A B S T R A C T Aluminium is a lightweight material with high strength and good corrosion resistanceamong other beneficial properties. Thanks to these properties, aluminium is more extensively used in the vehicle industry. High-pressure die casting of aluminium is a manufacturing process that makes it possible to attain complex, multi-functional components with near-net shape. However, there is one disadvantage of such castings, that is, the presence of various defects such as porosity and its effect on mechanical properties. The aim of this work was to investigate the influence of porosity on the fatigue strength of high-pressure die cast aluminium. The objective was to derive the influence of defect size with respect to the fatigue load, and to generate a model for fatigue life in terms of a Kitagawa diagram. The aluminium alloy used in this study is comparable to AlSi9Cu3. Specimens were examined in X-ray prior to fatigue loading and classified with respect to porosity level and eventually fatigue tested in bending at the load ratio, R, equal to −1. Two different specimen types with a stress concentration factor of 1.05 and 2.25 have been tested.It has been shown that the fatigue strength decreases by up to 25% as the amount of porosity of the specimen is increased. The results further showed that the influence of defects was less for the specimen type with the higher stress concentration. This is believed to be an effect of a smaller volume being exposed to the maximum stress for this specimen type. A Kitagawa diagram was constructed on the basis of the test results and fracture mechanics calculations. A value of 1.4 Mpa m 1/2 was used for the so-called stress intensity threshold range. This analysis predicts that defects larger than 0.06 mm 2 will reduce the fatigue strength at 5 × 10 6 cycles for the aluminium AlSi9Cu3 material tested. A = area of the defect a = 'crack length', defect radius da d N = crack growth rate N = number of cycles to failure K = stress intensity factor K t = elastic stress concentration factor R = stress ratio (=σ min /σ max ) K = stress intensity range K th = stress intensity threshold range at approximately 10 −9 m/cycles σ n = nominal stress range σ an = nominal stress amplitude σ FL = fatigue strength at N = 5 × 10 6 cycles

No abstract

No abstract

The hot crack sensitivity in metals is suggested to be caused by the supersaturation of vacancies created during the solidification process. Equations have been derived to predict the nucleation and growth of cracks by the condensation of vacancies. The transition temperature from brittle to ductile fracture was found to be related to the decrease in the supersaturation of vacancies due to an annealing process. The hot crack sensitivity was observed to be related to the supersaturation of vacancies, the diffusion rate, and the structure coarseness. The effect of surface active elements such as phosphorous and sulphur in steel alloys is discussed.

Some work has been done over the years to achieve understanding of the mechanisms behind air gap formation in casting processes. Experimental work has been done as well as work within the subject field of numerical modeling of air gap formation during solidification. It is, however, quite rare to find comparisons between experimental measurements and simulation results with commercial codes.There are a number of different commercial codes available on the market useful for example for qualitative identifications of various problem areas in complex casting geometries. But to get reliable quantitative predictions of the growth of an air gap in a casting process there still remains a lot of development and research to be done.To be able to quantitatively check the results from a simulation it is necessary to have measured data from an experimental procedure that is simple to model in a program. Within the MEBSP subgroup IV members such experiments are made. There are also four different simulation programs available within the group for calculating solidification as well as thermal stresses and strains during a casting process. The aim of the work of MEBSP subgroup IV within this area is to get a better understanding of the models implemented in the codes and see how well the existing models fit to experimental data. It is also of interest to see if the different models give quantitatively the same results and whether any of the four programs have special suitability for certain purposes.At the Division of Casting of Metals at the Royal Institute of Technology, KTH, a lot of experiment based research is taking place within the subject field of casting. In the area of air gap formation the work is focused on the behavior and properties of a solidifying metal during solidification and the influence these have on the shrinkage behavior, the solidification process and the air gap formation.In order to study the solidification process coupled with the air gap formation an experimental set-up has been constructed. It consists of a cylindrical mould made from low alloy steel which is insulated at the bottom and the top so that solidification occurs axisymmetric. In the center of the cylinder a cylindrical core is placed, made from quartz glass filled with oil bound sand. Figure 1 shows a schematic drawing of the set-up.The radial temperature distribution is recorded during cooling and solidification via thermocouples placed in the mould and the casting. Simultaneously, the movements of the mould walls and the outer surface of the casting are measured with linear variable differential transducers (LVDT). Figure 2 and 3 show an example of results from this kind of experiments. These particular curves are taken from experi-COMMUNICATIONS