Materials Science Forum Vols. 519-521

Abstract: In recent years, the horizontal continuous casting has been increasingly applied in a variety of aluminum products such as busbars, T-bars, foundry ingots, small diameter extrusion billets and forging feedstock, due to its high productivity and relatively low cost. By introducing the continuous process for long cast periods, feeding blockage as a unique process-related complex problem has emerged, which causes casting defects, a short cast duration and even an unexpected interruption of the cast. A literature review of the feeding blockage phenomena during continuous casting in the steel and aluminum industries is presented. The causes and mechanisms of feeding blockage in the aluminum horizontal continuous casting are proposed and discussed. Typical examples of feeding blockage from the industrial scale production with results of metallographic examination are illustrated.

Abstract: The needs for high-strength and lightweight structural materials have increased in automotive and aerospace structural applications. Semi-solid processed light alloys have satisfied these requirements because of processing advantages and significant weight reduction. Conventional semi-solid casting methods have got a wide variety of problems and difficulties. The cup-cast method that has been just developed is a novel process that make semi-solid casting as easy as pouring the water from a pitcher into a drinking glass, and avoid all the problem and difficulties of other semi-solid casting processes. Cup cast method is based on the heat and mass transfer and spherical equiaxed particles with controlling the nucleation and growth of solid particles were produced. In this study the different factor of this method was optimized by micro-structural investigation on the Al-A356 alloy. Pouring height and temperature, duration of pouring, and cup coating had played important roles in this method.

Abstract: The addition of grain refiners during industrial direct chill (DC) casting of aluminum billets promotes formation of smaller equiaxed grains with obvious advantages. However, the role of grain refining in the extent of macrosegregation in DC cast Al alloys is still unclear. This is particularly evident in the case of commercial aluminum alloys with various alloying elements. In this work, the structure and associated macrosegregation patterns in DC cast AA 2024 (Al–Cu–Mg) aluminum alloy billets were studied at different casting speeds. The concentration profiles of Cu and Mg, measured along the billet diameter, showed an expected negative segregation in the center and close to the surface. The severity of segregation increases at a higher casting speed. On the other hand, grain refining does not seem to have any dramatic effect on the macrosegregation patterns. The experimental results are correlated with microstructural observations such as grain size and morphology and the occurrence of “floating” grains across the cross-section of the billet.

Abstract: In order to extrude A7075 aluminum alloy soundly from melt without using feed stock billet, rheo-extrusion was tried by utilizing semi-solid slurry with fine solid granules made by employing cooling tube. When the melt moving down inside thin tube was adequately cooled in different ways and introduced into an extrusion container kept at semi-solid temperature of 873K, structure of solidified slurries were granular and mean grain sizes of about 60 to 120μm could be obtained. Subsequently, these slurries were extruded to round bars at various extrusion ratios (28 to 64) and press ram speed of 10mm/s, just after cooling to 833K. The newly developed slurries could easily be extruded to bars with smooth surfaces at lower forces. Although every tensile strength of extruded bars were lower than that of hot-extruded one, there was a tendency that finer the solid granules in slurry, higher the tensile strength of extruded bar.

Abstract: Grain refining experiments for casting of pure Al were conducted to evaluate the grain refinement performance of an Al-5mass%Ti alloy refiner before and after cold rolling. Al3Ti particles in the Al-Ti alloy refiner were efficiently fragmented by cold rolling. The size of the Al3Ti particles in cold rolled Al-Ti alloy refiner decreased from 280μm to 30μm with increasing reduction ratio of cold rolling. Mean size of α-Al grains in pure Al cast refined by the cold rolled Al-Ti alloy refiner decreased from about 500μm to 200μm with increasing the reduction ratio. In order to investigate mechanical property of the pure Al cast refined by the cold rolled Al-Ti alloy refiner, Vickers hardness test and tensile test were conducted. Strength of the pure Al cast refined by the cold rolled Al-Ti alloy refiner increased with increasing the reduction ratio. The strength improvement of pure Al cast refined by the cold rolled Al-Ti alloy refiner followed Hall-Petch relationship. From obtained results, it was concluded that cold rolling for refiner is useful practical application for pure Al cast.

Abstract: The effects of solution treatment time and artificial aging on the work hardening characteristics on Al-7%Si-0.6%Mg (D357) alloy castings were investigated. Four different solution treatment times at 540°C (1, 4, 16 and 64 hours) and six different artificial aging times at 160°C (0, 2.5, 5, 10, 20 and 40 hours) were used. Work hardening characteristics were investigated by Kocks-Mecking plots for each specimen. The effects of Si particle morphology (solution treatment) and matrix strength (aging) on Kocks-Mecking (Stage III) work hardening model parameters are discussed in the paper.

