Oddvin Reiso

The Effect of Intermediate Storage Temperature and Time on the Age Hardening Response of Al-Mg-Si Alloys

Specimens of three Al-Mg-Si alloys, 6060, 6005 and 6082, were solution heat treated, stored at different temperatures for different time, and artificially aged. Properties were measured before and after artificial ageing. The natural ageing response of the alloys is dependent on the storage temperature. Decreasing storage temperature leads to a delayed onset of natural ageing, but also to a higher strength after prolonged ageing, particularly for lean alloys such as 6060. The temperature and time of intermediate storage between solution heat treatment and artificial ageing has a significant effect on the strength of the artificially aged material. For the 6005 and 6082 alloys the processes that take place during natural ageing lead to a reduced strength after artificial ageing.

Dissolution and melting of secondary AI2Cu phase particles in an AlCu alloy

When up-quenching an Al-4.2 wt pct Cu alloy which has been equilibrated at 450 °C to a temperature at or above the eutectic temperature of 547 °C, liquid drops are found to form. Inside the matrix grains they have a globular shape, while they have a lenticular shape with a low dihedral angle when formed on grain boundaries. It is demonstrated metallographically that drops are formed by the melting of the Al2Cu phase particles together with the surroundingα matrix to form a liquid of chemical composition around the eutectic composition. On prolonged annealing, the drops are dissolved in theα matrix. The kinetics of this dissolution reaction of the drops, as well as the dissolution of Al2Cu phase particles at a temperature below 547 °C, is studied in some detail and compared with a simple mathematical model. Also, the thermodynamics of the melting and dissolution reactions are discussed by means of a free-energy diagram.

Improving Thermal Stability in Cu-Containing Al-Mg-Si Alloys by Precipitate Optimization

It is demonstrated that good thermal stability in Al-Mg-Si-Cu aluminum alloys correlates with a high density of fine lath-shaped, Cu-containing, disordered L-precipitates. Alloys optimized for L retained hardness above 90xa0HV after 3xa0weeks over-aging at 473xa0K (200xa0°C). Further improvement was achieved by substituting Si by Ge in one alloy. High-angle annular dark-field scanning transmission electron microscopy showed that at peak-hardness conditions, L coexists with more common needle-shaped precipitates, often with Cu-enriched interfaces.

Melting of secondary-phase particles in Al-Mg-Si alloys

The melting of secondary-phase particles—or, more precisely, the melting of such particles together with the surrounding matrix—in two ternary Al-Mg-Si alloys has been studied. In the quasi-binary Al-Mg2Si alloy, one melting reaction is found. In the alloy with an Si content in excess of that necessary to form Mg2Si, three different melting reactions are observed. At upquenching temperatures above the eutectic temperature, the reaction rates are very high, and it is assumed that they are controlled by diffusion of the alloying elements in the liquid. Melting is also observed after prolonged annealing at temperatures below the eutectic temperature in these alloys, which is explained by the different diffusion rates of Mg and Si. The rate of the melting reaction is in this case assumed to be controlled by diffusion of the alloying elements in the solid α-Al phase. It is shown that calculation of the particle/matrix interface composition, which determines when melting is possible, cannot be made solely on the basis of the phase diagram, but must also include the rate of diffusion of Mg and Si. The melting temperatures observed differ somewhat from the accepted eutectic temperatures for these alloys. On prolonged annealing, the liquid droplets formed dissolve into the surrounding matrix and their chemical composition is found to change during dissolution. The resulting eutectic structure after quenching of a droplet is explained by the phase diagram and the different diffusion rates of Mg and Si as well as by the nucleation conditions of the constituents involved.

Effects of Germanium, Copper, and Silver Substitutions on Hardness and Microstructure in Lean Al-Mg-Si Alloys

It is shown that strength loss in a 6060 Al-Mg-Si alloy caused by reduction in solute can be compensated by adding back smaller quantities of Ag, Ge, and Cu. Nine alloys were investigated. Ge was found to be the most effective addition, strongly refining the precipitation. The hardness is discussed in terms of statistics of the precipitates near a T6 condition, as acquired by transmission electron microscopy (TEM). Precipitates in some conditions were also investigated by high-angle annular dark-field scanning TEM. The added elements have strong influence on the main hardening precipitate, β″, changing its structure and promoting disorder.

