Shinji Kumai

Effects of dendrite cell size and particle distribution on the near-threshold fatigue crack growth behaviour of cast AlSiCp composites

Abstract Fatigue crack growth tests and crack closure measurements were performed for A356 cast aluminium alloys reinforced by 10 or 20% SiC particles and their matrix A356 alloys with systematically controlled dendrite cell size and particle distribution. The cell size dependence of the fatigue crack growth behaviour in the composite was found to be quite similar to that of the matrix alloy. This suggests that the cell size rather than the particle-crack tip interaction is the most important factor to control the fatigue crack growth of the composites. Near threshold fatigue crack growth properties were improved in the composites with coarser cell size and inhomogeneous particle distribution due to the enhanced roughness induced crack closure effect. These results were compared to those of powder-metallurgically-processed materials.

Solidification structure of monotectic alloys

The formation manner of the monotectic structure is shown from the observation of the evolution of crystallization and growth of CuPb and some aluminium-based monotectic alloys solidified in a free direction and unidirectionally. The morphological change of the monotectic composite structure is discussed in relation to the solid-liquid interfacial morphology at the monotectic growth front as well as the interfacial energy balance between the solid and two liquids in the monotectic reaction. In free-directional solidification, a characteristic monotectic cell with a spherical shape is formed which is unlike dendritic and lamella morphologies. The monotectic cell consists of a spherical solid and a separated L2 liquid lying along the radii of the solid sphere. In unidirectional solidification, the monotectic structure of AlPb, AlBi and AlIn alloys changes in the following sequence with decreasing growth rate under a constant temperature gradient; random dispersion of L2 droplets in the aluminium solid matrix → periodic regular array of L2 droplets → fibrous L2 composite → aluminium single-phase region without the L2 phase. In CuPb monotectic alloy the monotectic structure changes with decreasing growth rates as follows: irregularly shaped rod-like L2 composite → coalesced coarsened discontinuous L2 composite → periodic banded structure consisting of L2-rich regions and L2-poor regions. These morphological transformations of monotectic structure are strongly affected by the ratio of the temperature gradient to the growth rate, the volume fraction of liquid L2 separated through the monotectic reaction and the interfacial energies between the solid and two liquids at the monotectic growth front.

Interfacial microstructure of magnetic pressure seam welded Al-Fe, Al-Ni and Al-Cu lap joints

A new welding method, magnetic pressure seam welding, was used to lap join dissimilar metals (Al-Fe, Al-Ni and Al-Cu). The circuit for magnetic pressure seam welding consists of a capacitor, an electric discharge gap switch, and a plate-type coil. The overlapped metal plates are placed over the coil. When an impulse current from an energy-storage capacitor bank passes through the coil, a high-density magnetic flux is suddenly generated around the coil. The generated high-density magnetic flux lines cross the end of the overlapped plates. Eddy currents are induced mainly inside the Al plate because it has a high electrical conductivity. Both the Joule heat generated in the plates and the magnetic pressure applied from the Al side promote the joining of the lapped plates. The welding is normally achieved within 10 μs. This results in very little microstructural change in the parent plates aside from the area around the weld interface. Strong lap joints were obtained for every metal combination and no tensile fracture took place in the weld region. A characteristic wavy morphology was observed at the weld interface. An intermediate phase layer was also observed at the weld interface. TEM observation revealed that the intermediate layer consisted of fine Al grains and intermetallic compound particles dispersed among the Al grains. The growth direction of the wave, the welding condition dependency of the wavelength and the amplitude of the interfacial wave were intensively investigated in order to clarify the welding mechanism of this method.

Fatigue in SiC-particulate-reinforced aluminium alloy composites

The fatigue behaviour in SiC-particulate-reinforced aluminium alloy composites has been briefly reviewed. The improved fatigue life reported in stress-controlled test results from the higher stiffness of the composites; therefore it is generally inferior to monolithic alloys at a constant strain level. The role of SiC particulate reinforcement has been examined for fatigue crack initiation, short-crack growth and long-crack growth. Crack initiation is observed to occur at matrix-SiC interface in cast composites and either at or near the matrix-SiC interface or at cracked SiC particles in powder metallurgy processed composites depending on particle size and morphology. The da/dN vs ΔK relationship in the composites is characterized by crack growth rates existing within a narrow range of ΔK and this is because of the lower fracture toughness and relatively high threshold values in composites compared with those in monolithic alloys. An enhanced Paris region slope attributed to the monotonic fracture contribution are reported and the extent of this contribution is found to depend on particle size. The effects of the aging condition on crack growth rates and particle size dependence of threshold values are also treated in this paper.

Effects of the particle distribution on fatigue crack growth in particulate SiC/6061 aluminium alloy composites

Abstract Fatigue crack growth rates have been examined in powder-metallurgy-processed 6061 aluminium alloy composites containing 15 and 30 wt.% SiC particles with different particle distributions. Over the range 10 −9 to 10 −6 m/cycle the fatigue crack growth rates in the composite materials were lower than those reported for the monolithic alloy. SEM observations of the broken specimens showed very few SiC particles on the fatigue surfaces produced at low values of ΔK . An increased area fraction of the fractured coarse particles was evident on the fatigue surfaces of the bimodal distribution composites tested at high ΔK . Quantitative examination of the crack path profile was used to highlight the interaction between the particles and the crack path. The fatigue crack avoids SiC particles at low- ΔK ranges, but at high ΔK ranges the crack appears to proceed by linking fractured SiC particles ahead of the main crack front.

