Saden H. Zahiri

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.

Characterization of Cold Spray Titanium Deposits by X-Ray Microscopy and Microtomography

Cold gas dynamic spray (cold spray) is a rapid deposition technology in which particles deposit at velocities above the speed of sound (approximately 340 ms-1). Generally, porosity forms in cold spray deposits due to insufficient deformation of particles. In this study, the unique capability of the X-ray microscopy and microtomography is utilized to visualize the internal structure of deposited material. The results show that this characterization technique successfully reveals porosities in the cold spray commercial purity (CP) titanium structure. Furthermore, microtomography images confirmed the experimental results for porosity measurements in which helium (compared with nitrogen) as carrier gas significantly decreases porosity in cold spray CP titanium.

The static, dynamic and metadynamic recrystallisation of a medium carbon steel

Abstract Grain refinement during and after hot isothermal deformation of a medium carbon steel has been investigated. The average austenite grain size decreased with an increase in strain for the hot deformed and recrystallised material, with refinement extending beyond the strain for the peak stress. A window of strain that corresponds to transition from classical static to metadynamic recrystallisation was observed in respect to the recrystallised material. Within this post-dynamic transition window the strain at which strain independent softening occurs was different for different volume fractions of the recrystallised material. This led to a new terminology corresponding to initiation of strain independent softening. For the alloy of this study, strain independent softening for the start of post-deformation recrystallisation occurred near the strain to the peak stress. The strain corresponding to complete metadynamic recrystallisation, which was defined as when all levels of recrystallisation were strain independent, was much greater than the strain for the peak stress.

Simultaneous prediction of austemperability and processing window for austempered ductile iron

Abstract A model is developed for simultaneous prediction of the processing window and austemperability of austempered ductile iron (ADI). The processing window represents a frame of time and temperature in which ADI satisfies optimum mechanical properties defined by ASTM A897M:1990. Austemperability is the maximum section size of ductile iron that can be austempered without formation of pearlite during the austempering process. The outcome of the model presents the processing window and austemperability as a three dimensional diagram (processing - austemperability window). The processing window boundaries are estimated according to a model for prediction of the time for ausferritic reaction in ADI. The austemperability of ductile iron is predicted according to the estimated pearlite curve of the TTT diagram and a mathematical model that simulates conduction of heat in a solid cylinder. The heat transfer model is calibrated for a ductile iron of composition (wt-%) 3.63C, 2.4Si, 0.39Mn, 0.4Mo, 0.25Cu, 0.04Ni, 0.04Mg. The model for the processing - austemperability window is validated for a ductile iron of composition (wt-%) 3.41C, 2.46Si, 0.36Mn, 0.18Mo, 0.25Cu, 0.036Mg at 285, 380, and 400 ° C austempering temperatures. Results show that the material satisfies ASTM A897M:1990 standard for the chosen experimental points within the processing - austemperability window without formation of pearlite in the microstructure.

Model for prediction of processing window for austempered ductile iron

Abstract A computer model has been developed to predict the processing window (austempering window) for austempered ductile iron (ADI). The model is a modification of an existing model for the isothermal decomposition of austenite in bainitic steels. It was calibrated using experimental data from the literature. In order to validate the model, the processing window corresponding to a ductile iron of composition 3.41%C, 2.46%Si, 0.36%Mn, 0.18%Mo, and 0.25%Cu is predicted and compared to experimental data. Computer assisted image analysis was used to investigate the volume fraction of martensite at the lower boundary of the processing window. X-ray diffraction was used to calculate the normalised volume fraction of austenite at the upper boundary of the processing window. The results show that the model successfully predicts the processing window corresponding to the iron investigated in this study.

Hybrid metallic composite materials fabricated by sheathed powder compaction

A new process, sheathed powder compaction (SPC), was proposed to fabricate hybrid metallic composite samples at room temperature. The process involves densification of fine powder in an annulus sheath under combined moderate pressure and shear. Hybrid samples of copper annular with aluminum powder were produced to demonstrate the merits of the process. The fabricated samples were characterized and explored using electron microscopy, micro hardness, micro shear punch tests and density ratio measurements. Comparing “shear punch response” of the samples in the Al region with those of commercially pure aluminum and Al 5005-H34 samples showed an excellent shearing strength in the region. The electron backscatter diffraction analyses and scanning electron microscopy observations suggested that the substantially enhanced strength was facilitated by a significant SPC induced grain refinement. Moreover, the annular sheath (copper) also showed significant grain refinement due to the process. These demonstrated promising potentials of the SPC to manufacture hybrid metals with concurrent grain refinement at room temperature under moderate processing conditions.Graphical Abstract

Residual Stresses in Cold Spray Process Using Finite Element Analysis

In cold spray process, the simulation of coating deposition and the arising residual stress analysis are critical for an optimisation of process conditions. However, there are not many published literatures on the role of residual stresses in a cold sprayed coating. In addition, the multi-particles deposition behavior is also not well known, especially when coating of pure titanium powder is considered. This paper considers the development of an explicit finite element model of a cold spray process with defined initial and boundary environment. The explicit finite element model is then used to determine the optimum operating parameters to deposit titanium particles. It is also used to predict the residual stress developed in the coating by examining a fluid/structure interaction. The measured velocity and temperature from Computational Fluid Dynamics model are then use as initial input parameters to the explicit dynamic simulation. The predicted results reveal that Finite Element Method can be used to study the development of residual stress in a cold-sprayed coating as well as to find the optimum operating conditions to deposit coating of titanium particles before doing a real time fabrication.

Models for Static and Metadynamic Recrystallisation of Interstitial Free Steels

We have investigated the static and metadynamic behaviour of the interstitial free steels and in particular the effects of the steeling elements (phosphorous and boron) on kinetics of recrystallisation. The results showed that the strain for the initiation of strain independent softening (often referred to as metadynamic recrystallisation) varies with the Zener-Hollomon parameter and steel composition. Strain rate had a strong influence on kinetics of metadynamic recrystallisation. An increase in temperature from 930oC to 1100oC led to a decrease in time for 50% softening (about one order of magnitude) in the SRX region. However, for the same temperature range, the time for 50% MDRX did not change significantly.

PIV Validation of 3D Multicomponent Model for Cold Spray Within Nitrogen and Helium Supersonic Flow Field

This study presents the validation of a developed three-dimensional multicomponent model for cold spray process using two particle image velocimetry (PIV) experiments. The k-ε type 3D model developed for spherical titanium particles was validated with the measured titanium particle velocity within a nitrogen and helium supersonic jet. The 3D model predicted lower values of particle velocity than the PIV experimental study that used irregularly shaped titanium particles. The results of the 3D model were consistent with the PIV experiment that used spherical titanium powder. The 3D model simulation of particle velocity within the helium and nitrogen jet was coupled with an estimation of titanium particle temperature. This was achieved with the consideration of the fact that cold spray particle temperature is difficult and expensive to measure due to considerably lower temperature of particles than thermal spray. The model predicted an interesting pattern of particle size distribution with respect to the location of impact with a concentration of finer particles close to the jet center. It is believed that the 3D model outcomes for particle velocity, temperature and location could be a useful tool to optimize system design, deposition process and mechanical properties of the additively manufactured cold spray structures.