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plates were created. These designs were prone to the produc- tion of oxide entrainment into the bulk melt upon the conflu- ence of molten metal streams flowing in opposite directions. The plates, divided into two separate sets with different pouring temperatures, were produced by the EPGS process, linking up an electromagnetic pump for the controlled filling of green sand molds. The main results can be summarized as follows:


• The filling of the plates appeared to be free of bubble entrainment events as attested by the small size of pores found and the high average values of bending strength obtained (as far as non-heat treated material is concerned, see Table 8). This is consistent with the results of computer simulations, which showed flow velocities in the ingates below the critical values.


• Analogously, the simulations indicated the plate with the lowest filling temperatures, and therefore the highest propinquity to produce a superficial weld defect, was the B-type. This matched with experimental results, as only the plates of this kind showed this defect. This can be explained by the longer flow distances and enhanced cooling of the melt associated with the B-plates. Consistently, the increase of pouring temperature from 710C to 730C completely eliminated the defect in the ex- perimental plates.


• For a further analysis of the weld area of one of the plates, surface profile and area images of samples both in and away from the defected region were obtained using the Talyscan apparatus. The results from the surface image analysis showed the weld extended into the depth of the plate up to approxi- mately 100 microns (also confirmed by optical mi- croscopy), which is a minimal intrusion consider- ing the plate thickness.


• Consistently, out of four different test specimens extracted from the confluence weld area, only one of them broke within the weld. The optical, surface profiling, and bend test results indicate a success- ful joining of the two opposite metal streams for the B-plate took place. This is thought to be a re- sult of the continual movement of the streams from opposing ingates (none stop completely), which would ensure the oxide skins upon the surfaces of the streams remain thin and produce suitable con- ditions for a successful joining.


• After examining the fracture surfaces of the speci- mens with scanning electron microscopy, with par- ticular focus along the edges of the fractured area, regions of porosity were found in all of the sam- ples. These porous areas may have taken prece- dence over any confluence weld related oxide films formed during filling, thus leading to the fracture of the specimens in various locations.


• The simulation results indicate the highest prob- ability of oxide entrainment corresponds to the


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laterally-filled B-plates. For the early stage of the filling, a heavier collision of the metal streams and an enhanced swaying of the flow just afterward was observed. These two effects are explained by the longer distances of horizontal flow associated with these plates.


• The results of the Weibull analysis for the second set of castings confirms these effects, as the two B-plates (filled either by one or two ingates), con- sistently presented lower values of Weibull moduli versus the bottom-gated plates (A1 and A2). Con- sistently, the highest modulus was found for the plates (A2) where the horizontal flow is of least importance.


• These results indicate that in order to maximize the quality of a given A356 cast product, which fea- tures confluence welds and which is to be subjected to strong mechanical loading during service, a bot- tom gating design should be favored.


In summary, the results of this work preliminarily indicate the area in which the confluence weld occurs does not fea- ture lower mechanical reliability than the rest of the cast- ing, as long as proper flow conditions prevail in terms of sufficiently high temperature and non-stop movement of the metal front. Alternatively, the potential damage over the me- chanical integrity of the casting appears to be associated to the extent of horizontal flow, on the grounds of higher pro- pinquity to produce oxide entrainment versus an uphill flow typical of bottom gating.


As suggested future work for this research, the effect of flow velocities, porosity, microstructure, and grain size over me- chanical properties for casting Set 2 could be assessed. In ad- dition, specimen fracture surfaces from all four multiple gated systems could be examined using SEM technology. This would provide a more thorough study of the effect of solidifi- cation defects on the A356 alloy and the optimal gating sys- tem design to be used to deliver melt quiescently throughout the mold cavity with minimal to no entrainment events.


Acknowledgements


This work was funded by the National Science Foundation through Grant Numbers CTS-0553570 and CBET-0931801, and supplemental funding by the International Research and Education in Engineering (IREE) program. The authors are grateful to Aleaciones Ligeras Aplicadas S.L. of Vallado- lid (Spain) for making their casting facilities available for this research. The assistance of J.P. Juarez in the preparation of the CAD files and several images of this work, and J.C. Baird with SEM analysis is gratefully acknowledged. The aid of Professor E. William Jones with the Weibull analy- sis, and J. Coleman and H. Cole with Optical Microscopy and porosity analysis is also greatly appreciated. The authors at MSU also recognize the continual support of MAGMA Foundry Technologies (IL).


International Journal of Metalcasting/Spring 2012


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