The role of deposition temperature and scanning speed in the functional performance of laser assisted cold sprayed (LACS) coatings

Abstract: The functional performance of laser assisted cold sprayed (LACS) commercially pure ( CP) grade 1 titanium coatings was elucidated in terms of its mechanism of densification, microstructural evolution and corrosion resistance as the deposition temperature and scanning speed were altered by employing optical microscopy (OM) and potentio-dynamic polarization technique. The outcome of this study indicates that the densification mechanism of the coating was mainly influenced by the ratio of the processing temperature (T) and the scanning speed (SS) which is designated as . The attainment of the optimised functional properties of the coatings could be attributed to the thermal shear in the titanium film as well as its solid state inter-particulate consolidation resulting from localised thermal gradient which was induced between the ductile titanium particles and the brittle oxide film covering it at the optimum laser-gas-material interaction obtained at 600oC/10mm/s coupled with the adiabatic shearing of the particles upon impact at the deposition site. It was also established that microstructural porosity and cracks resulted from the increased lifetime of the liquid phase under suboptimal processing conditions which allowed more time for the propellant gas to initiate bubble formation within the coating’s microstructure. In addition, non-optimal parameters failed to attain the most desirable microstructural properties and corrosion resistance for the coatings. Finally, key factors in optimising LACS process parameters in order to achieve fully dense coatings are outlined

Investigation the effect of porosity on corrosion resistance and hardness of wc-co coatings on metal substrates

Abstract: Porosity is an important coating feature which strongly influences coating properties. Porosity creates poor coating cohesion and allows for higher corrosion rate and wear, and is generally associated with a higher number of unmelted or solidified particle that become trapped in the coating [1]. This investigation was conducted to investigate the effect of porosity on the hardness and corrosion resistance of WC-17Co coating on metal substrates. Coating of about 200μm were successful deposited by HVOF techniques unto four metal substrates, namely brass, 304L stainless steel, super-invar and aluminium. The corrosion behaviour was examined in chloride medium using direct current (DC) polarization test. The Vickers hardness was undertaken at loads of 5 kg for 10 s. The microstructures of the coatings were studied before and after the corrosion tests by scanning electron microscopy with EDX. The results indicated a strong correlation between porosity and corrosion rate, as well as hardness of the WC-17Co coatings

Synthesis of Ti-6Al-4V alloy with nano-TiN microstructure via spark plasma sintering technique

Abstract: The effect of nano-TiN dispersion strengthened Ti-6Al-4V via spark plasma sintering method has been investigated. Ti-6Al-4V with 4 vol. percent of nano-TiN were mixed in a Turbula shaker mixer for 8 h at a speed of 49 rpm and the admixed powders were sintered at sintering temperature range of 1000 - 1100 oC, holding time of 10-30 mins, heating rate of 100 oC/min under an applied pressure of 50 MPa. The morphology of the as-received and sintered compacts was examined by scanning electron microscopy (SEM) equipped with energy dispersive X-ray spectroscopy (EDS) and phase analysis was done by X-ray diffractometry (XRD). The sintered compacts without nano-TiN reveal lamellar structure while reinforced Ti- 6Al-4V with nano-TiN shows a bimodal structure and titanium nitride has a great influence on α grain growth at high temperature. Furthermore, the microstructural formation mechanism was investigated. With the addition of the content of Ti-6Al-4V with 4 vol.% of nano-TiN, the microhardness also improved and this was due to homogenous distribution of TiN in Ti-6Al-4V matrix

Influence of sintering temperature on hardness and wear properties of TiN Nano reinforced SAF 2205

Abstract: Conventional duplex stainless steel degrade in wear and mechanical properties at high temperature. Attempts have been made by researchers to solve this problems leading to the dispersion of second phase particles into duplex matrix. Powder metallurgy methods have been used to fabricate dispersion strengthened steels with a challenge of obtaining fully dense composite and grain growth. This could be resolved by appropriate selection of sintering parameters especially temperature. In this research, spark plasma sintering was utilized to fabricate nanostructured duplex stainless steel grade SAF 2205 with 5 wt.% nano TiN addition at different temperatures ranging from 1000 °C to 1200 °C. The effect of sintering temperature on the microstructure, density, hardness and wear of the samples was investigated. The results showed that the densities and grain sizes of the sintered nanocomposites increased with increasing the sintering temperature. The microstructures reveal ferrite and austenite grains with fine precipitates within the ferrite grains. The study of the hardness and wear behaviors, of the samples indicated that the optimum properties were obtained for the sintering temperature of 1150 °C

Effect of starting powder particle size and heating rate on spark plasma sintering of Fe- Ni alloys

Abstract: The effect of starting powder particle size and heating rate on spark plasma sintering of Fe-Ni alloys was investigated, with the particle powder size varying from 3 to 70 μm and heating rate from 50 to 150 °C/min. The effect of the starting powder particle size was more obvious when comparing 3-FeNi and 70-FeNi at all heating rates, with the former having better density and hardness than the latter. Sintered densities close to theoretical (≥ 99%) were achieved for a heating rate of 50°C/min for the different starting particle size powders, and decreased with increasing heating rate. The average grain size of alloys sintered at 150°C/min was ~34% smaller than those sintered at 50°C/min. The porosity content of the sintered samples increased with increasing heating for the same particle size. The shrinkage rate depends on both heating rate and particle size. At a particle size of 3 μm and a heating rate of 50oC/min, three peaks were observed indicative of the phenomena responsible for good densification. As the heating rate increases, only two peaks and one peak are observed at heating rates of 100 and 150oC/min, respectively. This suggests that, unlike high heating rates, the longer processing time at low heating rate allows the three phenomena to take place. The hardness measurement revealed a steady decrease with increasing heating rate. At a heating rate of 150°C/min the particles were well packed but no typical dimple structure of a ductile material was observed. However, for samples sintered at 50 and 100°C/min a typical dimple fracture morphology was observed

Corrosion behaviour of Al-Fe-Ti-V medium entropy alloy

Abstract: Alloys containing up to four multi-principal elements in equiatomic ratios are referred to as medium entropy alloys (MEA). These alloys have attracted the interest of many researchers due to the superior mechanical properties it offers over the traditional alloys. The design approach of MEA often results to simple solid solution with either body centered cubic; face centered cubic structures or both. As the consideration for introducing the alloys into several engineering application increases, there have been efforts to study the corrosion behaviour of these alloys. Previous reports have shown that some of these alloys are more susceptible to corrosion when compared with traditional alloys due to lack of protective passive film. In this research, we have developed AlFeTiV medium entropy alloys containing two elements (Ti and Al) that readily passivate when exposed to corrosive solutions. The alloys were produced in vacuum arc furnace purged with high purity argon. Open circuit potential and potentiodynamic polarisation tests were used to evaluate the corrosion behaviour of the as-cast AlFeTiV alloy in 3.5 wt% NaCl and 1 M H2SO4. The corrosion performance of the alloy was compared with Ti- 6Al-4V alloy tested under similar conditions. The results show that unlike in Ti-6Al-4V alloy, the open circuit potential of the AlFeTiV alloy move towards the negative values in both 3.5 wt% NaCl and 1 M H2SO4 solutions indicating that self-activation occurred rapidly on immersion. Anodic polarisation of the alloys showed that AlFeTiV alloy exhibited a narrow range of passivity in both solutions. In addition, the alloys exhibited lower Ecorr and higher Icorr when compared with traditional Ti-6Al-4V alloy. The traditional Ti-6Al-4V alloy showed superior corrosion resistant to the AlFeTiV alloy in both 3.5 wt.% NaCl and 1 M H2SO4 solutions