Yoji Awano

Thermo-mechanical Fatigue Behavior of Al-Si-Cu-Mg Casting Alloy

This paper describes the thermo-mechanical fatigue behavior of Al-Si-Cu-Mg casting alloy. The alloy is widely used for cylinder heads and pistons of automobile engines. Repeated cycling between driving and resting, or low-power and high-power driving cause thermal cycling in the engine materials. The thermo-mechanical fatigue property is therefore very important to develop high-performance engines. This study aims to characterize the cyclic stress-strain behavior of this alloy and to clarify the factors dominating the fracture life. Results obtained are : (1) The stress-strain behavior changes remarkably during thermal cycling. Cyclic hardening and cycling softening occur in the higher and lower strain ranges, respectively. (2) A thermo-mechanical fatigue fracture limit diagram is obtained by connecting fracture points on the inelastic strain range - number of cycles relationship. (3) Consequently, in 10 3 - 10 4 cycles, it is considered that poor ductility and inelastic strain increase due to overaging dominate the thermo-mechanical fatigue life of the alloy.

Improvement in the impact strength of JIS AC2B aluminum alloy castings solidified for a long time.

In order to improve the strength of Al-Si-Cu alloy castings solidified for a long time, solution heat treatment for extremely long time or non-equilibrium eutectic melting solution treatment has been performed. The reason is discussed why normal T6 heat treatment is not effective while long time solution heat treatment or non-equilibrium eutectic melting solution treatment is effective for increasing the strength. By normal T6 treatment, non-equilibrium Cu bearing compounds are little dissolved, and the shapes of eutectic Si phase as well as Fe bearing compound are not changed. On the other hand, both dissolution of Cu bearing compounds and spheroidization of eutectic Si are promoted by extremely long time solution treatment or non-equilibrium eutectic melting solution treatment. Needle-like iron bearing compounds are refined only by the latter treatment. It is concluded that these factors will be responsible for increasing of the impact strength.

Deformation and Densification of Porous Preforms in Sinterforging. (Part II)

Deformation and densification behavior of porous preforms in hot trapped die upsetting and closed die forging (double-cup and doulle-boss) was investigated with Fe-0.5C powder preforms.For forging processes so far studied including those described in the previous report, no marked difference in deformation behavior was observed between powder preforms and bar stock materials, except cracking tendency of the former. Below 8t/cm2 of forging pressure, the mean density of sinterforged specimens increased with increasing material flow during deformation, depending upon the deformation type, but above 8t/cm2 the density more than 99.5% of the theoretical value was obtained for any type of forging processes except upsetting. In closed die forging, densification in the specimens proceeded from the zone in initial contact with the punch, through high shear stress zone and then to the free surfaces with some delay.Residual pore defects were obtained on cracked surfaces, die chilled areas and insufficiently deformed surface layers. Re-sealing of in-process cracks was suggested to occur under suitable forging conditions.

Effect of Iron Powders on the Mechanical Properties of Sinterforged Irons

In order to clarify the effect of impurities contained in sinterforged irons on the mechanical properties, studies were carried out under various conditions using five kinds of iron powder.With the increase of the amount of nonmetallic inclusions, the tensile strength was little affected, but the elongation and reduction in area were linearly decreased, and the impact strength was more remarkably decreased. The impact strength was also affected by forging temperature and heating atmosphere. A peculiar phenomenon was observed in the sinterforged irons made at 900°C, in which the upper yield strength was higher than the ultimate tensile strength in the stress-strain curve.