Materials Science Forum Vols. 539-543

Abstract: The main aim of the metallurgical investigation was to enhance our understanding of the role Cu and Si have on the development of shrinkage porosity in the as-cast structure, which in turn can affect high cycle fatigue (HCF) properties of Al-9Si-1Cu (W328) and Al- 7Si-4Cu (W319) alloys. In order to achieve this objective a novel approach using thermal analysis and calculated fraction solid techniques was developed to assess mushy zone kinetics, the state between the liquidus and solidus where a solid skeletal α-Al phase and the Al-Si eutectic phase grow at the expense of an inter-dendritic liquid. Specifically, the cooling curve and calculated fraction solid curve were partitioned into segments, which reflect different stages of feeding through the entire solidification event of the alloy. Each partitioned segment corresponds to a stage of feeding, which in turn signifies a relative degree of pore growth susceptibility. Two thermal analysis techniques, both using calibrated thermocouples, were used to precisely understand the solidification path for both the W319 alloy and the W328 alloy.

Abstract: Traditional gravity pour down-sprue methods of filling moulds in the making of aluminum castings inherently lead to oxide and air bubble entrainment. The reason for this is found in the high velocities the metal flow experiences during the filling of a mould. The Nemak Windsor Aluminum Plant (WAP) produces cylinder blocks using the low-pressure Cosworth process, which includes low velocity up-hill filling of the sand mould package. This doctrine is followed in all except one part of the process: the runner system. The nature of the resulting defect is generally known as Head Deck Porosity. Runners were cast full in open production runners at three different velocities with the resulting quickly chilled castings analyzed using X-ray radioscopy, and Scanning Electron Microscopy. Results reveal that the subject bubble porosity is indeed the result of air entrained during initial transient flow within the production runner system whose velocity is higher than the critical value of 0.5ms-1. This theoretical value is corroborated by experimental results. In addition, a new "sessile" runner of optimized shape, filled at a velocity slower than the critical value, is proposed and analyzed using Magmasoft mould fill modelling software. The design can potentially replace the existing runner providing a casting free of entrained air.

Abstract: Laser irradiating process with Nd-YAG laser is investigated in order to improve the adhesion and wear resistance of low pressure plasma sprayed layer on the surface of aluminum extruded shape using the atomized powder of Al-50mass%Fe, Al-15mass%Fe-17mass%Si and Al-50mass%Si. The effect of pulse energy of laser beam on the microstructure, micro hardness and wearing rate of these laser irradiated layers are evaluated. Laser irradiated layers have appeared more smooth surface and better adhesion than as sprayed layer. Depth profile of micro hardness where laser irradiated is respectively kept constant. In the microstructure of laser irradiated layer of Al-50mass %Fe, fine needle-like Al3Fe and massive Al2Fe are dispersed. Micro hardness increases with decrease of the pulse energy of laser beam However, the wearing rate of laser irradiated layer increases due to the initiation of cracking. In the microstructure of laser irradiated layer of Al-15mass%Fe-17mass%Si, ultra fine needle-like and massive (Al, Fe, Si) ternary crystals are aggregated. In the microstructure of laser irradiated layer of Al-50mass%Si, ultra fine hyper-eutectic structure is observed. Micro hardness of these layers are HV250-350, HV150-200, respectively and wearing rate of these layer are 1/7 or less than anodized surface.

Abstract: In an attempt to optimize the friction spot joining process of Al alloys for automobiles (Al 5000 and Al 6000 series, thickness 1mm), effects of joining parameters such as tool rotating speed, plunging depth and dwelling time on the weld joints properties were evaluated. Experimental tests were carried out for lap joined Al plates. A wide range of joining conditions could be applied to join Al alloys for automobiles without defects in the weld zone except for certain welding conditions with an insufficient heat input. The microstructures and hardness variations in the weld regions are discussed. The microstructures of welds, corresponding to the pin hole, have dynamic-recrystallized grain similar to stir zone in FSW weld. In hardness distribution, minimum hardness region was located about 6-mm away from the weld center, corresponding to the shoulder radius of the tool. For each weld the results from tensile-shear tests are also presented. For sound joints without defects, tensile shear strength of weld joints was higher approximately 230% than acceptable criteria of tensile shear strength of electrical resistance spot-welded joints for aluminum (MIL-W-6858D).

