High-temperature Compressive Performance of Mg Alloy Foams Coated with Ni-P Layer

Mg/Ni hybrid foams were fabricated by the electroless method.The Ni-P(Nickel-Phosphorous)coatings were deposited on the surface of closed-cell Mg alloy foams.The composition,microstructure and phases of the Ni-P coatings were characterized by scanning electron microscopy(SEM),energy-dispersive X-ray spectroscopy(EDS)and X-ray diffraction(XRD),respectively.The compressive tests were performed on the Mg/Ni hybrid foams at 400℃using the Mg alloy foams as a reference.The experimental results show that the yield strength,plateau stress and energy absorption capacity of the closed-cell Mg alloy foams at high temperature were improved by the Ni-P coating.And there are four main modes for the Mg/Ni hybrid foam failure at 400℃,i e,shearing in cell wall,bending in cell edge,shedding and cracking in Ni-P coating.

Effect of grain size on the mechanical properties of Mg foams

The grain size of Mg foams was innovatively refined without alteration of pore structure and relative density by subjecting multi-axial forging(MAF)process to Ti-Mg composite,an intermediary product of the fabrication process of Mg foams where the spherical Ti particles were utilized as the replication material.The feasibility of the MAF process and the grain size effect on the mechanical properties of Mg foams were discussed.The results showed that,with the appropriate strain of 0.24 applied in the MAF process,Ti-Mg composites returned to original physical appearance without generating microcracks.And complete recrystallization was achieved after heat treatment,with the grain size of the MAFprocessed Mg foams two to three orders of magnitude smaller than that of as-cast foam.The mechanical properties of Mg foams were enhanced extensively after grain refinement with the yield strength and the plastic collapse strength increased by 147%and 50.7%,respectively.A revised model integrated by the Hall-Petch law and Gibson-Ashby model was proposed,which gave a good estimation of the yield strength and the plastic collapse strength of Mg foams from the compressive behavior of the corresponding parent material,though a knockdown factor of 0.45 was introduced for the yield strength.

Effects of porosity and pore size on the compressive properties of closed-cell Mg alloy foam

In our current work,AZ31 magnesium alloy foams with closed-cell were successfully fabricated by melt foaming method using Ca and CaCO3 as thickening and blowing agent,respectively.The influences of porosity and pore size on the quasi-static compressive properties of the foams were systematically investigated.The results showed that the yield strength,energy absorption capacity and ideality energy absorption efficiency were decreased with the increase in porosity.However,specimens with porosities of 60%,65%and 70%possessed similar total energy absorption capacity and ideality energy absorption efficiency.Meanwhile,experimental results showed that mean plateau strength of the foams was increased first and then decreased with increase in mean pore size.In addition,energy absorption capacities were almost the same in the initial stage,while the differences were obvious in the middle stage.From the engineering point of view,the specimens with mean pore size of 1.5 mm possess good combination of mean plateau strength and energy absorption characteristics under the present conditions.

Mg Alloy Foam Fabrication via Melt Foaming Method

For the first time AZ91 （MgAl9Zn1） and AM60 （MgAl6） Mg alloy foams with homogeneous pore structures were prepared successfully via melt foaming method using CaCO3 as blowing agent. It is revealed that the blowing gas to foam the melt is not CO2 but CO, which comes from liquid-solid reaction between Mg melt. The reaction temperature is more than 100℃ lower than CaCO3 decomposition, which makes Mg alloy melts foam into cellular structure much more easily in the temperature range from 690℃ to 750℃.

A novel approach to fabricate Zn coating on Mg foam through a modified thermal evaporation technique

Zn enriched coatings with distinct microstructures and properties were fabricated on Mg foams by a modified thermal evaporation technique using a tubular resistance furnace. As the temperature and kinetic energy of Zn vapor varied along the tubular system, a spatial variation of preparation conditions was created and the obtained coatings were found to follow two growth mechanisms： a thermal diffusion pattern in high-temperature zone and the a relatively low-temperature deposition model. AZn-based deposition coating with dense texture and nearly uniform structure was acquired while Mg foam was placed 20 cm far from the evaporation source, where the Zn vapor deposition model dominated the coating growth.Mechanical properties and bio-corrosion behaviors of the samples were investigated. Results showed that the Zn coatings brought dramatic improvements in compression strength, but exhibited differently in biodegradation performance. It was confirmed that the diffusion layer accelerated corrosion of Mg foam due to the galvanic effect, while the Zn-based deposition coating displayed excellent anti-corrosion performance, showing great potential as bone implant materials. This technique provides a novel and convenient approach to tailor the biodegradability of Mg foams for biomedical applications.

Fabrication of cellular Zn–Mg alloy foam by gas release reaction via powder metallurgical approach

By using CaCO3 as the ＂blowing agent＂, Zn–（10–50） wt% Mg alloy foams with close-pore structure were fabricated by the powder metallurgical route. The key of successful foaming is Mg addition and the proper sintering treatment, which lead to the formation of intermetallic compounds and can make the gas release reaction between Mg melt and CaCO3 happen the during foaming process.