Aluminum Alloys – Effects of Alloying Elements

---

Introduction

The properties and behavior of aluminum alloys are significantly influenced by both alloying elements and impurities. While many effects are well-documented, some impacts, particularly regarding impurities, may be specific to particular alloys or conditions.

Major Alloying Systems

Copper-Based Systems

Copper forms a crucial alloying element in aluminum, with concentrations ranging from 2 to 10%. Key characteristics include:

• Response to solution heat treatment and aging
• Maximum strengthening between 4-6% Cu
• Enhanced properties when combined with magnesium

Copper-Magnesium combinations provide:

• Increased strength after solution heat treatment and quenching
• Higher yield strength with artificial aging
• Improved dimensional stability when iron is present
• Reduced tensile properties if iron exceeds 0.5% with insufficient silicon

Magnesium-Based Systems

Magnesium serves as the major alloying element in the 5xxx series alloys. Its maximum solid solubility in aluminum is 17.4%, though current wrought alloys don't exceed 5.5%. Magnesium addition significantly increases aluminum's strength without unduly decreasing ductility, while maintaining good corrosion resistance and weldability.

Magnesium-Manganese combinations in wrought alloys provide:

• High strength in work-hardened condition
• Excellent corrosion resistance
• Good welding characteristics

However, increasing amounts of either element can complicate fabrication and increase tendency toward cracking during hot rolling, particularly with trace sodium present.

Magnesium-Silicon Systems

The 6xxx group wrought alloys contain up to 1.5% each of magnesium and silicon in approximately 1.73:1 ratio to form Mg2Si. These alloys feature:

• Maximum Mg2Si solubility of 1.85%, decreasing with temperature
• Formation of Guinier-Preston zones during aging
• Moderate strength increase compared to 2xxx or 7xxx alloys

Zinc-Based Systems

Zinc-containing aluminum alloys offer the highest combination of tensile properties in wrought aluminum alloys, despite historical challenges with hot cracking and stress-corrosion cracking.

Zinc-Magnesium combinations are particularly effective:

• Optimal strength development in the range of 3-7.5% Zn
• Formation of MgZn2, providing enhanced heat treatment response
• Increased tensile and yield strengths with MgZn2 concentration from 0.5 to 12%
• Further strength improvements with excess magnesium beyond MgZn2 stoichiometry

Zinc-Magnesium-Copper systems produce the highest-strength commercial aluminum alloys, featuring:

• Enhanced aging rate through increased supersaturation
• Improved stress corrosion resistance
• Reduced general corrosion resistance
• Critical influence of minor additions (chromium, zirconium) on properties

Major Alloying Systems

Manganese

Present as a common impurity (5-50 ppm), manganese:

• Decreases electrical resistivity
• Increases strength through solid solution or fine precipitation
• Maintains good corrosion resistance
• Exhibits limited solid solubility but remains in solution when chill cast

Chromium

Occurring as a minor impurity (5-50 ppm), chromium significantly affects:

• Electrical resistivity
• Grain structure control
• Recrystallization prevention in specific alloys
• Formation of fine dispersed phases

Nickel

Nickel's solid solubility in aluminum doesn't exceed 0.04%. Beyond this limit, it forms insoluble intermetallics, usually combining with iron. Its effects include:

• Increased strength in high-purity aluminum (up to 2% addition)
• Improved hardness and strength at elevated temperatures
• Reduced coefficient of expansion
• Decreased ductility

Boron

Boron functions as both a grain refiner and conductivity improver. Its characteristics include:

• Effective use range of 0.005 to 0.1% for grain refinement
• Enhanced effectiveness when combined with titanium (5:1 Ti:B ratio)
• Ability to precipitate vanadium, titanium, chromium, and molybdenum

Zirconium

Added in concentrations of 0.1 to 0.3%, zirconium:

• Forms fine intermetallic precipitates
• Inhibits recovery and recrystallization
• Controls grain structure in wrought products
• Increases recrystallization temperature

Minor Elements and Impurities

Common Impurities

Iron

As the most common impurity in aluminum, iron exhibits:

• High solubility in molten aluminum
• Very low solid-state solubility (~0.04%)
• Formation of intermetallic second phases
• Combination with aluminum and other elements

Silicon

Second highest impurity in commercial aluminum (0.01 to 0.15%), silicon:

• Combines with magnesium to form Mg2Si in 6xxx series
• Influences heat treatment response
• Affects mechanical properties

Trace Elements with Significant Effects

Antimony

• Present in trace amounts (0.01 to 0.1 ppm)
• Very low solid solubility (<0.01%)
• Used in bearing alloys (4-6%)
• Counteracts hot cracking in aluminum-magnesium alloys

