New High-Performance Aluminum Extrusion Alloys

By on July 31, 2019 in MATERIAL MATTERS

Two alloy families generating interest due to unique performance properties.

Automakers have relied on aluminum since the industry began. In 1899, a sports car with an aluminum body was introduced at the Berlin International Motor Show. In 1901, Carl Benz designed the first engine with aluminum parts. But it took the metal’s cost to drop after World War II for it to be inexpensive enough for mass-produced vehicles.

In 1961, the British Land Rover company came out with V-8 engine blocks made with aluminum cylinders. That was the breakthrough that led to aluminum automobile parts in wheels and transmission casings, and later cylinder heads and suspension joints.

Today, aluminum extrusions are widely used in the automotive industry. Aluminum has good extrusion characteristics and can be made in a wide variety of profiles. Extrusions are routinely produced from 1000, 2000, 3000, 5000, 6000 and 7000 series aluminum alloys.

The predominate alloy group by commercial volume is the 6000 series containing magnesium (Mg) and silicon (Si). This alloy system is used for automobile body structure, suspension and driveline components.

Ducker Worldwide estimates that the average vehicle will use about 42 pounds of aluminum extrusions by 2025. The Ford F-150 Crew Cab already uses 70 pounds of extruded product. Increased emphasis on lightweighting or special requirements has driven interest in improved extrusion alloys that are strong, have good extrusion characteristics, perform better at elevated temperatures or have improved corrosion performance.

This article highlights two recently developed alloy families that have generated interest due to a combination of unique performance properties.

HIGH-STRENGTH NICKALYN ALLOYS
These alloys have been developed around the (Al) + Al3Ni eutectic system. This eutectic forms around 6.1 weight percent nickel. The first Nickalyn alloys were produced using a basic composition of Al-6%Zn2%Mg-1%Cu-4%Ni. These compositions had ultimate tensile strengths as high as 540 MPa castings and 570 MPa in extrusions.

However, castings required high cooling rates in excess of 20 degrees Celsius/sec, which limited their use to processes such as liquid forging or squeeze casting. Extrusion processing was possible in simple shapes, but problematic in complex geometries.

It was initially assumed that Nickalyn alloys could be used only with the addition of copper (CU). This turned out not to be the case. Increasing the Mg and zinc (Zn) content to a combined level of 10 percent coupled with the removal of Cu developed equivalent strengths and decreased the corrosion susceptibility of the alloy. This variation of the alloy required very low levels of iron to avoid the formation of undesirable iron phases, and the high nickel (Ni) content added to the material cost.

As an alternative, a new alloy was designed based on the (Al) + Al9FeNi eutectic that uses less Ni, and the iron is an alloying element rather than an impurity. The composition chosen has .50 percent Ni and .40 percent iron (Fe) along with Mg and Zn with Si up to .50 percent as an allowed impurity.

Increased emphasis on lightweighting has driven interest in improved extrusion alloys that are strong, have good extrusion characteristics, perform better at elevated temperatures.

Extensive development and testing in both casting and extrusion has taken place in this alloy. Initial extrusion was done using a 52:1 extrusion ratio as shown in Figure 1. Parts were tested as extruded and with two variations of a T6 heat treatment. Those results are shown in Table 1.

Elemental mapping of the alloy phases after extrusion (prior to any heat treatment) shows the expected phases in the aluminum (Al) matrix. The light gray phases shown in Figure 2 are Al-Ni (aluminum-nickel) phases, and the dark gray phases are Al-Si phases.

A typical extrusion geometry as shown in Figure 3 was produced to better assess the extrusion characteristics of the alloy and for more extensive mechanical property testing.

Longitudinal and long transverse properties are shown in Table 2. The extrudability is equivalent to 6000 series alloy as measured by extrusion exit speed. Those comparisons are shown in Table 3.

The Nickalyn alloys offer an alternative to 6000 series alloys where higher strength is required along with good processability without needing to go to the stronger, but more difficult to process 7000 series alloys.

HIGH TEMPERATURE AL-CE ALLOYS
Al-Ce (aluminum-cerium) type alloys were initially developed as casting alloys. The casting characteristics of the binary Al-Ce systems are good to excellent depending on the additional alloying elements used. In general, production systems for melting, de-gassing and other processing of aluminum-silicon or aluminum-copper alloys can be used without modification for conventional casting of aluminum cerium alloys.

Extensive extrusion development has been done in the Al-Mg-Ce system. Extrusion improves the properties through a combination of work hardening and alignment of the intermetallic. Extrusions were produced at 300 degrees Celsius billet temperature at an extrusion ratio of 5.75 to 1 and 52 to 1 from an Al10Mg8Ce alloy.

A comparison of average casting and extrusion properties is shown in Table 4. As the extrusion ratio increases, tensile strength remains constant, with the elongation increasing and yield strength decreasing.

While these alloys do not have the extraordinary room temperature strength of Nickalyns, there are reasons these alloys should be considered for specialized applications. At long-term exposure at temperatures of 300 degrees Celsius, Al-Ce alloys retain about 80 percent of their yield strength and fully recover their baseline properties at room temperature regardless of length of exposure.

The intermetallic formed in this alloy is trapped by the zero solubility of cerium in the aluminum matrix. This trapping prevents the system from minimizing surface energy through diffusion, which blocks the alloys from traditional coarsening interactions.

The Ce also changes the intergranular corrosion behavior of aluminum alloys. Intergranular corrosion is caused by potential differences between the grain boundary region and the adjacent grain bodies. In copper-containing alloys, the anodic path is the copper-depleted zone on either side of the grain boundary, and in alloys containing magnesium, it is the anodic constituent Mg2Al3 when that constituent forms a continuous path around the grain boundary.

In the case of copper-containing alloys [Figure 4], the cerium acts as a diffusion barrier, preventing the formation of copper-depleted zones. In all alloys tested, the addition of small amounts of cerium increases their resistance to intergranular corrosion, as shown in Figure 5.

SUMMARY
There are several alloy choices that can be considered for high-performance extrusions. Nickalyn alloys are stronger than 6000 series and are easily extruded. Extrusion alloys containing Ce offer a unique combination of properties featuring very good strength at elevated temperature and superb corrosion performance. Both alloys are commercially available at a cost competitive with other high-performance alloys. Development continues in both alloy systems to meet the ever-changing requirements of the transportation industry

 

Authored by David Weiss and Joseph Demme

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