Effect of Mg on the Formation of Periodic Layered Structure during Double Batch Hot Dip Process in Zn-Al Bath

The article presents the results of studies on the influence of Mg on the formation of the periodic layered structure of the Zn-AlMg coatings. These coatings were produced by the double batch hot dip method in a Zn bath and then in a Zn-Al(Mg) bath with a content of 15, 23, 31 wt.% Al and 3, 6 wt.% Mg. The microstructure of the coatings (OM, SEM) was revealed and the phase composition (XRD) obtained in two-component Zn-Al baths and Zn-AlMg baths were determined. The periodic layered structure was found to consist of alternating FeAl3 phase layers and a bath alloy (Zn + Al + Mg). Moreover, it was observed that the addition of 3 wt.% Mg reduces the thickness of the coating in baths containing 23 and 31 wt.% Al. However, the addition of 6 wt.% Mg causes complete disappearance of periodic layered structure in a bath with 23 wt.% Al. In a bath with a content of 31 wt.% Al the addition of 6 wt.% Mg creates a compact layer consisting of the FeAl3 phase containing the precipitation of the MgZn2 phase and Fe2Al5 phase. Such a structure of the coating transition layer limits the growth of the periodic layered structure in the coating.

Role of Small Addition of Sc and Zr in Clustering and Precipitation Phenomena Induced in AA7075

A detailed characterization of phase transformations in the heat-treated commercial 7075 aluminum alloys without/with low Sc–Zr addition was carried out. Mechanical and electrical properties, thermal and corrosion behavior were compared to the microstructure development. The eutectic phase consists of four parts: MgZn2 phase, Al2CuMg phase (S-phase), Al2Zn3Mg3 phase (T-phase), and primary λ-Al(Mn,Fe,Si) phase. Strengthening during non-isothermal (isochronal) annealing is caused by a combination of formation of the GP zones, η’-phase, T-phase and co-presence of the primary and secondary Al3(Sc,Zr)-phase particles. Positive influence on corrosion properties is owing to the addition of Sc–Zr. Positron annihilation showed an evolution of solute Zn,Mg-(co-)clusters into (precursors of) the GP zones in the course of natural ageing. The concentration of the (co-)clusters is slightly negatively affected by the low Sc–Zr addition. A combination of both precipitation sequences of the Al–Zn–Mg–Cu-based system was observed. The apparent activation energy values for dissolution/formation of the clusters/GP zones and for formation of the metastable η’-phase, stable T-phase and stable η-phase were calculated.

Liquation Cracking Tendency of Novel Al-Mg-Zn Alloys with a Zn/Mg Ratio below 1.0 during Fusion Welding

The main obstacle for the application of high strength 7××× series aluminum alloys is that these alloys are susceptible to hot cracking during fusion welding. This study presents the liquation cracking susceptibility of the novel T-Mg32(AlZn)49 phase strengthened Al-Mg-Zn alloy with a Zn/Mg ratio below 1.0 by a circular-patch welding test, and compared the liquation cracking tendency with η-MgZn2 phase strengthened 7××× series alloys whose Zn/Mg ratios are above 1.0. It was found that all these novel Al-Mg-Zn alloys still have as low a liquation cracking susceptibility as traditional 5××× series alloys, surpassing that of traditional 7××× series alloys substantially. It was noticed that the increase of the Zn/Mg ratio will result in a larger difference between the fraction solids of the fusion zone and the partially melted zone during the terminal solidification stage, which can lead to a wider crack healing disparity between these two areas and thus result in different liquation cracking susceptibilities in different alloys.

Investigation on the Quench Sensitivity of 7085 Aluminum Alloy with Different Contents of Main Alloying Elements

The quench sensitivity of 7085 aluminum alloy with different contents of the main alloying elements (Zn, Mg and Cu) was investigated using time-temperature-transformation (TTT) curves and end quenching experiments. Then, the quenching microstructure was analyzed using transmission electron microscopy. With increasing the contents of the main alloying elements, the transitions and nose temperatures of the TTT curves are obviously increased, while the incubation time of 0.5% η (MgZn2) phase precipitation content is decreased. In addition, as the contents of the main alloying elements decrease, the conductivity of the quenched samples is increased, but the hardness of the quenched samples is decreased. Moreover, the size and area fraction of the η phase are increased with increasing the contents of the main alloying elements. Based on the experimental results, the increase of Mg and Cu contents can decrease the stability of supersaturated solid solution and increase the lattice distortion energy, which can increase the quench sensitivity of 7085 aluminum alloy.

Research on Multiple Plastic Deformations of Ultra-High Strength Aluminium Alloy

Research on multipass plastic deformation of 7A04 ultra-high strength aluminium alloy by isothermal compression experiments on the 6300KN extrusion press. Experiment results show that elongation reaches its maximal value 9.25% after the first deformation. It is obvious that fibrous tissues appeared along the metal flow direction in the deformed 7A04 ultra-high aluminium alloy, with heterogeneous distribution of precipitated η (MgZn2) phase in the matrix, which results in lower strength (Rm=335MPa, Re=212.5MPa). As the times of deformation increases, precipitated phases grow gradually and the plasticity of alloys decreases dramatically, which reaches its minimal value 5.17% after the fourth deformation. With η(MgZn2) phase disperses gradually, the strength of the alloy increases gradually, and reaches its maximal value 386.7MPa after the fourth deformations. It is proved that 7A04 high-strength aluminium alloy has better synthetic mechanical properties after four times deformation.

Microstructure and Properties of Hot Compacted Al-12 wt% Zn-3 wt% Mg-1.5 wt% Cu Melt Spun Ribbons

The alloys Al-12 wt % Zn-3 wt % Mg-1.5 wt % Cu with addition of Zr were melt spun and then hot pressed under 600 MPa pressure at 380 °C. Refinement of the microstructure and reduction of the volume fraction of the η (MgZn2) phase was observed in melt spun ribbon, as compared to the mould cast alloys. Good quality samples without pores and cracks obtained by hot pressing were composed of grains of size 0.2-0.5 µm. The particles of the η phase enriched in Zn, Mg and Cu were homogenously distributed in the matrix, while a few nanometres large precipitates of magnesium oxide were located at grain boundaries. Plate like precipitates of metastable h¢ phase appear after ageing at 120 °C and lead to microhardness increase up to about 195 HV as compared to 145 HV in pressed sample. Hot pressed ribbons showed compression strength of about 450 MPa which increased up to 630 MPa after ageing.

Effect of Heating Temperature on the Microstructural Evolution of Al-5.8%Zn-1.63%Mg-2.22%Cu-0.12%Zr Gas Atomized Powders

Al-5.8%Zn-1.63%Mg-2.22%Cu-0.12%Zr powders were manufactured by gas atomization process. The effect of temperature on microstructural evolution of the powders was investigated using X-ray diffraction (XRD), scanning electron microscopy (SEM) and energy dispersive spectroscopy (EDS). On reheating the atomized powders into semisolid state, the phase chemistry and quantity of liquid were typically changed as the system established equilibrium. As the heat treated temperature was increased from 550 °C to 620 °C, the amount of η (MgZn2) phase decreases and a great number of particles Al2Cu precipitates in the powders interior. The inter-diffusion of species will be the main factor. Considering the factors of microstructure, 600 °C is determined to be the best semi-solid forming temperature.