Hiroshi Yamagata

Development of Superplastic-Aluminum-Alloy Piston Formed with Controlled Forging Technology

Powder metallurgical aluminum alloys and forging technology have been developed to mass produce high-performance-engine pistons. The Al-Si base powder metallurgical alloys have shown much higher fatigue strength as well as better wear resistance than a conventional cast piston alloy. The alloys show high strain rate superplasticity at a limited temperature and strain rate range. In order to bring out the alloys formability, the forging has been programmed using an integrated process that involves the precise control on the die temperatures and the spray conditions of the lubricant. With these procedures, near-net-shaped pistons are efficiently obtained. The newly developed pistons have improved engine performance and are being put into practical use in Yamahas commercially available engines.

Effect of cooling rate on morphology of primary particles in Al-Sc-Zr master alloy

Al-1.0%Sc-1.0%Zr (mass fraction) master alloy was prepared at different cooling rates. The morphology and thermodynamics data of the primary particles of the master alloy were investigated by X-ray diffraction (XRD), scanning electron microscopy (SEM) and differential scanning calorimetry (DSC). It shows that the primary particles are dendrite-shaped particles comprised of several attached small cubic, cusped-cubic or crucifer shape particles at slow cooling rate. However, the primary particles are separated with crucifer shape at intermediate cooling rate, and they are cubic with cusped-cubic shape at high cooling rate. Meanwhile, the separated and attached particles present Al3Sc/Al3Zr1-xScx core-shell structure. The formation mechanism of the structure was systematically investigated by a mathematical model.

Effects of Al-Ti-B-RE grain refiner on microstructure and mechanical properties of Al-7.0Si-0.55Mg alloy

Abstract To investigate the effects of Al-Ti-B-RE grain refiner on microstructure and mechanical properties of Al-7.0Si-0.55Mg (A357) alloy, some novel Al-7.0Si-0.55Mg alloys added with different amount of Al-5Ti-1B-RE grain refiner with different RE composition were prepared by vacuum-melting. The microstructure and fracture behavior of the Al-7.0Si-0.55Mg alloys with the grain refiners were observed by X-ray diffraction (XRD), optical microscopy (OM), scanning electron microscopy (SEM), and the mechanical properties of the alloys were tested in mechanical testing machine at room temperature. The observation of Al-Ti-B-RE morphology and internal structure of the particles reveals that it exhibits a TiAl3/Ti2Al20RE core-shell structure via heterogeneous TiB2 nuclei. The tensile strength of Al-7.0Si-0.55Mg alloys with Al-5Ti-1B-3.0RE grain refiner reaches the peak value at the same addition (0.2%) of grain refiner.

High Temperature Deformation Behavior of Pre-Sintered Al-10Si-5Fe Based Alloy

The mechanical properties of pre-sintered Al-10Si-5Fe-1Cu-0.5Mg-1Zr (wt%) alloy were investigated in the temperature range from 673K to 813K and at initial strain rates from 10-4 to 100 s-1. In the high temperature range of 793K and 813K, the strain rate sensitivity index was close to that for superplasticity (0.3). Stress exponent was estimated to be 2 and 5 in the temperature range from 793K to 813K and from 673K to 773K, respectively. The activation energies for the plastic flow were calculated to 77kJ/mol for the n=2 region and 127kJ/mol for the n=5 region. These values were close to that for grain boundary diffusion and self-diffusion in pure aluminum, respectively. We found that a dynamic recrystallization (DRX) and grain boundary sliding (GBS) occur depending on the test temperature and stain rate. A filament-like phase containing Cu and Mg was observed in the cracked surface of the specimen deformed at 793K and 813K.

Functionally graded AlZr-based amorphous alloys

The application of sputtering in a mixed gas of Ar and N2 to Al80Zr20 resulted in the formation of compositionally graded amorphous Al(Zr, N) film. By changing continuously the partial pressure PN of N2 from 0.03 to 0.13 Pa in a total Ar + N2 pressure of 1 Pa, the resulting phase consists of only amorphous Al(Zr, N) in the whole PN range. As PN increases, the wavevector at the peak position of the amorphous halo in the X-ray diffraction patterns decreases continuously and the crystallization temperature increases to temperatures above 773 K. The Knoop hardness and the electrical resistivity measured along the amorphous film thickness increase from 660 to 2120 and from 4.0 × 10−6 to 1.3 × 10−4 Ω m respectively with increasing N2 content. Thus the sputtered amorphous film is concluded to have functionally graded characteristics resulting from the compositional gradients.