Effects of solidification structure and aging condition on cyclic stress-strain response in Al-7% Si-0.4% Mg cast alloys

Abstract Stress–strain response under cyclic loading at fixed plastic strain amplitude condition was examined for age-hardenable Al–7% Si–0.4% Mg (A356) cast alloys. The specimens examined include the cast alloys with ordinary dendrite structure and semi-liquid die-cast alloys with fine effective grain structure. Al–0.5% Mg–0.4% Si (6063) alloy was also tested for comparison. Special attention was paid to the effect of solidification structure and aging condition on cyclic hardening behavior. Cyclic hardening behavior was sensitive to the solidification structure. The refined grain size, DAS and unmodified acicular eutectic Si particles increased stress levels of cyclic hardening curves. Drastic change in cyclic hardening behavior was obtained by changing aging condition. After the initial rapid hardening, the stress amplitude kept increasing steadily until fracture in as-quenched and under-aged materials containing shearable GP zone. In contrast, for over-aged (and peak-aged materials), which were hardened by non-shearable β′-precipitates, the initial hardening occurred more rapidly and the stress amplitude reached the saturation stress in a quite early stage of the fatigue life. The attained stress level was almost constant until fracture. Essentially the same aging condition dependence was obtained for the 6063 alloy with no dendrite structure and eutectic Si particles. This indicated that nature of strengthening precipitates controlled the cycle hardening behavior of the present cast alloys.

Effects of solidification structure on short fatigue crack growth in Al-7%Si-0.4%Mg alloy castings

Abstract Crack initiation and successive short fatigue crack behavior of the peak-aged Al–7%Si–0.4%Mg alloy castings were examined using rectangular smooth bar specimens under the axial loading condition. Tests were performed for a permanent mold cast alloy with ordinary dendrite structure and a semi-liquid die-cast alloy with fine effective grain structure. In the alloy containing few casting defects, fatigue cracks preferentially initiated at the Si particle/matrix interface in the inter-dendritic region and grew tracing eutectic cell boundaries to form the meshy crack growth path. In the ordinary dendrite structure, grain boundaries acted as effective obstruction to the growing fatigue crack. On the other hand, the effective grain of the semi-liquid die-cast was a moderate obstruction to the crack growth. Short crack growth behavior in the semi-liquid die-cast was comparable to that of the long crack. Fine effective grain structure is considered to promote homogeneous cyclic deformation at the fatigue crack tip in spite of short crack length and the corresponding small plastic zone size.

Fatigue crack growth behavior in semi-liquid die-cast Al-7%Si-0.4%Mg alloys with fine effective grain structure

Abstract Structural control was performed on Al-7%Si-0.4%Mg alloy castings by three different fabrication routes: sand mold cast and HIP, permanent mold cast and semi-liquid die-cast. In contrast to the ordinary dendrite structure in the sand and permanent mold cast alloys, the microstructure of the semi-liquid die-cast alloy was characterized by colonies consisting of single or several dendrite cells. Crystallographic misorientation among these colonies is large and they can be regarded as ‘effective grains’. Fatigue crack growth tests were performed using CT specimens at a stress ratio of 0.1 and effects of microstructure were examined. Difference in the overall growth rates was not evident in three samples. However, crack growth path and fatigue fracture surface of the semi-liquid die-cast alloys was different from those in the others. The fatigue crack grew straight even at small Δ K levels. Large area of the fatigue fracture surface was covered with striations. These characteristic features are correlated with the relationship between the plastic zone size at the crack tip and the effective grain size in the semi-liquid die-cast alloys.

Analysis of microstructure evolution and precise solid fraction evaluation of A356 aluminum alloy during partial re-melting by a color etching method

Spheroidization of Al grains is required for the production of semi-solid slurry either by a partial solidification route or partial re-melting route. In this research, A356 aluminum alloy was deformed and partially re-melted to semi-solid state. A segregation sensitive reagent (Weck’s reagent) was used to reveal the inner microstructure of Al grains for the better understanding of the microstructure evolution during partial re-melting. Optical microstructure observation showed that the previously compressed Al dendrites were actually “fractured” during heat treatment and such “fractured” dendrites contributed to the refinement and spheroidization of Al grains. Further study of this phenomenon indicates that the “fractures” are actually migrating high-angle grain boundaries, which was related to the recrystallization that occurred during heat treatment. Moreover, the growth layer of Al grains formed during water quenching is clearly visualized after being etched by Weck’s reagent. Consequently, precise evaluation of solid fractions through image analysis was realized by excluding growth layer when measuring the area of solid phase.

Ferromagnetic properties of cyclically deformed Fe3Ge and Ni3Ge

The magnetisation behaviour of cyclically deformed and non-deformed Fe3Ge and Ni3Ge is examined at sufficiently low temperatures below the Curie point. Despite these two intermetallics having the same L12 structure, they are found to show quite different behaviour in their ferromagnetic properties; the spontaneous magnetisation (M S) remains unaffected in the former whereas it decreases notably in the latter after cyclic deformation. The origin of the difference is investigated and attributed to the difference in operative shear planes. These are mainly on {001} planes without the introduction of notable amounts of anti-phase boundary (APB) tubes in Fe3Ge and mainly on {111} planes with the introduction of a high density of APB tubes composed of {111} APBs in Ni3Ge. The effects of cyclic deformation on the high-field susceptibility χ and the coercive force (H C) are also discussed by taking into account the dislocation distributions introduced by the {001} and {111} slips.