Abstract: Atomic bonding of the GPB zone and S′′ phase of Al-Cu-Mg alloys in early aging stage are calculated using the empirical electron theory (EET) in Solid. The results show that not only the covalence bond-net is very strong in GPB zone, but the whole covalence bond energy of S′′ phase is also very large, all the primary bond-net framework of these precipitates can consolidate the matrix of alloy. Phase transformation from GPB zone to S′′ phase is explained reasonably based on atomic bonding and total binding capacity of Al and Cu atoms in these precipitates.

Abstract: Small pieces of metal are generated during trimming of automotive body panels. Commonly referred to as slivers, these pieces can be imprinted into the surface of stamped panels. This may require metalfinish of every stamped exterior panel. The objective of the paper is to study the influence of trimming conditions on quality of trimmed surface and to modify the trimming process to eliminate slivers and burrs from the trimmed surface. Suggested solution includes two measures: 1) building an elastic support of the offal eliminating bending of the area of the blank being trimmed off; 2) creating the preference of crack propagation from the lower shearing edge by machining a small radius on the upper shearing edge.

Abstract: Leak path defects in brake caliper castings were observed and an efficient method of detecting them was needed. The standard detection technique used a differential decay method at pressures up to 1000 psi but this technique could not detect all of the leak path defects. The only known method that could detect all of the leak path defects was a technique that used brake fluid, and required pressures up to 3,000 psi. Because a fluid medium used to conduct this test, regulations precluded the calipers from being used on a vehicle after the inspection process. This was the motivation for a Capstone Project in the Mechanical Engineering Technology Program at Central Washington University. For safety and economic reasons, a nondestructive test which utilized acoustics was chosen. It was decided that the main focus for this project would be the design of the fixture. The prototype fixture was designed to utilize a toggle clamp and a backing plate (manifold) with O-rings. The clamping system was produced at INTERMET Corporation. After the fixture was completed it was sent to Central Washington University. To verify predictions, strain gauges were used at different parts of the system during normal use. Then the placement of the acoustic listener was determined, completing the test fixture. In order to obtain a good seal (O-ring to cavity lip) the fixture required deflections less then .0045” (as determined by tolerance stacking and O-ring deformation requirements). A magnetic pin gauge was used to measure this displacement as it the fixture was pressurized to 82psi (available shop pressure). To demonstrate the safety of the device, a strain gauge was mounted on the backing plate to measure the force being counteracted by the toggle clamp. The maximum stress on the backing plate was approximately 10% of the material's yield strength (~ 36 ksi for A36 steel); producing a safety factor of about 10. No more than 3750 psi was measured. The maximum strain of the backing plate was 0.003 in/in. Upon completion, the fixture was sent back to INTERMET Corporation where a production model was designed and implemented.

Abstract: Al alloys with Mg as the major alloying element constitute a group of non-heat treatable alloys with medium strength, high ductility, excellent corrosion resistance and weldability. However, the segregation of Mg may adversely affect the performance of these materials if they are exposed to rapid heating and cooling environments such as resistance spot welding. The formation and migration of vacancy is an important factor affecting Mg segregation. In this paper, the amount and distribution of Mg were measured by electron probe microanalysis and the vacancy formation energy in AA5754 alloys was measured by positron annihilation lifetime spectroscopy. The results indicated that the segregation of Mg at cracks, occurring under suitable temperature and stress conditions, is related to the formation and migration of vacancies, and may promote crack initiation and propagation.

Abstract: The present paper outlines the effect of small addition of Sc,Ag and Mg on the ageing characteristics and microstructural evolution in some microalloyed Al alloys,such as Al-Cu-Mg, Al-Ag and Al-Li alloys.Small quantities of these microalloying elements can modify the ageing characteristics of these alloys,and significant changes have been found in their microstructural evolution after small microalloying element additions. The observed effects are explained in terms of the atomic behaviour during the early ageing stage,and one of the key factors governing this effect is Sc/Vacancy mechanism.