Microstructural evolution during reheating of A356 machining chips at semisolid state

The microstructural evolution of A356 machining chips in the semisolid state was studied at different temperatures and holding times. The results showed that the elongated α-Al grains first recrystallized in the semisolid state and then became globular with a high shape factor (SF). Both the temperature and the holding time clearly affected the grain size and SF. When the heating temperature or holding time was increased, the grain size and SF gradually increased and finally became stable. Moreover, the Vickers hardness of primary α-Al grains gradually decreased with increasing heating temperature. The optimal slurry for semisolid processing, with a good combination of grain size and SF, was obtained when the chips were held at 600°C for 15 min. The semisolid slurry of A356 chips exhibited a lower coarsening rate of α-Al grains than those produced by most of the conventional semisolid processes. The coarsening coefficient was determined to be 436 μm3·s−1 on the basis of the linear Lifshitz–Slyozov–Wagner (LSW) relationship.

The Effects of Two-Stage Aging on the Microstructure and Mechanical Properties of the New Al-Si-Cu-Mg Alloy

Two-stage aging treatments were applied to Al-6Si-2Cu-0.5Mg casting alloy, and the influence of aging treatment parameters on the microstructure and mechanical properties was investigated. The experimental results indicated that the microstructure and mechanical properties of Al-6Si-2Cu-0.5Mg alloy were significantly influenced by the aging time and temperature, and the elongation remarkably increased with the aging time increasing before 7 h. In the case of the alloy aged at 200°Cfor 2h, higher hardness and tensile strength were obtained, which may be attributed to precipitation of a large amount of Cu-rich phases. However, the higher elongation was achieved in the alloy under 200°C aging treatment for 5h, while its hardness and tensile strength slightly decreased. It is mainly due to the amount of the Cu-rich phases decrease slightly, but the morphology of the phases evolved from plate-like to bulk-like structure.

Effect of Iron-Rich Intermetallic and Eutectic Si Accumulation on Al-Si-Mg Alloy

Al-Si-Mg alloys, an important aluminum cast alloys, are excessively used inmanufacturing of critical components due to their high strength to weight ratio, flexibility ofmanufacturing designs, economic processing and capital material cost for automotive industry. Thisresearch is aimed to study microstructure evolution including distribution and morphology ofiron-rich intermetallic, as well as eutectic Si accumulation and their effect on mechanical propertiesof Al-Si-Mg (A356) casting alloy after artificial ageing. The results show that formation of iron-richintermetallic and eutectic Si accumulation resulted in surprisingly opposite mechanical properties,especially ductility. The elongations deceased with increase of area of accumulated eutectic Si and theamount of needle-like iron-rich intermetallic. When the area of accumulated eutectic Si reached 31%of the microstructure of the A356 alloy, the strength and elongation were respectively damaged to129.69 MPa and 1.05%. Moreover, the amount of needle-like iron-rich intermetallic increased, thestrength and elongation respectively decreased.

Porosity Variation on the Compressive Deformation of Pre-Sintered Al-Si-Fe Based Alloy

The effects of temperature and strain rate on porosity variation have been studied in order to obtain an optimum processing condition in the powder forging of pre-sintered Al-Si-Fe based alloys. Compressive test was carried out at temperatures from 673 K to 813 K and at initial strain rates from 10 0 s -1 to 10 -3 s -1 . The prior porosity of 15% in the pre-sintered alloy decreased with compressive deformation depending on temperature and strain rate. We have found that the amount of liquid phase containing Cu and Mg is closely associated with porosity changes of the present alloys deformed. It is suggested that the occurrence of liquid phase during the compressive deformation may facilitate the particle boundary sliding resulting in decreasing porosity in the pre-sintered Al-Si-Fe based alloys.

High Strain Rate Superplasticity in Al-16Si-5Fe Based Alloys with and without SiC Particulates

Superplasticity at a high strain rate of 1.4×10 -1 s -1 was exhibited at 783K in the powder metallurgy processed Al-16Si-5Fe-1Cu-0.5Mg-0.9Zr (wt%) alloys with and without 5 wt% addition of SiC particulates. The maximum elongations of 350% and 270% were obtained for the monolithic and composite material, respectively, where high m values were found. The high strain rate superplasticity in these materials was attributed to the presence of a thermally stable fine microstructure and the operation of grain boundary sliding. The lower superplastic elongation of the composite specimen compared to that of the monolithic specimen was suggested to the result of an increased density of cavities owing to the SiC decohesion at the SiC/matrix interface.