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Metal Casting Technologies : March 2009
TECHNICAL FEATURE Figure 2 which shows that up to 30KN their strains are elastic, therefore they can not eliminate contraction loads. Reference to Figure 2 shows that up to 30 KN deformations of these plates are linear thus they do not have plastic deformation. At this load, their bend is about 2.9mm. Reference to test condition count of bend for each plate is about 1.45mm. Early studies have shown that hot tearing occurs at very low loads thus, effect of deformation plates on hot tearing is very small and can be overlooked. During solidification of casting in order to investigate proximity bend test to work condition a micrometer is set on back of plates to measure their displacements. This displacement really is the bend of plate. Results support the bend test condition accuracy. For example when load cell contraction load is 30 KN, measured displacement by micrometer is 1.5mm for each plate. These plates connect together by four pipes and other equipment set in this rigid flask like Figure 1. Observation Figure 3 illustrates an example of graph that get from hot tearing test on Al-9%Si alloy. Observed that at the initial time of pouring melt, the load graph is negative, one of the reasons is expansion of the bolts in the melt. Following with forming solid shell on the mold surface and its contraction, load graph is being moved to positive. Whereof the hot tearing occurs at the low loads, this behavior may cause error in data and eliminates slope changes or probable peaks in the early stage of cooling. This phenomenon in the other references observed but eliminated . Al-9%Si that modified the temperature of bolts and expansion is very small. In this Figure negative parts of graph are very small and their error is small. Figure 4. Load-time and temperature-time graph at second test Discussion Figure 3 shows pressure load at the load Figure 3. Load-time and temperature-time graph at first test In order to minimize negative load, use originality method to minimize the temperature of bolts. Figure 4 shows another kind of graph, extract from cell in onset of casting and then with increasing contraction load and graph moves to positive. Applied primeval pressure on the load cell is due to warming of instrument and its expansion. At the next steps the solid fraction increases and the solidified shell is being coarse and contraction load overcome on expansion load. At this stage the graph is moving to positive region. In the first test, due expansion of instrument and prison bolts in the melt, load cell shows a negative number. Following the solidification and forming Figure 2. Bend test graph of rigid plates Onset of casting, rapid solidification around bolts fixes them to the casting and transfers contraction load during solidification to copulated rod and load cell, then this load will be converted to signal and shown as a load-time graph. Preventing free contraction in other solidified zones focuses the strain in the mushy zone of casting, in a proper condition hot tearing will not occur. This tear presents in the graph as a peak or change slope. With temperature- time graph can be determine time of occurrence tearing and solid fraction. 44 www.metals.rala.com.au Figure 5. Optical micrograph of surface crack, first test