties of cold spray Titanium coatings deposited using nitrogen gas at different gas temperature and pressure were examined. In general, it was found that gas-atomised CP-titanium powder is capable of producing dense coating structures on aluminium alloy (Al6061) substrates. The micro-hardness, oxygen and nitrogen content of the coatings were found to be slightly higher than powder in the as-received condition. It was also found the coating residual stress was purely compressive when cold spray is conducted at high gas pressure and temperature.

Thermal Fatigue Studies Using HF Induction Heating of Die Materials for Light Metals Casting

In this study a new thermal fatigue test rig has been developed that can apply a net energy input to materials with different physical properties using HF induction heating. Several commercially available hot work ferrous and non-ferrous die materials were evaluated with the aim of providing a basis for selecting an appropriate die material with good thermal fatigue resistance for a given HPDC application. The results show materials with high thermal conductivity such as tungsten-based materials are more resistant to thermal fatigue cracking than conventionally used hot work tool steels for HPDC dies. The initiation and growth of thermal fatigue cracks were examined and periodically evaluated using computer image analysis, for crack morphology, and hardness on each material tested.

Utilization of Titanium Particle Impact Location to Validate a 3D Multicomponent Model for Cold Spray Additive Manufacturing

Cold spray is a solid-state rapid deposition technology in which metal powder is accelerated to supersonic speeds within a de Laval nozzle and then impacts onto the surface of a substrate. It is possible for cold spray to build thick structures, thus providing an opportunity for melt-less additive manufacturing. Image analysis of particle impact location and focused ion beam dissection of individual particles were utilized to validate a 3D multicomponent model of cold spray. Impact locations obtained using the 3D model were found to be in close agreement with the empirical data. Moreover, the 3D model revealed the particles’ velocity and temperature just before impact—parameters which are paramount for developing a full understanding of the deposition process. Further, it was found that the temperature and velocity variations in large-size particles before impact were far less than for the small-size particles. Therefore, an optimal particle temperature and velocity were identified, which gave the highest deformation after impact. The trajectory of the particles from the injection point to the moment of deposition in relation to propellant gas is visualized. This detailed information is expected to assist with the optimization of the deposition process, contributing to improved mechanical properties for additively manufactured cold spray titanium parts.

Processing TiROTM Powder for Strip Production and other Powder Consolidation Applications

The TiROTM process has been developed at CSIRO for the continuous direct production of Ti powder. The process has two main steps; a reaction step where the Ti is produced as very fine particles dispersed in larger particles of magnesium chloride. The MgCl2 is separated from the Ti powder in a continuous vacuum distillation unit. The Ti product from this unit comprises a lightly sintered “biscuit” of Ti particles that can be broken up into individual particles powder with a d50 around 200 μm. These particles have a unique morphology which is a function of the process.For many powder metallurgical applications TiROTM powder will require further processing to tailor its morphology for the specific application. A small sample of Ti strip has been produced from ring milled TiROTM powder by a CSIRO patented combination of direct powder rolling (DPR) followed by hot roll densification (HRD). The Ti strip was annealed and characterised in terms of microstructure and chemistry.A powder manipulation technology (PMT) has been developed to modify TIROTM particulates without the need of more expensive hydride-dehydride (HDH) or gas atomization routes to improve density, flowability, size, distribution and shape for cold spray and additive manufacturing applications.