Effect of Melt Overheating on Structure and Mechanical Properties of Al-Mg-Si Cast Alloy

The paper discusses the complex effect of melt overheating with subsequent fast cooling down to the pouring temperature on the crystallization process, microstructure and mechanical properties of Al-Mg-Si aluminum alloy. The results obtained facilitated the establishment of rational modes of melt overheating, leading to a significant change in the dispersion and morphology of structural components. In particular, with an increase in the melt overheating temperature to 900 °C with holding and subsequent rapid cooling to the casting temperature, a decrease in the average size of dendritic cells of the aluminum solid solution from 39 μm to 13 μm was observed. We also noticed the refinement of eutectic inclusions of the Mg2Si phase with compact morphology. An increased level of mechanical properties was noted; the maximum values of tensile strength and elongation reached 228 MPa and 5.24%, respectively, which exceeded the initial values by 22.5% and 52.3%, correspondingly. The microhardness of the aluminum solid solution sequentially increased from 38.21 to 56.5 HV with an increase in the temperature during melt overheating. According to the EDS linear scanning, an increase in the superheat temperature of the melt is accompanied by an increase in the degree of saturation of the solid solution with magnesium.

Properties of welded joints of the Al – Zn – Mg – Ca alloy doped by microadditivies of zirconium and scandium

This work is devoted to the study of the structure and properties of welded joints of hot-rolled sheets from the aluminum alloy Al – 4%Zn – 2.5%Mg – 2.5%Ca – 0.2%Zr – 0.1%Sc. The joints were obtained by manual argon-arc method (TIG-welding) using a wire of own production from the base metal composition (sample AlCa) and standard welding wire Of SvAMg5 alloy (sample AlMg). It is shown that in the cast state the alloy structure consists of an aluminum solid solution (Al) (which contains 2.9% Zn, 2.5% Mg, 0.28% Zr and 0.12% Sc) and eutectic crystals of the phase (Al, Zn)4Ca with a volume fraction of ~7.6%. Microstructural investigation of wrought products showed that the formation of a structure consisting of an aluminum matrix and uniformly distributed spherical particles with a diameter of less than 2 microns occurs as a result of hot rolling with a compression ratio of 95%. The quality of welded joints produced using different additives meets the requirements for the absence of internal defects. The structure in the middle of the joints corresponds to the cast structure of the alloy and has hardness (~80 HV) inferior to the hardness (~105 HV) of the main deformed alloy. Heat-affected zone retains a relatively high hardness (at 95 HV), which is associated with a stabilizing effect of dispersed particles of the eutectic phase (Al, Zn)4Ca, and also presence of nanoparticles of the phase Al3(Zr,Sc) in the original sheet. Additional annealing of welds at 350 oC resulted in degradation of the hardness of the AlMg sample to less than 70 HV and relative cross-sectional hardness equalization in the AlCa sample to more than 90 HV. This effect can be explained by the dispersion hardening in case of the AlCa sample due to the separation of coherent nanoparticles of the Al3(Zr, Sc) phase with the L12 structure during the decay of a supersaturated aluminum solid solution. The results of mechanical tests confirmed the advantages of the strength properties of the AlCa sample over the values of the AlMg sample. All samples in the initial state and annealed sample AlMg have strength coefficients relative to the base metal from 72.4 to 75.3. The greatest mechanical properties can be achieved in the welded joint AlCa in the annealed state: UTS = 274±2 MPa, YS = 181±20 MPa, El = 2.9±0.8 %, which corresponds to a coefficient of more than 80%. This paper was prepared in the framework of the Assignment No. 11.2072.2017/4.6 for implementing a project on the following subject: Developing a process for obtaining deformed semi-finished products made of aluminium-matrix eutectic composites hardened with L12 phase nanoparticles with no quenching applied.

