Metal Casting Technologies : MCT-2NDQRT-2017
28 www.metals.rala.com.au METAL Casting Technologies 2nd Quarter 2017 29 TECHNICAL FEATURE is normally used for many castings. However, even with quenching in hot water distortion can still occur in some casting designs. Hence, as mentioned above, forced air is preferred for larger and complex castings such as engine blocks and heads. With respect to the casting design and the alloy used for its production a balance has to be achieved such that the casting can be quenched at a rate fast enough to prevent precipitation of any damaging second phases, but not so fast that it causes excessive residual stress or distortion. Attempts to optimize the quenching operation include the use of polymer quenchants , and quenching into fluidised beds . The most widely accepted polymers used in aqueous solutions as quenchants are poly-alkylene-glycols (PAG) since their use can result in improved wetting and give more uniform, less steep temperature gradients, which in turn result in significantly lower residual stresses and less distortion when compared to both cold and hot water quenchants . Slower quenching rates can reduce both strength and intergranular corrosion resistance of heat treated Al alloys. Corrosion resistance must therefore also be monitored when using forced air quenching, polymer quenchants or fluidised beds to prevent distortion at the expense of acceptable reductions in strength. The effects of rate of cooling during quenching together with the effects of solution treatment and ageing conditions have been examined via quench factor modelling using Jominy type end quench testing  and by plotting continuous cooling precipitation diagrams, e.g. for Al-7Si-0.3Mg alloy . Simulation modelling can predict not only the porosity distribution and microstructure in the as-cast condition but also indicate mechanical properties after heat treatment with respect to casting geometry. Alloy composition, casting method and heat treatments can then be optimized . To obtain uniform heat treatment response and to minimize distortion in each casting in a heat treatment batch it is essential to use correct racking arrangements which ensure correct support without restriction and which give predictable heating and cooling behaviour at each position [10, 11]. One other approach that has been considered to reduce residual quenching stresses is the use of uphill quenching [12-14]. This involves treating Al alloy that has been solution treated by cold water quenching by immediate further sub-zero cooling in dry ice or liquid nitrogen. The alloy is then reheated in boiling water or high pressure steam and then aged. Residual stresses induced during water quenching are said to be offset during the subsequent rapid heating – the uphill quench. The process is said to be relatively expensive and difficult to apply  but has been assessed for treatment of forgings and to improve resistance to fatigue crack growth in heat treated Al – Si – Mg castings . To date the main application of uphill quenching is for precision machined components for aerospace, optical and military uses . Solution Treatment Various studies [e.g. 4, 15-18] have shown that it is possible to reduce solution treatment times while still achieving satisfactory mechanical properties, particularly for gravity die castings and for sand castings having small secondary dendrite arm spacings and well modified structures. In particular, the use of “3 in 1” type plant is said to enable total heat treatment time to be reduced from 19 to 6.5 hr. . Producing high pressure diecastings via semi-solid processes not only enables full solution treatment to be applied without blistering but also allows effective use of short holding times e.g. 1 hr. for A356 brake calipers . For conventional castings in A319 with dendritic microstructures, raising the solution treatment temperature from 500 to 545oC can reduce solution treatment time from 8 to 4hr. The higher temperature also tended to cause increased rounding of the eutectic Si improving fracture resistance . Clearly there is a greater danger of incipient melting if higher solution treatment temperatures are not adequately controlled with respect to composition, notably Mg content in Al-Si-Cu-Mg alloys. There has also been interest in the use of two-step temperature solution treatments. For example, in the case of A319 treatment at 495oC for 2hr is followed by 4hr at 515oC . The higher temperature stage is