Materials Science Forum Vols. 610-613

Abstract: Deformation of the material during 90° change-channel angular extrusion (CCAE) process is analyzed using upper-bound theorem. The mathematical model was established which includes the effect of friction between the sample and inner surface of the die, radius of inner and outer corner of the die, and the dead metal zone on the deformation patterns during change-channel angular extrusion deformation process. The model of parameters is explored in relation to the deformation of the material during the change-channel angular extrusion process. Results showed that the extrusion force increased with the increase of extrusion ratio (a2/b2), friction factor m, out corner and the reduce of inner corner. Further directions for progress in upper-bound analysis in change-channel angular extrusion deformation process are outlined.

Abstract: The AZ31 thin sheet (minimum thickness less than 1mm) was obtained by rolling the cast strip which was prepared by vertical twin roll casting in this paper. Since the absolute deformation during rolling was smaller,due to the small thickness of the strip of 3mm, the rolling of cast strip was different from the conventional rolling process. It was found that homogenizing time at 400°C for the cast strip was 4h and reduction per pass should be 8-10% for producing thinner (1-1.5mm) rolled AZ31 sheet at 350°C. Mechanical properties of the sheet were equivalent to conventional rolling sheet’s.

Abstract: More and more research has been focused on the improvement of the mechanical properties and the optimal design of the new excellent Mg-based alloys. In spite of many experimental investigations, the theoretical studies of the mechanical properties are very scarce. First-principle calculations of the elastic constants and mechanical properties of typical Mg-based alloys become necessary to understand the fundamental mechanism governing the observed mechanical properties. In this paper, the single-crystal elastic constants Cijs of the typical fcc and hexagonal structured Mg-based alloys (Mg3Zn3Y2 and CaMg2) were calculated, using density functional theory within the generalized gradient approximation. Then the bulk modulus B, shear modulus G, Young’s modulus E, Poisson’s ratio ν and anisotropy value A were derived from single-crystal elastic constants. The mechanical properties such as the ductility and stiffiness of the alloys are analyzed and discussed in comparison with experimental observations.

Abstract: The nano-indentation response and the friction/wear properties of DLC/SiC (diamond-like carbon/silicon carbon) double layer thin films deposited on Mg alloy (AZ91D) substrate using magnetron sputtering technique at room temperature were investigated. The results show that the DLC films displayed low nano-hardness (3.05 GPa), low Young's modulus (24.67 GPa) but high hardness-to-modulus ratio (0.124). The films-substrate system exhibited a good friction and wear properties with the mean friction coefficient of about 0.175, the special wear rate in the magnitude order of 10−6 mm3 m−1 N−1 together with little film-cracking and interface-delaminating, when sliding against Si3N4 (silicon nitride) ball using ball-on-disc wear tester under dry frictional condition. The high wear-resistance is in accordance with high ductility of the films, good modulus match in the films-substrate system, and high hardness-to-modulus ratio of the films. The underlying factors are discussed and are believed to be due to the substrate is Mg, a metal with high activity.

Abstract: The mechanical testing for magnesium alloy AM60 was conducted on an MTS servohydrolic material testing system. In order to examine the effects of impact velocity on fracture toughness, finite element method was applied to compute the impact fracture toughness of AM60 specimens with single notched edge at room temperature. The specimens were impacted by steel balls (Diameter 20 mm) at the velocities of 80m/s, 120m/s and 160m/s. The simulation results well described the effects of impact velocities on fracture toughness. The finite element analysis performed in this work offered an effective method to compute dynamic fracture toughness for engineering applications.

Abstract: AZ91D magnesium alloy and Al metal were bonded successfully at 450 oC with load of 25 or 31 MPa by using Mg-Al eutectic alloy as the solder in atmosphere. The microstructure of the AZ91D/Al joints was characterized by SEM equipped with EDS, and the bond strength was measured through three-point bending test. It was revealed that reactive diffusion occurred obviously at the jointing part. The diffusion layers are composed of α-Mg + β-Mg17Al12, β-Mg17Al12, γ-Mg2Al3 and α-Al. The average bend strength of AZ91D/Al joints is 24 MPa with maximum value 28 MPa. To demonstrate the potential engineering application of this technology, AZ91D magnesium alloy plate was coated with Al foil and its corrosion resistance was tested. Electrochemical test results showed that the corrosion resistance of Al coated AZ91D magnesium alloy plate was greatly improved compared with the uncoated one.

Abstract: TiN/CrN multilayer coating was deposited on AZ91D magnesium alloy by pulsed bias cathodic arc PVD process. Effects of the coating on wear and corrosion behaviours of magnesium alloy were investigated. The stick-tearing for adhesion evaluation, nanoindentation for hardness measurement, ball-on-disc testing for wear tests, and potentiodynamic polarization for corrosion tests were used. The results show that the friction coefficient is decreased and the wear resistance is improved remarkably by TiN/CrN multilayer coating on AZ91D magnesium alloy. The corrosion resistance of the alloy is increased at a certain degree.

Abstract: Aluminum-magnesium alloys were prepared from magnesium oxide by molten salt electrolysis method. MgF2-BaF2-LiF was taken as electrolysis.Since the density of MgF2-BaF2-LiF was high than aluminum liquid, upload cathode of aluminum liquid was used in the tests. Current efficiency during melting was more than 80%, and the maximum was up to 87.7%. Some amount of magnesium oxide flowed into electrolytic cell. The content of Mg in alloys increased gradually and up to maximum of 18.6%(w) with electrolyzing time. The experiment results revealed that the addition of KCl improved the physical-chemical property of the melten salt.

Abstract: The solution mainly containing Zn2+, Mn2+, Ni2+ and H2PO4- was used to prepare phosphate film on AZ31 magnesium alloys. The process was adjusted to the following parameters, PH value 3～4, temperature 25～40°C, treatment time 3～10 min. Surface characteristics and microstructure of the phosphate film were also investigated by SEM and XRD. The film can fit for organic coatings prepared with electrophoresis and painting.It is found that the coatings adhered well to the phosphate film, and the anti-corrosion abilities were improved. The coating has no change in 3% NaCl solution for 360h and in neutral salt fog for 168h, respectively.

Abstract: The effect of Ga on the microstructure and mechanical properties of Mg-8wt.%Al alloy at room temperature were investigated by SEM and X-ray analysis. The microstructure of as-cast Mg-8wt.%Al alloy was consist of α-Mg matrix, β- Mg17Al12 and α+β eutectic phases. The primary β- Mg17Al12 phase precipitate along the α-Mg matrix grain boundary and in the discontinuous reticular shape. Ga addition in the as-cast Mg-8wt.%Al alloy change the microstructure significantly. When the addition is less then 2% Gd could be detected and distributed mainly within β- Mg17Al12 phase and in the interface of α-Mg matrix and β- Mg17Al12. With increasing Ga content α-Mg matrix grains and β- Mg17Al12 phase are refined. The amount of α+β eutectic structure increase as well. The holes within β phase are diminished. When Ga content was more than 2% with increasing of Ga content β phase was broken along the concentration of Ga. With the increase of Ga content to 4%, a new phase of Ga2Mg form in the microstructure and β phase precipitate in the shape of smooth baboon along the grain boundary. The mechanical properties test showed that as-cast Mg-8wt.%Al-2wt.%Ga alloy has the optimum combination of ultimate tensile strength of 248 MPa and elongation of 10%.