Effect of Cerium and Magnesium on Surface Microcracks of Al–20Si Alloys Induced by High-Current Pulsed Electron Beam

The effect of Ce and Mg on surface microcracks of Al–20Si alloys induced via high-current pulsed electron beam (HCPEB) was studied. Mg was revealed to refine the primary Si phase in the pristine microstructure by forming a Mg2Si phase, leading to the suppression of microcrack propagation within the brittle phase after HCPEB irradiation. The incorporation of Ce into the Al–Si–Mg alloys further refined the primary Si phase and reduced the local stress concentration in the brittle phase induced by HCPEB irradiation. Ultimately, the surface microcracks were observed to be eliminated by the synergistic effects between the two elements. For Al–20Si–5Mg–0.7Ce alloys, Ce demonstrated a homogeneous distribution in the Al matrix on the HCPEB-irradiated alloy surface, while the Mg and Si exhibited a certain degree of aggregation in the Mg2Si phase. Metastable structures were formed on the HCPEB-irradiated alloy surface, including the nano-primary silicon phase, nano-cellular aluminium structure, and nano-Mg2Si phase. Compared with alloy specimens containing Mg, the Al–20Si–5Mg–0.7Ce alloy specimens exhibited an excellent anticorrosion property after HCPEB irradiation mainly due to the combined effects of the grain refinement and microcrack elimination.

Wear and Friction properties of H-Al-17Si alloy with dry, lubrication and coated (DLC-Star) conditions under HFRR

Dry, lubrication (SAE15W40), and coated (DLC-Star) reciprocating tribological tests on rapid solidified AlSi17Cu3.5-4Mg0.6-0.8 alloy was conducted using a high frequency linear reciprocating rig (HFRR) at ambient temperature. The alloy fabricated with the rheo-stir squeeze casting procedure under T-6 condition. However, at different loading (0-30 N) conditions, wear and friction properties of rapid solidified H-Al-17Si alloy are investigated. It is observed that the lower friction coefficient value obtained for DLC-Star coated H-Al-17Si alloy compared to dry and lubrication conditions. Though, for dry and lubricated sliding, the obtained wear coefficient values are 2.9X10-3 mm3/N.m and 4.0X10-4 mm3/N.m. A lower coefficient of wear value of 5.4X10-5 mm3/N.m was recorded with DLC-star coating under dry conditions. The alloy wear coefficient values first increases with applied load (up to 20 N) and then decreases (20 N to 30 N). EDS, AFM surface roughness profilometer, SEM, and advanced metallurgical microscope (AMM) analysis techniques used for the characterization of surface morphologies. The developments in friction and wear coefficients were fundamentally ascribed to the dispersion and size of primary Si elements and the development of tribo-oxide films on the rapid solidified AlSi17 alloy coated (DLC-Star) surfaces.

Microstructure and morphology of Si crystals grown in pure Si and Al-Si melts

Microstructure of Al-40 wt%Si samples solidified in electromagnetic levitation furnace is studied at high melt undercooling. Primary Si with feathery and dendritic structures is observed. As this takes place, single Si crystals either contain secondary dendrite arms or represent faceted structures. Our experiments show that at a certain undercooling, there exists the microstructural transition zone of aceted to non-faceted growth. Also, we analyze the shape of dendritic crystals solidifying from liquid Si as well as from hypereutectic Al-Si melts at high growth undercoolings. The shapes of dendrite tips grown at undercoolings >100 K along the surface of levitated Al-40 wt%Si droplets are compared with pure Si dendrite tips from the literature. The dendrite tips are digitized and superimposed with theoretical shape function recently derived by stitching the Ivantsov and Brener solutions. We show that experimental and theoretical dendrite tips are in good agreement for Si and Al-Si samples.

Nucleation Behavior of a Single Al-20Si Particle Rapidly Solidified in a Fast Scanning Calorimeter

Understanding the rapid solidification behavior characteristics, nucleation undercooling, and nucleation mechanism is important for modifying the microstructures and properties of metal alloys. In order to investigate the rapid solidification behavior in-situ, accurate measurements of nucleation undercooling and cooling rate are required in most rapid solidification processes, e.g., in additive manufacturing (AM). In this study, differential fast scanning calorimetry (DFSC) was applied to investigate the nucleation kinetics in a single micro-sized Al-20Si (mass%) particle under a controlled cooling rate of 5000 K/s. The nucleation rates of primary Si and secondary α-Al phases were calculated by a statistical analysis of 300 identical melting/solidification experiments. Applying a model based on the classical nucleation theory (CNT) together with available thermodynamic data, two different heterogeneous nucleation mechanisms of primary Si and secondary α-Al were proposed, i.e., surface heterogeneous nucleation for primary Si and interface heterogenous nucleation for secondary α-Al. The present study introduces a practical method for a detailed investigation of rapid solidification behavior of metal particles to distinguish surface and interface nucleation.