Structure and Properties of Al–0.6wt.%Zr Wire Alloy Manufactured by Direct Drawing of Electromagnetically Cast Wire Rod

The method of electromagnetic casting (EMC) was used to produce the long-length rod billet (with a diameter 12 mm) of aluminum alloy containing 0.6 wt.% Zr, 0.4%Fe, and 0.4%Si. The combination of high cooling rate (≈104 K/s) during alloy solidification and high temperature before casting (≈830 °C) caused zirconium to dissolve almost completely in the aluminum solid solution (Al). Additions of iron and silicon were completed in the uniformly distributed eutectic Al8Fe2Si phase particles with an average size of less than 1 µm. Such fine microstructure of the experimental alloy in as-cast state provides excellent deformability during wire production using direct cold drawing of EMC rod (94% reduction). TEM study of structure evolution in the as-drawn 3 mm wire revealed the onset of Al3Zr (L12) nanoparticle formation at 300 °C and almost-complete decomposition of (Al) at 400 °C. The distribution of Zr-containing nanoparticles is quite homogeneous, with their average size not exceeding 10 nm. Experimental wire alloy had the ultimate tensile strength (UTS) and electrical conductivity (EC) (234 MPa and 55.6 IACS, respectively) meeting the AT2 type specification. At the same time, the maximum heating temperature was much higher (400 versus 230 °C) and meets the AT4 type specification.

Simultaneous Increase of Electrical Conductivity and Hardness of Al–1.5 wt.% Mn Alloy by Addition of 1.5 wt.% Cu and 0.5 wt.% Zr

The effect of Cu and Zr additions and annealing temperature on electrical conductivity and hardness of the Al–1.5 wt.% Mn alloy in the form of as-cast ingots and cold rolled sheets has been investigated. It is shown that due to the formation of low alloyed aluminum solid solution and Al20Cu2Mn3 and Al3Zr (L12) phase nanoparticles, the 1.5MnCuZr alloy is superior to the base 1.5Mn alloy both in the hardness (up to two times) and electrical conductivity (up to 30%) after metal processing and annealing. A new alloy can be considered as a replacement for existing 6201 type conductive alloys.

Microstructural Characterization and Formation Mechanism of Nitrided Layers on Aluminum Substrates by Thermal Plasma Nitriding

Nitrided layers on 6082 aluminum alloy substrates and 1060 aluminum substrates are formed at atmospheric pressure using thermal nitrogen plasma, which only takes seconds to form a millimeter-level layer. The nitrided layers are composed of aluminum nitride (AlN) and aluminum solid solution phases. Microstructures in these nitrided layers can be divided into three regions from bottom to top: the transition region, the dendrite region, and the lamella region. These regions are formed in sequence. The formation mechanisms and processes of the three regions are discussed in detail. Furthermore, we found that Al melt is transported upward through the voids and the capillaries in the AlN structures, and reacts with N plasma in the melt surface. The growth of the AlN structures promotes this transport. With the increase of N2 flow rates from 1 L/min to 7.5 L/min, both the hardness and the wear resistance of the nitrided layers are improved, and the nitrided layer becomes thicker.

Superplastic-Like Elongation by Transition of Deformation Mechanism from Grain Boundary Sliding to Solute Drag Creep in Fine-Grained Al-Mg Solid Solution Alloy

It is widely accepted that the dominant deformation mechanism of fine-grained superplasticity is through grain boundary sliding (GBS) that occurs in fine-grained materials. However, it has been reported that in “Class I” solid solution alloys, superplastic-like behavior controlled by trans-granular deformation occurs by solute drag creep. In this study, we have investigated superplastic behavior in a fine-grained aluminum solid solution alloy with a thermally unstable microstructure. To obtain fine-grained microstructure, friction stir processing (FSP) was applied to a commercial 5083 aluminum (Al−Mg) alloy. An equiaxial fine-grained microstructure with a grain size of 7.4 μm was obtained after FSP; however, this microstructure was unstable at high temperatures. Generally, for fine-grained superplasticity or GBS to occur or continue, the fine-grained microstructure must be smaller than 10 μm during high-temperature deformation. However, a large elongation of over 200% was observed at high temperatures despite the occurrence of grain growth. From microstructural observations, it was determined that a fine-grained microstructure is maintained in the early stage of deformation, but at strain levels greater than 100%, trans-granular deformation occurs. The microstructural feature of this trans-granular deformation is similar to the deformation microstructure of solute drag creep observed in “Class I” solid solution alloys. This indicates that a change in the deformation mechanism from GBS to solute drag creep takes place during high-temperature deformation. Here, based on our observations on our model system, which is a thermally unstable aluminum solid solution alloy, we discuss the possibility of a superplastic elongation occurring by means of a transition of the deformation mechanism.

