This study aims to clarify the effects of the external notch on the impact and bending characteristics of pearlitic spheroidal graphite cast iron (PDI) and pearlitic compacted vermicular graphite cast iron (PCV). We prepared Charpy-type specimens with five kinds of notches whose stress concentration factors (α) varied from 1.0 (unnotched) to 4.8. These specimens were subjected to the instrumented Charpy impact test in the temperature range between 173 K and 423 K and also to the 3-point bending test with a slow loading speed at room temperature. The results of the Charpy impact tests and 3-point bending tests indicate that the absorbed energy greatly decreases with an increase in α from 1.0 to 2.3, but decreases only slightly above 2.3 in both samples. When the absorbed energy is divided into the crack-initiation energy and crack-propagation energy, a decrease in the crack-initiation energy mainly accounts for the decrease in the total absorbed energy. In comparison of PDI and PCV, the former is more sensitive to the notches than the later in both testing methods. The impact characteristics are more sensitive to notches than the 3-point bending characteristics in both samples. PDI and PCV show considerably smaller absorbed energy and higher transition temperatures, and have larger sensitivity to the external notch compared with ferritic matrix cast irons.
The algorithm for mold-filling simulation with consideration of surface tension has been developed based on a high-accuracy numerical scheme. As the governing equations, the Navier-Stokes equations for incompressible, unsteady laminar flows were used. In this study, we proposed a way of considering surface tension in mold-filling simulation. The proposed scheme for surface tension was based on the continuum surface force (CSF) model, which is a typical model of treating surface tension as the volume force. As a test problem, squeeze casting simulation was taken up. When the physical properties of the molten metal were assumed to be the same as those of zinc alloy, we could confirm the remarkable effectiveness of the surface tension. As an application to a practical casting problem, we also applied this scheme to die-casting simulation and compared the numerical results with experimental data on the size and position of blow holes. Numerical results both with and without surface tension predicted the existence of blow holes. However, it was found that simulation without surface tension tended to underestimate the size of blow holes. These numerical results confirm that the consideration of surface tension is very important for predicting the size and position of blow holes remaining in mold cavities.
Compo-casting of ceramics and cast iron is expected to be one of the major casting technologies that can expand the application fields of cast iron. The largest problem in compo-casting technology is the generation of cracks caused by thermal shock. Although this crack generation can be prevented by reducing thermal stress by means of preheating ceramics, the necessary preheating temperature is considerably high and precise controlling is difficult at foundry working sites. In this study, we tried to numerically predict the critical preheating temperature of ceramics using thermal stress analysis in unsteady heat transfer and the Newman's diagram, and found that the preheating of ceramics to reduce thermal stress can be substituted with placing an appropriate cast iron cover around the ceramics. We introduce our newly developed compo-casting method which requires no preheating of ceramics with several experimental results of the method.
The effects of the ferrite around the graphite on fracture toughness in gray cast iron were investigated. Four kinds of gray cast iron were prepared. Austempered gray cast iron (AGI) for bainite matrix was made by austempering at 653 K after heating at the austenitizing temperature of 1143 K. AGI contained about 35% residual austenite (γR). AGI-S2 subject to subzero treatment after the same austempering process as AGI showed about 5% residual austenite γR in the bainite matrix. α-AGI was made by the austempering after slow cooling from the austenitizing temperature to the range of ferrite and austenite coexistence temperature, 998 K. In this process, the ferrite thickness around the graphite was about 15 μm, and the matrix was a bainite structure. Fully pearlite matrix cast iron was used as casting. Precipitated ferrite around the graphite in austempered gray cast iron was found to be effective for improving the fracture toughness. There was good correlation between the stress intensity factor (K) calculated by the load which appeared at high amplitude AE and other mechanical properties such as elongation, Charpy impact strength, and elastic-plastic fracture toughness (J). K calculated by AE of α-AGI was 15.0 MPa⋅m1/2, AGI was 11.5 MPa, m1/2, AGI-SZ was 8.5 MPa⋅m1/2, although K of the as cast specimen was 7.8 MPa⋅m1/2.
Al-4 mass%Cu alloy was melted in a quartz tube with a small nozzle at the bottom. By pressing the melt surface with argon gas pressure of 0.12 to 0.13 MPa, the alloy melt flowed out through the nozzle onto the copper substrate and was rapidly solidified. A rectangular tube specimen was formed by moving the sub. strate in the X-Y horizontal directions and downwards simultaneously. With increasing the moving speed of the substrate, the thickness of one layer deposited reduced and the spacing between eutectics in the interdendritic regions increased, while the width of the castings remained almost constant. With increasing the number of layers, the cooling rate was decreased, and the weldability between the layers improved. Under optimum conditions, a coexisting solid-liquid region was formed between the layers, resulting in smooth side surface of the castings due to the surface tension of the liquid phase in the region.