Coatings applied on steel molds used for casting aluminum parts have two main purposes: avoid mold metal reaction and control thermal transfer to obtain directional solidification. The coatings widely known to foundry operators are water-based sodium-silicate bonded ceramic suspensions; they are air sprayed and cured on mold surfaces and typically last for 100 casting cycles. Although thermal sprayed coatings have been shown to last more than 5000 casting cycles, they are not yet the preferred mold protection method. This study addresses the issue by developing a knowledge base of thermal transfer properties that can be achieved with air plasma sprayed magnesium zirconate powders. The properties are assessed on an instrumented mold using the inverse technique for different coating compositions.

Aluminum castings have limited strength and stiffness and tend to exhibit brittle fracture behavior under fatigue loading. These properties can be significantly improved, however, as this study shows, by reinforcing cast aluminum parts with magnesium metal-matrix composite structures. In order to obtain a bond between the cast Al and fiber-reinforced Mg composite surfaces, Al alloy (Al 99 and AlSi 12 ) layers were deposited on the Mg structures by thermal spraying. The mechanical properties of the bonding were assessed via single-lap shear and adhesive tensile tests along with optical microsection analysis. Hybrid aluminum alloy AlSi 10 Mg castings incorporating coated Mg-MMC inserts were also produced and examined, validating the general approach, while revealing that heat input to the MMC structure must be reduced through design or process adaptations.

This study compares the dielectric properties of annealed forsterite (Mg 2 SiO 4 ) and alumina coatings deposited on mild steel substrates by atmospheric plasma spraying. As-sprayed coating samples were electrically characterized then submitted to a series of one-hour annealing treatments at temperatures from 300 to 800 °F. After each treatment, impedance measurements were recorded over a frequency range of 30 to 100 kHz. An electrical model was fitted to Nyquist data (Im Z vs. Re Z) using a least-mean-square algorithm with a weighting function. Although impedance spectroscopy measurements were obtained at different temperatures, this paper focuses on the acquisition, modeling, and comparison of room temperature properties, particularly electrical resistivity and dielectric constant. It also compares the microstructure of as-sprayed and annealed forsterite and alumina coatings and discusses coating degradation mechanisms stemming from differences in CTE.

Magnesium alloy AZ91D-SiC composite coatings were fabricated by cold spraying to study the effects of SiC particle size and volume fraction on microstructure and mechanical properties. The results show that coatings with large SiC particles have higher microhardness and bonding strength. Mechanically blended powders with fine SiC particles, on the other hand, are difficult to deposit. SiC volume fractions in the starting powders were 15, 30, 45, and 60 vol%, resulting in coatings with SiC volume fractions of 19, 27, 37, and 51 vol%. Based on test results, coating hardness and bonding strength increase with increasing volume fraction of SiC particles.