Research on a Low Melting Point Al-Si-Cu (Ni) Filler Metal for 6063 Aluminum Alloy Brazing

A new type of low melting point Al-Si-Cu (Ni) filler metal for brazed 6063 aluminum alloy was designed, and the microstructure and properties of the filler metal were systematically studied. The results show that when the content of Cu in the Al-Si-Cu filler metal increased from 10 wt.% to 20 wt.%, the liquidus temperature of the filler metal decreased from 587.8 °C to 533.4 °C. Its microstructures were mainly composed of the α-Al phase, a primary Si phase, and a θ(Al2Cu) phase. After a proper amount of Ni was added to the Al-Si-20Cu filler metal, its melting range was narrowed, the spreading wettability was improved, and the microstructure was refined. Its microstructure mainly includes α-Al solid solution, Si particles, and θ(Al2Cu) and δ(Al3Ni2) intermetallic compounds. The results of the shear strength test indicate that the shear strength of the brazed joint with Al-6.5Si-20Cu-2.0Ni filler metal was 150.4 MPa, which was 28.32% higher than that of the brazed joint with Al-6.5Si-20Cu filler metal.

Microstructural Evolution and Hardness of Rapidly Solidified Hypereutectic Al-Si Surface Layers Produced by Laser Remelting

A laser rapid solidification technique was employed to remelt and refine the microstructure of Al-25wt.%Si and Al-30wt.%Si alloyed layers produced by laser melting. The microstructure of the as-fabricated Al-Si layers consisted of irregular polygonal primary Si crystals of size 5 to 7 µm, fine α-Al dendrites, and Al-Si eutectic. Laser rapid remelting results showed a significant refinement of all the solidified phases with increasing scan speed and decreasing laser power. At the lowest laser power (800W), the sizes of the primary Si crystals were reduced to a sub-micron level and an interwoven network of nano-sized eutectic colonies was obtained. The higher cooling rates, resulted in a reduction in the amount of the α-Al phase especially those surrounding the primary Si, thereby stimulating the eutectic Si fibers to grow from the pre-existing primary Si crystals and increased the proportion of the fibrous eutectic. Transmission electron microscopy revealed fibrous eutectic, which was internally nano-twinned, with a diameter approaching as low as 10-15 nm for the highest cooling rate. The hardness measured by nanoindentation of the eutectic in the remelted Al-25wt.%Si layer increased with decreasing the eutectic spacing (ʎ) reaching a maximum value of 3.15GPa.

Effect of ultrasonic melt treatment on the microstructure and tensile properties of Al-25% Si alloy assisted by phosphorus addition

The effects of ultrasonic melt treatment (UST) on the microstructure and tensile properties of Al-25% Si alloys with and without phosphorus addition were investigated. The average size of the primary Si firstly decreased and then increased with the rise of the ultrasonic intensity. The minimum value is about 25.4 and 32.8 μm when the ultrasonic intensity is 900 W with phosphorus addition and 1 200 W without phosphorus addition. The average number of primary Si per unit area firstly increased and then decreased in both alloys with increasing of ultrasonic intensity. Both tensile strength and elongation were improved mainly due to the refinement of microstructure by UST. Finally, the mechanism of UST on the primary Si is discussed.

Effect of pulsed magneto oscillation and phosphorus on microstructure and tensile properties of as-cast Al-25% Si alloy

The effect of pulsed magneto oscillation (PMO) on the microstructure and tensile properties of Al-25% Si alloys with and without phosphorus addition was investigated. When PMO was applied to the melt during solidification, the results show that the PMO treatment is an effective method for refining the primary Si of Al-25% Si alloy with and without phosphorus addition. The morphology of primary Si was considerably refined via coarse dendritic and polygon shape to fine and regular block under the PMO treatment, and the average size of primary Si was refined to 30.8 μm and 45.3 μm under the optimal processing conditions for the alloy with and without phosphorus addition, respectively. The tensile strength and elongation of the alloys first increased and then decreased with rise of current. Analysis shows that application of PMO decreases Gibbs free energy and critical nucleus radius of the Al-25% Si alloy during solidification. The nucleation rate first increased and then decreased with increasing PMO intensity. The variation tendency of average number of primary Si per unit area is consistent with the theory analysis.