Spray forming of tubular deposits with close-coupled atomizer (CCA)

Abstract

The spray forming process is a rapid solidification process for production of metal preforms. In spray forming a superheated molten metal stream is atomized into small droplets by means of high-speed protective gas (e.g. N2, Ar or He) jets. The atomized semi-solid particles are deposited on a substrate into “near-net-shape” deposit, depending on the demands, e.g. billet, plate, ring, tube, strips. The refined grain structure of the as-sprayed materials due to its high cooling rate (103 – 106 K/s) provides a better mechanical behaviour than materials via conventional ingot casting or powder metallurgy techniques. Intermetallic refinement and macro segregation free microstructure allow to produce numerous new or adapted alloys with specific alloying elements. Spray forming of tubes may provide even faster cooling of the deposited materials compared to other forms like billets. This process can be used to produce coating on rods or tubes, clad tubes at a high production rate compared to other production techniques. In practice, spray-formed tubes or rings typically are produced by free-fall atomizers (FFA), which typically have low yield. In addition, the larger particles (d50.3 = 100 – 200 µm) of a FFA spray may induce high porosity in early stage of deposition. A recent study [ELLE14] showed that high yields and low porosities for small diameter tubes can be achieved with close-coupled atomizers (CCA). The aim of this PhD thesis is to understand the effect of the process conditions and to develop a process route of spray-formed tubes by CCA. A central hypothesis is proposed that less porosity and increased yield can be achieved by higher impact velocities of the spray droplets in CCA spray. Based on the central hypothesis four working hypotheses are derived: i) the deposit porosity can be reduced by smaller droplets by a CCA, ii) the droplet velocity can be increased further by using hot gas atomization, iii) a decreased deformation time during impact will result in high density materials and will extend the process window, and iv) the developed knowledge is transferable to other alloy systems. Therefore, a CCA needs to be adopted for spray forming of tubes with a provision of using hot gas in order to achieve higher droplet velocities. In-situ temperature and particle velocity measurement should be introduced during the atomization process to support the process understanding and development. As a result of the CCA adaptation, better porosity level have been achieved by smaller particles of a CCA compared to a FFA for AISI 52100 tubular deposits. However, high porosities are found at the deposit end positions due to lower melt mass flow rate and shadowing effects by the rebound particles. The porosity increases in the vicinity of the substrate with increasing gas-to-melt ratio (GMR), where mostly cold porosities are found. In-situ deposit surface temperature measurements show that the maximum deposit surface temperature mainly depends on the deposit thickness compared to the GMR. The grain size in the as-sprayed materials also increases at higher maximum deposit surface temperatures. Comparison between porosity and the maximum deposit surface temperature reveal that the porosity increases with decreasing maximum deposit surface temperatures below the solidus temperature of the alloy. An empirical prediction model is derived to calculate the maximum deposit surface temperature that is validated with the experimental data for AISI 52100 steel. Furthermore, for transferring the acquired knowledge to other alloying systems a dimensionless parameter DTT (ratio of the maximum deposit surface temperature and solidus temperature) is introduced. Introduction of hot gas atomization show that smaller particles with high velocity and low temperature can be produced at lower gas consumption by a CCA. With smaller particles sizes and higher velocities, the particles deformation time is lowered. The spray-formed tubes with hot gas atomization show less porosity even at lower deposit surface temperatures with hot gas atomization. The grain sizes also decrease due to lower maximum deposit surface temperatures. However, the microstructures of the as-sprayed materials show hardly any difference. Substrate preheating by the hot process gas lowers the porosity level in the vicinity of the substrate. Larger tube diameter result in additional cooling of the deposit and resulting lower maximum deposit surface temperature and smaller grain size. Comparison between the relative density at the deposit center and the maximum deposit surface temperature depicts that higher density can be achieved by hot gas atomization even well below the solidus temperature, which subsequently extended the process window of the spray forming of tubes. For cold gas a relative density > 0.95 is achieved at 1 < DTT < 1.13 and for hot gas atomization at 0.85 < DTT < 1.13. Spray forming of Al-alloy tubes with CCA show similar materials quality like the steel tubes, suggesting that the knowledge from the spray forming of steel tubes is transferable to other alloy systems. Refined grain structures with nano-meter-sized intermetallic phases are observed. High amount of porosity is found in the vicinity of the substrate (about 20% of the deposit thickness). The DTT of the Al-alloys deposits are calculated with the proposed empirical model. A minimum porosity is found for 1.02 < DTT < 1.05. However, the model needs to be validated with experimental data for Al-alloys.