Beryllium

• Reduces oxidation at elevated temperatures in magnesium-containing alloys
• Improves adhesion of aluminum film in aluminizing baths (up to 0.1%)
• Restricts formation of iron-aluminum complexes

Bismuth

Used in free-machining alloys, bismuth:

• Has restricted solubility in solid aluminum
• Forms soft, low-melting phases
• Compensates for lead shrinkage
• Used in 1:1 ratio with lead in alloys 2011 and 6262
• Can counteract sodium effects on hot cracking (20-200 ppm)

Specialized Applications

Gallium

• Typically present at 0.001 to 0.02%
• Minimal effect on mechanical properties at normal levels
• Influences corrosion characteristics at 0.2% level
• Affects etching and brightening responses

Lithium

• Affects aluminum foil discoloration at <5 ppm
• Increases oxidation rate of molten aluminum
• Alters surface characteristics of wrought products

Process-Specific Elements

Cadmium

In aluminum alloys, cadmium:

• Accelerates age hardening in aluminum-copper alloys (up to 0.3%)
• Increases strength and corrosion resistance
• Reduces aging time in aluminum-zinc-magnesium alloys (0.005 to 0.5%)

Calcium

With very low solubility in aluminum, calcium:

• Forms intermetallic CaAl4
• Creates superplastic properties when combined with 5% Zn
• Forms nearly insoluble CaSi2
• Decreases age hardening in aluminum-magnesium-silicon alloys
• Increases strength but decreases elongation in aluminum-silicon alloys

Carbon

Appears infrequently as an impurity:

• Forms oxycarbides and carbides (primarily Al4C3)
• May form carbides with other impurities like titanium
• Can lead to surface pitting due to Al4C3 decomposition in water

Rare and Specialized Elements

Cerium

• Added experimentally as mischmetal (50-60% Ce)
• Increases casting fluidity
• Reduces die sticking

Cobalt

Though not common, cobalt:

• Transforms acicular β (aluminum-iron-silicon) into rounded aluminum-cobalt-iron phase
• Improves strength and elongation in aluminum-silicon alloys
• Used in powder metallurgy for aluminum-zinc-magnesium-copper alloys (0.2 to 1.9%)

Indium

• Significantly influences age hardening of aluminum-copper alloys (0.05 to 0.2%)
• Particularly effective with low copper contents (2-3%)

Environmental and Processing Considerations

Hydrogen

Critical in processing due to:

• Higher liquid-state solubility at melting point
• Formation of gas porosity during solidification
• Causing secondary porosity and blistering
• High-temperature deterioration during heat treating
• Potential role in stress-corrosion cracking
• Control through fluxing or vacuum degassing

Mercury

• Used at 0.05% level in sacrificial anodes
• Causes rapid corrosion in most aluminum alloys when present as metal or salt

Phosphorus

• Present as minor impurity (1-10 ppm)
• Very low molten aluminum solubility (~0.01% at 660°C)
• Even lower solid-state solubility

Vanadium

• Present at 10-200 ppm in commercial-grade aluminum
• Typically precipitated from electrical conductor alloys using boron
• Affects conductivity

Advanced Alloying Interactions and Special Considerations

Molybdenum and Niobium Effects

Molybdenum, present in trace amounts (0.1 to 1.0 ppm), historically served as a grain refiner at 0.3% concentration. While it can modify iron constituents, its current industrial use is limited. Similarly, niobium's potential for grain refinement through peritectic reaction has been explored but shows limited effectiveness compared to modern refiners.

Lead and Processing Optimization

Lead's role extends beyond its presence as a trace element in commercial-purity aluminum. When added at 0.5% with bismuth in specific alloys (2011 and 6262), it creates optimal machining conditions through:

• Formation of soft, low-melting phases
• Enhanced chip breaking characteristics
• Improved tool lubrication properties

Complex System Behaviors

The zinc-magnesium-copper system exemplifies how multiple elements interact to create superior properties. This system achieves the highest commercial strength through:

• Controlled aging processes via zinc and magnesium
• Enhanced supersaturation through copper additions
• Formation of specific intermetallic phases
• Balanced corrosion resistance properties

Conclusion: Understanding Aluminum Alloying Complexity

The effectiveness of aluminum alloys stems from the intricate balance of multiple elements and their interactions. Several key principles emerge from this comprehensive review:

Property Development

• Major elements (Cu, Mg, Zn) establish basic strength and performance
• Secondary additions fine-tune specific properties
• Trace elements significantly impact processing and final characteristics

Processing Considerations

• Element interactions affect heat treatment response
• Formation of intermetallic compounds influences properties
• Processing parameters must account for element combinations

Performance Optimization

• Balanced compositions achieve desired property combinations
• Understanding element interactions enables targeted improvements
• Control of impurities remains crucial for consistent performance