EFFECT OF ANNEALING AT DIFFERENT TEMPERATURES ON THE STRUCTURE AND HARDNESS OF AL85Y8NI5CO2 ALLOY AMORPHOUS STRIPS

Aluminum-based metallic glasses are the new promising family of materials. However, the effect of heat treatment on the structure and properties of Al–Y–Ni–Co amorphous alloys has not been widely studied so far. In this paper, Al85Y8Ni5Co2 amorphous alloy strips were obtained by hardening on a rotary copper wheel. The effect of vacuum annealing at temperatures ranging from 100 to 500 °C for 30 minutes on the structure and hardness of these strips was investigated. Transmission electron microscopy, X-ray diffraction analysis, and differential scanning calorimetry were used to study changes in the structure of strips after heat treatment. Vickers microhardness was measured to investigate the effect of annealing on the mechanical properties of strips. The results obtained allowed for the conclusions made about changes in hardness depending on the Al85Y8Ni5Co2 alloy strip structure. It was found that as the temperature rises, strip microhardness increases reaching a maximum value of 575±7 HV after annealing at 350 °C, then it decreases with a further increase in the annealing temperature. It was shown that the Al85Y8Ni5Co2 alloy strips remain completely amorphous and no crystalline phases are detected in their structures after annealing at temperatures up to 250 °C for 30 minutes. A sharp increase in hardness after annealing at 350 °C is associated with 10–30 nm nanocrystals of an aluminum solid solution formed in the amorphous matrix and surrounded by a residual amorphous matrix, while further hardness decrease is associated with the increasing sizes of these crystals and Al3Y and Al19Ni5Y3 intermetallics formed in the structure.

INVESTIGATION OF POSSIBILITY TO PRODUCE HIGH-STRENGTH BORON ALUMINUM SHEETS WITHOUT HOMOGENIZATION AND QUENCHING OPERATIONS

Al–Cu–Mn (Zr) alloys feature high strength and processability without any thermal treatment operations. Al–2%Cu–1,5%Mn–2%B and Al–2%Cu–1,5%Mn–0,4%Zr–2%B alloys were obtained in order to investigate the possibility of producing a aluminum boroncontaining alloy in the form of high-strength sheet rolled stock without thermal treatment. Melting was performed in the RELTEK induction furnace with intense melt stirring to eliminate sedimentation of boride refractory particles. Melting temperature was 950– 1000 °С. Melt was poured into 40×120×200 mm graphite casting molds. Calculation methods (Thermo-Calc) were used to demonstrate that manganese forms complex borides with aluminum and zirconium at a melting temperature while there is enough manganese in liquid and there is practically no zirconium left. Experimental methods (electronic scanning microscopy and electron microprobe analysis) proved the formation of the complex AlB2Mn2 boride, however, manganese remained in a solid solution is enough to form the Al20Cu2Mn3 phase particles in the amount up to 7 wt.%. In the alloy with zirconium, boron stimulates primary Al3Zr crystal separation and, therefore, zirconium content left in the aluminum solid solution is not sufficient for hardening. It is shown that it is possible to produce thin-rolled steel with a thickness of less than 0,3 mm with uniformly distributed clusters of the boride phase with a particle size of less than 10 μm. A high level of strength up to 543 MPa is reached without the use of hardening and aging due to the precipitation of Al20Cu2Mn3 phase dispersions during hot deformation (t =450 °C).