Novel method of screw extrusion for fabricating Al/Mg (macro-) composites from aluminum alloy 6063 and magnesium granules

A novel method of screw extrusion was used for producing a bimetal composite Al/Mg from granules containing aluminium alloy 6063 (AA6063) and commercial pure magnesium. Up to 12.5% (mass fraction) pure magnesium was added to the aluminium alloy. In general, the material consisted of a fine grained microstructure. In addition to the phases originating from the input materials, intermetallic phases were observed as islands consisting of the Al2Mg3 phase surrounded by γ-Mg17Al12, throughout the microstructure. The mechanical properties of the extruded material showed a gradual increase in strength with increasing the addition of Mg. The highest registered UTS, well above 350 MPa, was observed for the material containing 10% Mg. Examinations of the fracture surfaces indicated that increasing the magnesium content led to a higher degree of brittle fracture and a gradual change of the fracture micro-mechanisms. The optimization of the post-extrusion processing conditions is still ongoing.

Directionality and Column Arrangement Principles of Precipitates in Al-Mg-Si-(Cu) and Al-Mg-Cu Linked to Line Defect in Al

In the structures of all metastable precipitates in Al-Mg-Cu and Al-Mg-Si alloys, we find that column surrounding of an element column in the needle/lath direction order according to simple principles. Advanced transmission electron microscopy and DFT calculations support the principles originate with a line defect, which is a segment of a <100>Al column shifted to interstitial positions. We propose the defect aids solute decomposition by partitioning the FCC matrix locally into columns of fewer and higher number of nearest neighbours, which suit smaller and larger size solute atoms, respectively. The defect explains how <100> directionality of the precipitates can arise in a cluster. Ordering of a few defects leads naturally to GPB zones in Al-Mg-Cu and to β in Al-Mg-Si.

The Effect of Heating Rate on the Density and Spatial Distribution of Dispersoids during Homogenisation of 6xxx Aluminium Alloys

Two 6xxx alloys with different Mn-content have been homogenised in a furnace at 575 oC for 2 hours and 15 minutes. Three different heating rates to the homogenisation holding temperature were chosen, as this was expected to affect the precipitation behaviour of the dispersoids. The study focused on developing a reliable procedure for the characterization of the density and spatial distribution of dispersoids in aluminium alloys; both in terms of sample preparation, microscopic techniques and quantitative analyses of results. Scanning electron microscopy (SEM) has been used to evaluate the dispersoid characteristics for the different alloys and heating rates. The results indicate an increase in dispersoid number density and a more uniform distribution of dispersoids for the lowest heating rate, as compared to the more rapid heating rates, for the alloy with 0.05 wt% Mn. For the alloy with 0.15 wt% Mn the number density increased with the heating rate. This is suggested to be due to particle coarsening as an effect of the low heating rate where the samples spend longer time in the furnace.

The effect of Cu and Ge additions on strength and precipitation in a lean 6xxx aluminium alloy

It has been demonstrated that the strength loss in a lean Al-Mg-Si alloy due to solute reduction could be compensated by back-adding a lower at % of Ge and Cu. Nanosized precipitate needles which are the main cause of strength in these alloys, and material hardness has been correlated to parameters quantified by TEM. It was found that additions of Ge and Cu strongly affect the precipitation process by increasing precipitate density and reducing precipitate size. Investigations of precipitate atomic structure by HAADF-STEM indicated that they contain mixed areas of known phases and disordered regions. A hexagonal Si/Ge-network was found to be present in all precipitate cross sections.

On the Mechanism of Surface Cracking in DC Cast 7xxx and 6xxx Extrusion Ingot Alloys

When applying the Hydro variant (Hycast Gas Cushion) of the Showa Denko gas slip technology for casting extrusion ingots of 7xxx alloys surface cracks occasionally occurred. Especially one alloy with 0.3 wt.% Cu caused problems. In order to identify the problem, the casting process for these alloys was simulated by a coupled stress, thermal and fluid flow model (ALSIM/ALSPEN). The simulations were designed as a factorial trial where casting speed, ramping of the speed, casting temperature, cone height of the starting block, cooling water efficiency and primary cooling were systematically varied. The hoop stress in the surface at the temperature when 97.5% of the material was solidified was used as a crack sensitivity indicator. Three stages were identified: (I) At the start a maximum hoop stress evolved, (II) then a minimum stress occurred before (III) the stress reached a stable level. For an AA6060 alloy the stress was found to be zero in the stable stage while the AA7108 alloy experienced tension stress also during the steady state regime. Based on the factorial analysis it was found that the stable stress increased most rapidly with increasing casting speed and decreased with an increased primary cooling and a reduced melt temperature.