Zhan Yong Zhao

Microstructure formation mechanism and properties of AZ61 alloy processed by melt treatment with vibrating cooling slope and semisolid rolling

A melt treatment with a vibrating cooling slope and a semisolid rolling process to produce an AZ61 alloy strip was proposed. The microstructure formation mechanism and the properties of the AZ61 alloy produced by the proposed process were investigated. Due to the high cooling rate and stirring action caused by the vibration cooling slope, the nucleation rate was greatly improved, which caused the formation of fine spherical or rosette primary grains. During the rolling process, the solid fraction increased from the entrance to the exit of the roll gap, and under the shearing action of the roller, the distribution of solute in the melt was homogenous, and the primary grains grew further. When the casting temperature was 680 °C, a strip with a cross section of 4 mm×160 mm was produced and a homogeneous microstructure was obtained. The ultimate tensile strength of the AZ61 alloy strip produced by the proposed method reached 242 MPa, and the corresponding elongation to failure was 4%, which were better than those achieved in previous similar studies.

Microstructure formation mechanism and properties of a Mg-3Sn-1Mn (wt%) magnesium alloy processed by a novel semisolid continuous shearing and rolling process

A novel semisolid Continuous Shearing and Rolling (CSR) process for producing a Mg-3Sn-1Mn (wt%) alloy strip is developed, and the microstructure formation mechanism and properties of the Mg-3Sn-1Mn (wt%) alloy processed by this process are investigated. At a casting temperature of 690°C and a roll speed of 0.052 m·s−1, a Mg-3Sn-1Mn (wt%) alloy strip with a cross section size of 4×160 mm was produced by the proposed process. Under strong cooling and shearing actions, eruptive nucleation, direct globular grain growth and dendrite arm breakage took place during the process, which caused formations of fine spherical grains. The grain size and roundness of the Mg-3Sn-1Mn (wt%) alloy strip increased with increasing increments of the casting temperature. In this perspective, roll speed obviously affects grain shape. The ultimate tensile strength and elongation of the Mg-3Sn-1Mn (wt%) alloy strip reached 205.93 MPa and 7.2%.

Effect of Heat Treatment on the Microstructures and Mechanical Properties in AZ31 Magnesium Alloy Processed by Continuous Rheo-Extrusion

AZ31 magnesium alloy profiles were prepared by continuous rheo-extrusion process, and effects of solution and aging treatments on the microstructures and mechanical properties was investigated by OLYMPUS optical microscope, scanning electron microscope, energy dispersive spectroscopy device and tensile machine. The results reveal that saturated solid solution was formed in the profile at a solution temperature of 415 °C and 16 hours. During aging treatment of saturated solid solution, β-Mg17Al12 phase nucleates firstly at grain boundaries and takes on globular growth, which is mainly due to the lower interfacial energy in coherent or semi-coherent interface between β-Mg17Al12 phase and matrix phase. Lower interfacial energy is favorable to the nucleation and growth of β-Mg17Al12 phase at grain boundary. With the increase of aging time or the rise of aging temperature, β-Mg17Al12 phase precipitates out gradually from grain interior and starts to grow in globular shape. After β-Mg17Al12 phase grows to a certain extent, it grows in lamellar shape along the orientation of lower mismatch between β-Mg17Al12 phase and matrix phase. After solution at 415°C and 16h and aging at 180°C and 8h, the ultimate tensile strength and elongation to failure of the profile are 305MPa and 13%, respectively.

Microstructure Formation and Mechanical Properties in AZ31 Alloy Processed by Continuous Rheo-Extrusion

Based on continuous casting and extrusion (CAXTEX) process, a semisolid metal forming process, continuous rheo-extrusion of magnesium alloy, was proposed. Effect of casting temperature on semisolid region distribution, microstructure formation and stability of forming process, as well as microstructure and mechanical properties of the AZ31 alloy fabricated by the process were investigated. Microstructure evolution from dendrite to rosette or spherical grains was observed with the application of large shearing force provided by the roll. The results indicate that semisolid region in the roll-shoe gap moves downward gradually with the increase of casting temperature, and proper casting temperature range of 730~750°C is suggested. Under the suggested casting temperature, 10×15mm sectional bar of AZ31 alloy with smooth surface and homogeneous striped microstructure has been obtained. As the product was aged for 14h at 180°C after 16h solution at 415°C, the ultimate tensile strength and elongation could reach 305MPa and 11.5%, respectively.

Characterization of Magnesium Alloy Degradation in Whole Blood and Platelet Rich Plasma

Magnesium (Mg) is an attractive biomaterial due to its desirable biodegradable and mechanical properties. In this study, we compared the degradation behavior of Mg and a new Mg alloy incubated in both whole blood and platelet rich plasma (PRP) for two hours under standard cell culture conditions. To avoid settling of red blood cells, tubes with whole blood were under constant rotation during the incubation. Post-incubation solutions were collected, centrifuged, and analyzed for pH and Mg ion concentration. Mg and Mg alloy samples were fixed with a 3% glutaraldehyde solution, dehydrated using an ethanol series, critical point dried, sputter coated, and imaged with a field emission scanning electron microscope. Analysis of the post-incubation solutions showed PRP had greater concentrations of Mg ions and higher pH values when compared with whole blood. This indicated that the Mg and Mg alloy degraded faster when incubated in PRP than in whole blood. When comparing the surface of the materials after incubation with whole blood and PRP, the surfaces of Mg and Mg alloy that was incubated in PRP had larger cracks and grain boundaries than the samples incubated in whole blood. Additionally, more particulate microstructures were observed on the samples incubated in PRP as opposed to whole blood. Further studies are still needed to elucidate the differences in degradation of Mg alloys in whole blood and PRP.

