As the cornerstone of modern industry, the performance variations of metallic materials directly influence their selection for specific applications. Magnesium, zinc, titanium, and aluminium alloys-the four principal lightweight metallic materials-exhibit significant differences in core metrics such as density, strength, corrosion resistance, and workability.
I. Comparison of Fundamental Physical Properties
1. Density and Specific Strength
Magnesium alloys, with a density of 1.7–1.9 g/cm³, rank as the lightest structural metals. Their specific strength (strength/density) reaches 150–250 MPa/(g/cm³), substantially exceeding that of aluminium alloys (80–120 MPa/(g/cm³)). For instance, AZ91D magnesium alloy maintains a tensile strength of 280 MPa while weighing only 68% of 6061 aluminium alloy. Although titanium alloys possess higher density (4.5 g/cm³), their specific strength still exceeds 300 MPa/(g/cm³), achieving a 30% weight reduction in aerospace engine blades.
Zinc alloys possess a high density of 6.6–7.2 g/cm³ with a specific strength of merely 40–60 MPa/(g/cm³). However, their high specific gravity enables precision gear moulding to 0.5 mm thickness in die-casting applications-a feat unattainable with aluminium alloys (requiring 1.2 mm wall thickness).
2. Thermal-Physical Properties
Magnesium alloy thermal conductivity (156 W/(m·K)) is 23 times that of titanium alloy (6.7 W/(m·K)). In laptop cooling modules, magnesium alloy casings can reduce CPU temperatures by 8–10°C. Aluminium alloy exhibits superior thermal conductivity (237 W/(m·K)), yet magnesium alloy's lightweight advantage ensures its dominance in mobile device thermal management.
Titanium alloy retains 80% of its room-temperature strength at 500°C, whereas aluminium alloy loses 40% of its strength at 200°C. This disparity in heat resistance makes titanium alloy the material of choice for aircraft engine combustion chambers, whereas aluminium alloys are predominantly used in ambient-temperature structural components.
II. Chemical Properties and Corrosion Resistance
1. Oxidation Behaviour
Magnesium rapidly forms a 0.5–1μm thick MgO film in air, but this film is porous and fragile, exhibiting pitting corrosion within 24 hours in 3.5% NaCl solution. Micro-arc oxidation technology can generate a 20μm-thick ceramic coating on magnesium surfaces, enhancing corrosion resistance tenfold.
The naturally formed Al₂O₃ film (3-5nm) on aluminium alloy surfaces possesses self-healing properties, maintaining service life exceeding ten years in marine environments. Anodised 6061 aluminium alloy achieves a coating thickness of 25μm, with salt spray resistance surpassing 2000 hours.
The TiO₂ film (2-10 nm) formed on titanium alloy surfaces exhibits perfect passivation properties, remaining stable even in highly corrosive media such as aqua regia and concentrated sulphuric acid. The corrosion rate of industrial pure titanium in seawater is merely 0.001 mm/a, one-twentieth that of 316L stainless steel.
2. Electrochemical Corrosion
Zinc alloys are prone to intergranular corrosion in humid environments. When impurity elements (Pb, Cd) exceed 0.005% content, the corrosion rate increases threefold. Adding 0.1% Mg forms a Zn-Mg phase, significantly inhibiting electrochemical corrosion.
Magnesium alloys possess a significantly lower standard electrode potential (-2.37 V) than aluminium alloys (-1.66 V) in electrolytes, leading to galvanic corrosion at magnesium/aluminium interfaces. Implementing insulating coatings or sacrificial anode protection can control corrosion rates below 0.1 mm/a.
III. Machinability and Process Adaptability
1. Casting Properties
Magnesium alloys have a melting point (650°C) 10°C lower than aluminium alloys (660°C), yet exhibit lower viscosity, superior fluidity, and better filling capability. In die-casting production, magnesium alloy moulds achieve a service life of 200,000 cycles, twice that of aluminium alloys.
Zinc alloys possess the lowest melting point (385°C), enabling continuous production via hot-chamber die-casting machines. This yields a 40% increase in production efficiency compared to aluminium alloy cold-chamber die-casting. However, zinc alloys exhibit a higher shrinkage rate (0.6%) than magnesium alloys (0.5%), necessitating more precise mould design.
2. Deformation Processing
Aluminium alloys can achieve over 90% deformation through processes such as rolling and extrusion, with 6061 aluminium alloy in T6 condition reaching a yield strength of 290 MPa. Magnesium alloys, however, exhibit poor plastic deformation capability at room temperature due to their hexagonal close-packed (HCP) crystal structure. This necessitates the use of equal-corner angle extrusion (ECAP) to achieve an ultra-fine grain structure, increasing elongation from 8% to 25%.
Titanium alloys exhibit exceptionally high work hardening rates (n=0.4), with cutting forces 1.5 times those of steel. High-temperature forging (900–1000°C) yields β-phase microstructures, though this increases equipment energy consumption by 30%. Novel β-type titanium alloys (e.g., Ti-5553) enhance room-temperature formability by 50% through controlled β-stabilising element content.
IV. Analysis of Typical Application Scenarios
1. Aerospace Sector
Titanium alloys constitute 41% of the F-22 fighter jet's structure. Its landing gear beams, fabricated from TC4 alloy, maintain stable performance across temperatures ranging from -55°C to 600°C. Magnesium alloy AZ31B achieves 40% weight reduction in satellite mounts, though requires nickel plating to meet space environment corrosion resistance requirements.
Aluminium alloy 7075-T6 constitutes 15% of the Boeing 787 aircraft. Its wing spars, joined via friction stir welding (FSW), achieve joint strengths reaching 90% of the base material, compared to only 70% for traditional riveted structures.
2. Automotive Industry
Magnesium alloy wheels (e.g., AM60B) reduce weight by 35% compared to aluminium alloy wheels, though at double the cost. Semi-solid injection moulding (SSM) technology can lower magnesium alloy wheel production costs by 40%.
Zinc alloy die-cast components hold an 80% market share in automotive door locks. The ZA8 alloy, after T5 heat treatment, achieves a hardness of 120 HB and exhibits three times the wear resistance of aluminium alloys. However, zinc alloys suffer from poor dimensional stability at elevated temperatures (>120°C), limiting their application in engine components.
3. 3C Electronics
Magnesium alloys command a 65% market share in laptop casings. The AZ91D alloy achieves a surface hardness of 1200 HV after micro-arc oxidation, surpassing stainless steel in wear resistance. Aluminium alloy 6063 constitutes 80% of smartphone mid-frames, enabling texture processing with 0.1mm precision via nanoimprinting technology.
Titanium alloys are utilised in folding screen mobile phone hinges. The β-type Ti-3Al-2.5V alloy undergoes cold spinning, increasing its elastic modulus from 105 GPa to 120 GPa, meeting the requirement for 200,000 folding cycles.
