Metallurgical Factors Affecting High Strength Aluminum Alloy Production

The major application of high strength aluminum alloys is in airframe construction. Over the last 50 years, the degree of control of their proportion has been increasing as now insights and test techniques have become available. Today, the final properties of particular interest are strength, toughness, fatigue crack growth rate and exfoliation and stress corrosion resistance. Since strength is usually a primary consideration, it will not be treated as a variable. In discussing the effect of microstructure on other properties, it may be presumed that adequate strength was achieved as a necessary prerequisite.

Only aluminum alloys from two systems, both precipitations hardenable, are serious contenders for aircraft structures: the Al-Zn-Mg-Cu (7000 series) and the Al-Cu-Mg (2000 series) systems. While there is some interest in the Al-Mg-Li system, no real benefits have been demonstrated and the potential casting problems with such a volatile alloying element as lithium are not encouraging.

Of the two major systems, 7000 series alloys are usually chosen for their high strength, while 2000 series alloys are generally selected where fatigue is a problem and for applications where higher service temperatures may be encountered. The alloys 7075 and 2024 are, by far, the most often used, and offer very useful properties. Improved versions of these alloys, such as 7475 and X2048, have been recently introduced for improved toughness. For specific applications other alloys have been developed, such as 2618 for elevated service temperatures or 2219 for weldability.

The effects of processing on stress corrosion or exfoliation performance are well known, although considerable doubt still exists as to the mechanisms involved. The most important microstructural features are grain shape and orientation with respect to the applied stress, degree of recrystallization and electrochemical differences between grain boundary precipitates and the matrix. It is worth mentioning at this point that the corrosion resistance of Al-Zn-Mg alloys containing approximately 1.5% or more Cu can usually be assured when aging is carried on to a sufficiently overaged (T73) condition. Electrical conductivity has been found to be a very good measure of degree of overaging, and is incorporated in many corrosion resistant temper specifications.

Effect of Microstructural Features on Mechanical Properties

We will first consider which microstructural features influence the most important properties, particularly toughness.

The important microstructural features, as far as toughness is concerned, are second phase particles and grain structure. The second phase particles of concern are of 3 types:

Other microstructural features, such as precipitate free zone, may be important, but often they are difficult to dissociate from other effects.

Grain orientation also plays a large part in determining toughness of alloys containing coarse particles. In a rolled product, the insoluble phases are broken up and strung out in the rolling plane. Thus, when fracture toughness is measured in the short transverse direction or, to a lesser extent, in the long transverse direction, toughness is greatly reduced due to the presence of the aligned weak particles.

The Effect of Intermediate Particles and Grain Structure. The principal role of the intermediate size Cr-, Zr- or Mn-bearing particles is to retard recrystallization and grain growth. In fact, little is known about the intermediate particles acting on their own. The most effective elements are Cr or Zr in Al-Zn-Mg alloys and Mn in Al-Cu-Mg alloys. The Cr, Zr or Mn additions remain in solution during casting and are precipitated during homogenization or other high temperature treatment. Their size and spacing, which are largely controlled by temperature and time, have important effects.

The Effect of Hardening Precipitates. In commonly used tempers, the greatest size difference in hardening precipitates occurs in 2000 series alloys between the T3 and T8 tempers. In the former temper, aging is carried out at room temperature and results in a fine distribution of GP zones. The T8 temper is achieved by aging T3 temper material at elevated temperatures, and contains coarse, partially coherent laths of S’ (metastable Al₂(CuMg)). Smaller effects of the hardening precipitates have been noted for 7000 series. Toughness decreased as the peak of the aging curve was approached, as might be expected on the basis of increasing yield strength.

The effect of age hardening precipitates on stress corrosion behavior of Al-Zn-Mg alloys is complex; but, in general, the presence of the metastable M’ hardening precipitates produced by overaging is preferable to the GP zones present in an underaged condition. The presence of the M’ particles appears to mainly improve the deformation mode and electrochemical homogeneity.

The deformation mode in the underaged condition is one of narrow slip bands and coplanar arrays of dislocations due to shearing GP zones. These result in dislocation pile-ups at grain boundaries and, consequently, high local stresses which may assist preferential fracture at the boundaries. In the overaged condition, such differences are small, and a more uniform pitting attack, rather than stress corrosion cracking, is promoted.

Copper content and quench rate after solution heat treatment play important roles in determining stress corrosion resistance. Only above about 1.0% Cu is good resistance to stress corrosion achieved in all grain orientations in an overaged condition. The effect of quench rate is markedly different for high- and low-Cu Al-Zn-Mg alloys. The stress corrosion performance of low-Cu alloys (below about 1.0 pct) benefits from as slow a quench rate as is consistent with strength requirements. However, these alloys are only resistant to stress corrosion when their grain structure is not equiaxed and when the applied stress direction is not normal to a continuous high-angle grain boundary path.