Abstract: A hot dip aluminizing process to simulate the continuous galvanizing line (CGL) was carried out in three successive steps by a hot dip simulator: the pre-treatment for removing scales on the 200 × 250 mm2 and 1mm in thickness cold rolled steel sheet, the dipping in 660°C Al-Si melt for 3s and the cooling. In a pre-treatment, the steel specimen was partly coated by Au to confirm the mechanism of intermetallic compound (IMC) formation. Scanning electron microscopy (SEM), energy dispersive X-ray spectroscopy (EDX), and transmission electron microscopy (TEM) analyses were followed to observe the cross-section and the distribution of the elements. The specimen was analyzed in the boundary of the dipped-undipped part to see the formation mechanism of the aluminized steel. An intermetallic compound (IMC) is rapidly developed and grown in the steel-liquid interface. It has been usually reported that the IMC was formed by the dissolution of iron in the steel substrate toward the melt and the diffusion of aluminum in an opposite direction. The specimen is covered with aluminum-10 wt.% silicon, forms the IMC in the part that was not Au coated. However, IMC is not formed in the Au-coated part. The interface of the dipped-undipped is also analyzed by EDX. At the interface of the steel-IMC, it is clearly shown that the IMC is only formed in the dipped part and exists in the steel substrate as well, and contributes by iron, aluminum and silicon. The result clearly shows that only aluminum diffuses into the steel substrate without the dissolution of iron and forms the IMC between the steel substrate and the melt. Au coating and the short dipping time prevent the iron from dissolving into the aluminum melt. By TEM combined with focused ion beam (FIB) sample preparation, the IMC is confirmed as Fe2SiAl8, a hexagonal structure with space group P63/mmc.

Abstract: In order to improve the understanding of hot tearing during laser welding of aluminium alloys, the rheology of the alloys in the mushy state must be characterized. The present work investigates the mechanical behaviour of the aerospace alloy AA6056 using a specially designed isothermal tensile test in the mushy state. Using a Gleeble thermo-mechanical machine, two different tests have been performed: i) tests during partial remelting and ii) tests after partial solidification at a high cooling rate. These tests have been carried out not only on the 6056 alloy but also on a mix between 6056 and 4047 Al-Si alloy which corresponds to the composition of the nugget of a laser using a filler wire. The increase of the solid fraction results in an increase of the maximum stress and a change on the fracture surface from a smooth dendritic to a more ductile one. Moreover, the alloys exhibit a typical visco plastic behaviour with an increase of the maximal stress with the strain rate. When the test is performed at a particular solid fraction of 0.97, the fracture is more erratic and the ductility is low. The results show the existence of a ductile/brittle/ductile transition with the fraction of solid. The fracture stress is shown to be higher when testing after partial remelting as compared to partial solidification for the same solid fraction. This is due to the difference in microstructure of the mushy zone and more particularly in the connectivity of the solid skeleton. An adapted creep law is used to describe the mechanical behaviour of alloys during the partial remelting test using the fraction of grain boundary wetted by the liquid given by Wray. This law is shown to be irrelevant to the partial solidification tests, as a result of the modified geometry of the liquid phase. From these tests, we have determined a new law relating the solid fraction to the fraction of grain boundaries wetted by the liquid. This law is a useful tool to predict the mechanical behaviour when mechanical loading occurs during solidification.

Abstract: The aging of Al-Zn alloys has been vastly studied for decades. In the previous paper, 0hta et al. studied carefully the hardness of the alloy during aging and revealed the existence of softer regions near the surface and the grain boundary than the interior of the specimen even after aging for a long time. Electrical resistivity measurement and X-ray small angle scattering experiment together with hardness test suggested that in these regions vacancy decay to the surface and grain boundaries was severe, thus the growth of GP zones were suppressed and therefore age hardening was retarded. Also, it is well-known that an addition of a small amount of Ag raised solvus temperature of GP zones. In this paper, soft surface layer formed in an Al-12mass%Zn alloy is studied by adding small amount of Ag by means of hardness test and resistometry. Addition of Ag more than 0.1% decreases the thickness of soft surface layer as well as accelerates age hardening rate and suppresses the formation of soft region near the grain boundaries. Higher quenching temperature also reduces the thickness of soft surface layer. Together with the behavior of aging curves of the specimen with various thicknesses, the origin of the soft surface layer is confirmed to be the effective role of surface as sinks for vacancies.