Metal Casting Technologies : MCT MAR 2018 (1ST QRT)
Temperature(°C) conv abl2 abl1 abl-0.5 0 200 400 600 650 600 550 500 450 400 Time(s) slurry generated from ablation by sedimentation in case a soluble binder, whereas there is possibility of recovery and reuse of the binder from the liquid part separated from the slurry by drying. In case of green sand, similar procedure can be followed to reclaim the base sand, clay and the liquid for recycling. Ablating fluid Water is the most common ablating fluid due to its cheapness, availability and non-toxicity. In most of the trials so far, water has been used as the ablating fluid. To increase the effectiveness of ablation, it can be acidic, alkaline or a solution containing salts depending on the type of binder in the mould material. In case of resin bonded sand mould, an organic solvent needs to be used as the ablating fluid, although the cost of such medium could be quite expensive. In commercial production, the recycling of the fluid is economical. The temperature of the fluid needs to be reduced by cooling before recycling for effective cooling of the sections of the casting. A typical temperature in case of water is 65°C that has been used in production of ablation castings. Ablation process The major objectives of the ablation process are to remove the mould material in a specified manner, cool the sections of the casting at fast rate for refinement of the microstructure and achieve directional solidification for maximum soundness of the casting. Spraying of the ablating fluid through nozzles is the common technique of mould removal and cooling. The spray of the fluid can be from the top or bottom of the casting, although other directions may be necessary for a complex casting shape. Fig. 1a) shows the progress of ablation from far end of the casting towards the feeder and Fig.1b) shows the casting exposed after ablation. The speed of the jet of fluid is significant in the sense that too low speed may be insufficient to ablate the mould material and remove the vapour blanket on the surface of casting. On the other hand, excessively high speed of the jet may distort the casting which is not strong enough in the hot condition. Hence the speed of the ablating fluid needs to be optimized for effective ablation producing a casting free from distortion. To achieve directional solidification, the ablating fluid jet should move from colder part of the casting towards the feeder at a controlled speed. For Complex shaped castings and having more than one feeder, the number of nozzles and their speed need to be optimized by use of cooling curve for effective directional solidification. Thermal simulation using software such as MAGMASOFT, PROCAST etc. may not be possible, as the cooling by spray of fluid is much more complicated than a stable mould. Microstructure It is well established that increased rate of cooling produces finer microstructure in metals and alloys by the mechanism of more number of nuclei in solidifying liquid metal. The ablation technique results in fast cooling of sections of a casting, comparable to permanent mould casting or even better depending on the process parameters. Fig. 2 compares the cooling rate of conventional sand casting with ablation castings. It is observed that the rate of cooling is increased considerably by ablation and as the time gap between pouring and start of ablation decreases, the rate of cooling increases. 32 www.metals.rala.com.au METAL Casting Technologies 1st Quarter 2018 33 Table 1. Comparison of Mechanical Properties of Al alloy 356 in T6 condition Mechanical properties Sand Casting Permanent mould casting Squeeze casting Pressure counter pressure casting Ablation casting Yield Strength (MPa) 179 207 243 239 261 UTS (MPa) 228 262 312 334 325 Elongation % 3.5 4 11 14.3 12.5 TECHNICAL FEATURE In conventionally cast Al-Si alloys, the microstructure consists of primary aluminium dendrites and the Al-Si eutectic. Similar structure is observed in the surface of the casting produced by ablation, since in the initial phase of solidification the mould exists around the liquid metal in the mould. Subsequently the ablation of the mould and contact of the fluid over the surface of the casting increases the rate of heat extraction by a factor of 100 to 1000 times. As a result the secondary dendrite arm spacing (DAS) reduces by a factor of 10 approximately in the subsurface of an ablated casting. The eutectic is also refined substantially by ablation and a typical inter-particle distance in the eutectic is of the order of 1 μm. In complex castings with varying section thickness, the finer microstructure may not be achievable in all its sections, i.e., some of the sections will have finer structure, whereas a relatively coarse structure will form in others. Mechanical properties Ablation cast Al alloys exhibit much improved mechanical properties compared to conventional sand castings. Table 1 shows the mechanical properties of Al alloy 356 castings under different processes. The yield strength, ultimate tensile strength and elongation % increases significantly by ablation casting and the properties are comparable to pressure-counter pressure castings. Improvement in yield strength and ultimate tensile strength (UTS) can be attributed to the finer microstructure with lower DAS. However under such mechanism, the elongation % should decrease. From Table 1, since both strength and elongation % has improved, other factors are to be taken into account. Reduction in porosity in ablated castings is another major factor for overall improvement in the mechanical properties. Under fast cooling in ablation, the temperature gradient is very high that results in reduced depth of mushy zone and hence reduces porosity. Moreover, the dissolved gases such as hydrogen have less scope to coalesce and form gas porosity under fast solidification, since diffusion of hydrogen to form porosity is a time dependent phenomenon. Oxide bi-films appearing as cracks present in Al alloy castings reduces its mechanical properties. When the rate of solidification is slow, the bi-films unfold and become more deleterious to the mechanical properties. In ablation castings, the fast rate of solidification makes the bi-films compact and minimizes the harmful effect of it on mechanical properties. Presence of finer particles of intermetallic based on Fe under fast cooling in ablated castings is another effect to improve the mechanical properties of the castings. Conclusion Ablation casting process produces Al alloy sand castings with considerably improved mechanical properties that are comparable or even better to the properties of castings produced by special casting processes such as squeeze casting and pressure counter pressure casting. The process is being developed for light alloy castings that are used in automobile and aerospace applications. High strength- to- weight ratio of the ablation castings have great potential for various light weight structural applications in future. n REFERENCES 1) Grassi et al. US Patent No. 7,216,691 2) J. Grassi, J. Campbell, M. Hartlieb and F. Major, Aluminum Alloys: Fabrication, Characterization and Applications, TMS (The Minerals, Metals and Materials Society), 2008, pp.73-77. 3) www.Solutionsfonderie.com/2017/02/02/what-is-ablation-casting/ 4) Grassi et al. US Patent Application Publication No. US 2017/0314110 A1 FIGURE 1. Ablation Casting Process; a) ablation in progress towards feeder. b) ablation complete showing exposed hot casting . FIGURE. 2 Comparison of cooling rate of an aluminium alloy 356 casting at its same critical section. Black cooling curve shows conventional casting without ablation, blue curve is for ablation started after 2 minutes of pouring, yellow curve for ablation after 1 minute of pouring and red curve for ablation after 0.5 minute after pouring . A B THE SPRAY OF THE FLUID CAN BE FROM THE TOP OR BOTTOM OF THE CASTING, ALTHOUGH OTHER DIRECTIONS MAY BE NECESSARY FOR A COMPLEX CASTING SHAPE.
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