In summary, it appears that coarser particles of all types and recrystallized coarse grain structures both lead to lower toughness. The actual effects of the intermediate particles is uncertain since it appears that they influence toughness more by their effects on grain size than due to their own size per se. The effect of microstructure on fatigue is less well understood, though in the case of 2000 series alloys, the presence of coarser age hardening precipitates is undesirable. Stress corrosion and exfoliation resistance are fairly well understood. They depend primarily on the state of aging though quench rate and grain structure can also play an important role.

Control of Microstructure

Composition. Obvious gains in toughness can be obtained by reducing Fe, Si and other trace elements. Limits to such reductions are set by cost and availability of high purity materials. Alternatively, minor benefits can be derived from achieving a more uniform distribution of whatever second phase particles are present.

In a unilaxially-worked wrought product, the second phase particles break up during working and appear in the final product as rows of particles or "stringers". Such stringers tend to have their worst effect on deformation and fracture when stresses are applied normal to them, i.e. particularly in the short transverse direction.

More homogeneous properties can be obtained by not allowing these stringers to form. The reduction of Fe and Si should be treated with some caution since it may also affect the precipitation process in some not very well understood manner. It has been observed that Fe and Si can influence the nucleation of new phases or change precipitation kinetics.

More readily attainable reductions in the amount of second phase particles can be achieved by control of the major alloying elements to avoid exceeding the solubility limit. The composition limits of 2024, for example, have been fixed for years, yet approximately half its composition range lies above the solubility limit.

As detailed above, composition is important if stress corrosion resistant tempers are to be produced in 7000 series alloys. An alloy with a Cu level below about 1% will not fully respond to overaging for SCC resistance. For such an alloy, it is desirable to reduce the quench rate and to seek an unrecrystallized grain structure for improved longitudinal (L) and long transverse (LT) stress corrosion resistance. Short transverse tensile stresses in exposed surfaces must be avoided.

Quench sensitivity of 7000 series alloys can be influenced by composition. Specifically, the minor addition elements (Cr, Zr, Mn), which are added to control recrystallization, cause a loss in strength on reducing quench rate. Cr results in the greatest quench sensitivity.

In heavy plate applications, quench rate is naturally slow and it is often desirable to quench at an even slower rate to minimize residual stresses. To do this and still achieved adequate strength, Zr can be substituted for Cr since it allows the attainment of strength at much lower quench rates and yet is a very effective recrystallization retardant.

Casting. Faster freezing rates would be desirable to reduce the dendrite arm spacing and, hence, facilitate the solutionizing of soluble second phases during homogenization. Also, by faster freezing, larger amounts of the low solubility phases can be kept in solution. These will subsequently be precipitated, but as relatively fine particles. Usually, however, there is little that can be done to increase freezing rates since the ingot must be of sufficient thickness for the economical production of a wrought material and the avoidance of ingot cracking is often achieved by reduced cooling. For certain products, such as those produced by powder metallurgy or from small ingots, it may be possible to significantly alter the distribution of insoluble particles.

Homogenization. It is desirable to achieve adequate homogenization, though this is not always necessary since, to eliminate soluble second phase particles, it is the ability to return the soluble elements to solution during solution heat treatment that is important. However, good solutionizing is more readily achieved if the soluble elements have been in solution at some point during processing. For thinner products, this may not be necessary since coarse particles may be broken up during fabrication, and so can be more readily returned to solution during an intermediate anneal or during solution heat treatment. A homogenization or intermediate anneal is also necessary to precipitate Cr, Zr or Mn from solid solution. This is an important processing step since the size and distribution of these particles determines final degree of recrystallization.

Fabrication Practices. In general, while wishing to avoid hot shortness, it is desirable to keep a high working temperature. This helps to minimize recrystallization after solution heat treatment, which benefits strength and stress corrosion resistance.

A combination of quench rate and the presence of subgrains or hot working structure can influence strength. They showed that the outer layers of a press quenched extrusion, although not recrystallized, had lower properties than the core. This they attributed to the nucleation of MgZn₂ on high-angle boundaries in the outer regions during quenching. That is, the outer layers were more quench sensitive than the core.

Solution Heat Treatment and Aging. For optimum toughness, solution heat treatment should be carried out at as high a temperature as possible, though undue melting should be avoided. Either increased time or temperature can be used to increase the solutionizing of the soluble elements. Generally, after a few hours, further improvements are not accomplished. More solutionizing can generally be gained from using higher temperatures rather than longer times.

Small, coherent precipitates are the most desirable, particularly for Al-Cu-Mg alloys. If, however, over-aged tempers are necessary for good stress corrosion and exfoliation resistance, there is little choice of aging practice. Some benefits may be expected from using as low an aging temperature as possible, consistent with a reasonable aging time. For Al-Zn-Mg-Cu alloys, a two-step aging practice is desirable.