Al-Fe Conductor Alloys: Part One

Aluminum alloys with 0.5-0.9% Fe content have largely supplanted 1350 EC alloy for building electrical circuits because the latter frequently suffered from gradual loosening at terminals, which gave rise to overheating. This problem has been completely overcome in the new conductor alloys without sacrifice of conductivity.

Aluminum 1350 Electrical Conductor alloy with a conductivity 61% of International Annealed Cu Standard has a mass resistively (0.0764 Ω for a 1 gm conductor 1 m long) which is only one half that of Cu (0.15328 Ω g/m2). The primary application is electrical distribution in buildings with potential in automobiles, aerospace, telephone lines and magnet winding.

To economically realize the weight advantage aluminum wire must be capable of attaching securely to standard fixtures without special handling techniques. However, EC wire on binding screw terminals tightened to a standard torque can become loose. When the connection heats slightly due to a minor overload, the wire expands more than the Cu-alloy fixture and creeps to relax the added stress.

On cooling it contracts to a smaller dimension, thus reducing the area of contact and allowing oxide to form at the interface. On a subsequent large current flow the overheating increases so that additional plastic flow and loosening occur to further diminish the integrity of the joint. EC wire, annealed for adequate bend ability, has a substructure softening at 200°C and consequently fails due to repetition of such cycles.

The new alloys (8000 series) of 0.5-0.9% Fe have greatly improved microstructural stability and creep resistance and are not subject to such junction failure. At 180°C, the strength of annealed from falls 125 to 116 MPa in 500 hrs and to 100 in 2000 hrs, whereas EC-AI falls rapidly to 104 and 82 MPa, respectively.

When annealed to the same ductility or bendability, the high Fe alloys are about twice as strong. This capability has been confirmed by field trials of several years in the United States, Europe and South Africa after these alloys were introduced in 1968.

More advanced alloys which not only give high integrity to terminations but are suitable for magnet wire after standard hot annealing, have been developed by additions of a third element to improve the distribution and morphology of the stabilizing particles; examples are 0.5% Fe with 0.5% Co and 0.5% Fe with 0.2-0.4% Si.

The development and patenting of such high Fe additions were remarkable since industry generally considered Fe as impurity to be limited to less than 0.4%. These alloys, with operating tensile strengths of 110-130 MPa, are not intended to complete against high strength wire for overhead lines such as 6201 alloy (0.7% Si, 0.8% Mg) in which 305-330 MPa are attained with a sacrifice in conductivity to 54% lACS.

Processing and Microstructure

In continuous casting, as typically practiced, a bar of about 50 cm² is produced at 16 m/min on a 2.5 m diameter copper wheel. The rapid solidification results in a 20 μm dendrite arm spacing and eutectic rod cpacing of about 0.2 μm with a supersaturation of about 0.1% Fe. As explained later these very fine particles play important role in stabilizing the substructure while being incapable of nucleating recrystalization.

The bar, direct from the casting wheel is reduced 98.6% to 0.7 cm² in a 13 stand rolling mill. The cast structure is broken down as the grains are elongated and reduced in thickness to about 2.5 μm. Within them, a substructure is formed which becomes progressively refined as the temperature declines from 485 to 180°C.

The eutectic colonies extended and dispersed longitudinally, are brought closer together transversely. The particles are rotated bent and fractured into shorter segments. The rod is cooled before coiling to avoid nonuniform sub grain coalescence. In continuous practice, the rod goes directly to wire drawing without intermediate annealing. As a result the hot-worked substructure, instead of statically recrystallized grains, is carried into the drawing process.

Strain hardening in hot working decreases in rate as temperature but increases as strain rate. The lower rate compared to cold working is the result of dynamic recovery, i.e. reduced accumulation of dislocations because of annihilations and combinations.

The presence of sub grains in hot worked aluminum has been known for some time but without quantitative determination of the dimensions or the effects on properties. As temperature rises from 200-450°C, the cold yield strength of the hot worked product substantially decreases from the strengthening produced by 97.5% cold rolling.

As has been observed in many hot worked metals, the yield strength is inversely proportional to sub grain diameter. Because the temperature is lower and the strain rate higher in a given pass than those in the previous one, substructure "inherited" from, i.e. carried forward from, the latter is altered by addition of dislocations to the existing walls to raise their density and by formation of new walls to subdivide the subgrains reducing their size.

Since the strength varies as the square root of the dislocation density, the strength increases by a factor of 1.3. The 0.5 Fe-0.5 Co alloy has a higher density of particles and a subgrain size of 0.85 μm with a yield stress of 140 MPa. The electrical conductivity is reduced by the dislocation substructure but, because of the dynamic recovery, not as much as for metal cold worked to the same strain. The conductivity of 0.65 Fe as rolled rod is 60.4% lACS whereas that of 0.5 Fe-0.5 Co is 59.8% IACS.