ALUMINUM AND HEAT TREATMENT OF ALUMINUM BASIC INFORMATION AND TUTORIALS

Aluminum is an important commercial metal possessing some very unique properties. It is very light (density about 2.7) and some of its alloys are very strong, so its strength-weight ratio makes it very attractive for aeronautical uses and other applications in which weight saving is important.

Aluminum, especially in the pure form, has very high electrical and thermal conductivities and is used as an electrical conductor in heat exchangers, etc. Aluminum has good corrosion resistance, is nontoxic, and has a pleasing silvery-white color; these properties make it attractive for applications in the food and container industry, architectural, and general structural fields.

Aluminum is very ductile and easily formed by casting and mechanical forming methods. Aluminum owes its good resistance to atmospheric corrosion to the formation of a tough, tenacious, highly insulating, thin oxide film, in spite of the fact that the metal itself is very anodic to other metals.

In moist atmospheres, this protective oxide may not form, and some caution must be taken to maintain this film protection. Although aluminum can be joined by all welding processes, this same oxide film can interfere with the formation of good bonds during both fusion and resistance welding, and\ special fluxing and cleaning must accompany welding operations.

Commercially pure aluminum (99+%) is very weak and ductile: tensile strength of 90 Mpa (13,000 lb/in2), yield strength of 34.5 MPa (5000 lb/in2), and shearing strength of 62 MPa (9500 lb/in2). Extrapure grades (electrical conductor grade) are 99.7+% pure, and are even weaker, but have better conductivity.

Heat Treatment of Aluminum Alloys.

Alloys of the 1000, 3000, and 5000 series cannot be hardened by heat treatment. They can be hardened by cold working and are available in annealed (recrystallized) and cold-worked tempers.

The 5000 series alloys are the strongest non-heat-treatable alloys and are frequently used where welding is to be employed, since welding will generally destroy the effects of hardening heat treatment. The remaining wrought alloys can be hardened by controlled precipitation of alloy phases.

The precipitation is accomplished by first heating the alloy to dissolve the alloying elements, followed by quenching to retain the alloy in supersaturation. The alloys are then “aged” to develop a controlled size and distribution of precipitate that produces the desired level of hardening. Some alloys naturally age at room temperature; others must be artificially aged at elevated temperatures.

TITANIUM AND TITANIUM ALLOYS BASIC INFORMATION AND TUTORIALS

Titanium alloys are important industrially because of their high strength-weight ratio, particularly at temperatures up to 427°C. The density of the commercial titanium alloys ranges from 4.50 to 4.85 g/cm3, or approximately 70% greater than aluminum alloy and 40% less than steel.

The purest titanium currently produced (99.9% Ti) is a soft, white metal. The mechanical strength increases rapidly, however, with an increase of the impurities present, particularly carbon, nitrogen, and oxygen.

The commercially important titanium alloys, in addition to these impurities, contain small percentages (1% to 7%) of (1) chromium and iron, (2) manganese, and (3) combinations of aluminum, chromium, iron, manganese, molybdenum, tin, or vanadium.

The thermal conductivity of the titanium alloys is low, about 15 W/m ⋅ K at 25°C, and the electrical resistivity is high, ranging from 54 mΩ ⋅ cm for the purest titanium to approximately 150 mΩ ⋅ cm for some of the alloys.

The coefficient of thermal expansion of the titanium alloys varies from 2.8 to 3.6 x 10–6 per degree Celsius, and the melting-point range is from 1371 to 1704°C for the purest titanium. The tensile modulus of elasticity varies between 100 to 120 GPa (15 to 17 # 106 lb/in2).

The mechanical properties, at room temperature, for annealed commercial alloys range approximately as follows: yield strength 760 to 965 MPa (110,000 to 140,000 lb/in2); ultimate strength 800 to 1100 MPa (116,000 to 160,000 lb/in2); elongation 5% to 18%; hardness 300 to 370 Brinell.

On the basis of the strength-weight ratio many of the titanium alloys exhibit superior short-time tensile properties as compared with many of the stainless and heat-resistant alloys up to approximately 427°C.

However, at the same stress and elevated temperature, the creep rate of the titanium alloys is generally higher than that of the heat-resistant alloys. Above about 482°C, the strength properties of titanium alloys decrease rapidly. The corrosion resistance of the titanium alloys in many media is excellent; for most purposes, it is the equivalent or superior to stainless steel.