needed to increase dissolution of the Cu rich intermetallics and homogenize the structure . Such double solution treatment has been shown to increase the corrosion resistance of A319 . Work by Lumley [20, 21] has shown that conventional high pressure diecastings can be solution treated by using lower than normal solution treatment temperatures of 450-460oC at which surface blistering and part distortion can be avoided. This enables higher strengths to be obtained after ageing when compared to non-solution treated material. This was said to offer the possibility of 10-40% weight savings if the parts can be re-designed with respect to the higher attainable strength levels. Interrupted Ageing For given alloy and solution treatment variables the time and temperature of ageing control the type, size, shape, distribution, volume fraction, coherency and the crystallographic nature of the precipitate that is formed, and hence determine the mechanical and other properties of a heat-treated component, including its dimensional stability. Figure 4 illustrates how hardness changes during artificial ageing at temperatures from 150 to 230oC in solution treated A319 alloy. The as-cast and solution treated (503oC for 8hr.) microstructures of this alloy are shown in Figure 2. The STEM image in Figure 3 shows the precipitate after ageing at 170oC for 24 hr., the condition which gave maximum hardness [2, 3]. Natural ageing after solution treatment (called T4 treatment) does not provide stable dimensions and properties since ageing will continue with time giving gradual structural changes, especially if service temperature is raised. Artificial ageing (T6 treatment) gives better structural and dimensional stability but controlled over-ageing (T7 treatment) must be applied to ensure thermal stability in components to be used at higher relative temperatures, e.g. pistons. Treatment of as-cast structures with no prior solution treatment by artificial ageing is also referred to as stabilization and this is denoted by T5 temper. This can reduce costs of heat treatment but final properties depend on the as-cast microstructure which can result in their wide variation. T6 and T7 artificial ageing treatments are normally applied soon after solution treatment but they may be delayed, usually for 8hr. or more, to allow straightening and natural ageing as T61 and T71 tempers. This delay period is said to result in reduced variation in final strength levels. It has been found that if conventional artificial ageing is interrupted by an intermediate period at a lower temperature, typically between 25 and 65oC, before resuming ageing at the original or other artificial ageing temperature then mechanical properties can be improved by an average 10% compared to conventional non-interrupted ageing treatments [22-25]. To differentiate it from conventional T6 treatment, interrupted treatment was signified as T6I6 . A modified interrupted treatment, denoted as T6I4 can also be applied, in which treatment involves under-aging at a higher temperature then quenching and holding at a lower temperature to complete ageing without re-heating to a higher ageing temperature . Beckwith Macbro Resin Coated Sands Contact : Rob Dalla Via 30 Devon Road Devon Meadows, Melbourne Telephone: +61 3 5995 4244 Mobile: 0417 332 723 Fax: +61 3 5995 5030 E-mail: firstname.lastname@example.org Website: www.beckwithmacbro.com.au All grades of resin coated sand used for shell molding and shell cores for ferrous and non-ferrous applications PRODUCTS n Range of resin strengths from 1.0% to 5.0%. n Silica, Zircon, Chromite coated sands or blended mixes. n Coated Sands of different AFS typically from 50-90 AFS. n Thermal Reclaimed Coated Sands. n Frac Sand. SERVICES n Full technical and trouble shooting service. n On-site shell core and shell molding facility to evaluate the product applications. n Laboratory facility to ensure product quality. FORSALESHELLCOREMACHINESSHELLMOULDMACHINES SIMULATION MODELLING CAN PREDICT NOT ONLY THE POROSITY DISTRIBUTION AND MICROSTRUCTURE IN THE AS-CAST CONDITION BUT ALSO INDICATE MECHANICAL PROPERTIES AFTER HEAT TREATMENT WITH RESPECT TO CASTING GEOMETRY. NATURAL AGEING AFTER SOLUTION TREATMENT (CALLED T4 TREATMENT) DOES NOT PROVIDE STABLE DIMENSIONS AND PROPERTIES SINCE AGEING WILL CONTINUE WITH TIME GIVING GRADUAL STRUCTURAL CHANGES, ESPECIALLY IF SERVICE TEMPERATURE IS RAISED. FIGURE 4. Effect of ageing temperature and time on hardness of solution treated A319 .