Boundary layer and cooling rate and microstructure formation on the cooling sloping plate

During melt treatment by cooling sloping plate, laminar flow and turbulent flow exist on sloping plate surface commonly. The thickness of velocity boundary layer and the critical transfer distance from laminar flow to turbulent flow increase with the decrease of initial flow velocity. The thickness of temperature boundary layer increases with the increment of flow distance and the decrease of initial flow velocity. The melt cooling rate and melt thickness have an inverse proportion relationship. The melt cooling rate of cooling sloping plate process can reach 102–103 K/s and belongs to meta-rapid solidification scope. Uniform solute field and high cooling rate can lead to eruptive nucleation. In addition, a large quantity of heterogonous nuclei appears on the sloping plate surface, and vibrating flow can enable heterogonous nucleus to escape off the plate, which leads to nucleus multiplication. Under relative uniform solute field and high cooling rate, some grains can keep stable growth surface, go on growing with the round surface and finally maintain their globular structure. However, there are always some grains that grow along a certain preferred direction, but under vibrating flow their dendritic arms break and transform into near spherical structure.

Preparation of HA-Coated Mg-4.0Zn-1.0Ca-0.6Zr(wt%) Alloy and its Biodegradation Behaviors

Magnesium and magnesium alloys, as biomaterials, possess many properties that are superior to those of other metals. However, magnesium and magnesium alloys have strong chemical activity and porous and brittle surface oxide film, as degradable implantation materials, their degradation rates are too fast. Hydroxyapatite (HA) has good biocompatibility and biological activity and has become one of the replacement materials of biomedical stiff hemopoietic tissue, but the application of HA biomaterial is hindered because HA is brittle and has low strength. Integrating good mechanical properties of metallic materials with excellent biological performance of HA, the composite obtained by coating HA to the surface of metallic matrix is ideal rehabilitation material of bone tissue. In the present study, a new Mg-4.0Zn-1.0Ca-0.6Zr (wt%) was designed according to the requirements of biocompatibility. The microstructures and the mechanical properties of the new alloy were investigated by experiment. The excellent mechanical properties fully meet the service requirements of human bone tissue for mechanical property. Flat and dense hydroxyapatite coating was prepared on the surface of magnesium alloy matrix by preceding alkali heat treatment, electrodeposition and post alkali heat treatment. The Structure and constituent of HA coating and the biodegradation behavior of HA-coated Mg-4.0Zn-1.0Ca-0.6Zr (wt%) alloy were evaluated. Resuls showed that the degradation rate of HA-coated Mg-4.0Zn-1.0Ca-0.6Zr (wt%) alloy in SBF biomimetic solution decreased obviously and tended to be stable after 10 days. As degradable implantation materials, HA-coated Mg-4.0Zn-1.0Ca-0.6Zr (wt%) alloy fully meets the service requirements of human bone tissue.

Effects of the Parameters of Sloping Plate Process on Microstructures of A2017 Alloy

In this paper, effects of the parameters of sloping plate process on microstructures of A2017 alloy were investigated, and semisolid ingot of A2017 alloy was prepared. The grain size of the ingot prepared by the wavelike sloping plate is much smaller than that prepared by the flat sloping plate. The primary grain becomes rounder and smaller with the increment of the sloping plate length. High preheating temperature of the plate reduces the nucleation rate of the melt and leads to coarse microstructure. When the casting temperature is 720°C, the sloping angle is 45°, and the wavelike plate length is between 400mm and 500mm, semisolid ingot of A2017 alloy with fine and homogeneous microstructures can be obtained. The reheated microstructure of semisolid ingot is mainly composed of spherical solid grains and the melted liquids and is suitable for semisolid forming.

Effects of Technical Parameters on Process Stability and Microstructure during Continuous Rheo-Extrusion Process of 6201 Alloy Wire

In this paper, effects of technical parameters on process stability and microstructure during continuous rheo-extrusion process of 6201 alloy wire were investigated. The optimal process parameters for producing 6201 alloy wire are obtained. The casting temperature is from 720°C to 740°C, and the roll rotating speed is 15r/min. The 6201 alloy wire with good performance has been successfully manufactured. After heat treatment, the tensile strength of the product reaches 325 MPa and the resistivity reaches 32.47 nΩm, which is better than LHA2 conducting wire of China.

Melt Flow Type and Physical Model of Semisolid Rheo-Casting Process by Cooling Sloping Plate

In this paper, a physical model of semisolid rheo-casting process by cooling sloping plate is established, and the features and parameters of this process are analyzed. The calculation results show that there exists lamellar flow and turbulent flow on the sloping plate surface commonly. The critical transfer distance from lamellar flow to turbulent flow decreases with the increment of the initial flow velocity gradually. The critical transfer distance decreases with the increment of the sloping angle slowly. The effect of the sloping angle on the critical transfer distance is relatively not obvious.