The structural material for low temperature
Aluminum alloys are very important class of structural metals for use in the negative and even cryogenic temperatures. They are used in the items to work at such low temperatures like -270 ° C. At low temperatures, most aluminum alloys exhibit only slight changes its strength properties.
Tensile strength (temporary resistance) and yield stress aluminum alloys with temperature decrease may only slightly decrease, and the impact strength remains approximately constant. therefore aluminum It is a useful material for many low temperature applications.
Aluminum or Steel?
The main deterrent wide use of aluminum alloys in cryogenic technology is their rather low elongation compared to some austenitic-ferritic steels. Therefore, in critical applications with high demands on security, apply all the same, these steels.
A good example of the use of aluminum alloys at low temperatures is the production of pressure vessels, which operate in the temperature range of from -195 by 65 ° C. There is used aluminum alloys 5083 and 5456. In these alloys in the range from room temperature to -195 ° C, the tensile strength increases by 30-40 %, yield point - on 5-10 % and elongation - at 60-100 %.
Durability at low temperatures
Aluminum and its alloys, Unlike steel, They do not have the cold brittleness threshold. in front of, their strength increases with a decrease in temperature. Prolonged exposure at low temperatures does not influence the level of strength properties of the thermally stable aluminum alloys reinforced directly at these low temperatures, and upon return to room temperature.
Svezhezakalennye thermally hardenable alloys can be maintained in this state for a long time, when stored at low temperature to delay the aging process. Aircraft rivets made of AlCuMgSi alloys (duralumin) are heated for quenching at 495 ° C for 5 by 60 minutes depending on the size and number of rivets, after which they were quenched in cold water. At room temperature the rivet remain plastic for two hours, -5 ° C during this condition persists 45 hours, and at -15 ° C - 150 hours!
The increase of strength characteristics of aluminum alloys the temperature decreases almost not noticeable to -15 ° C, but it begins to increase significantly below -100 ° C. The figure shows the behavior of alloy 6061 in the temperature range of from -250 ° C to room temperature.
Relative extension most aluminum alloys also increases with decreasing temperature down to -196 ° C. Some Al-Mg alloys with a Mg content 4,5 % with reduced elongation first increased temperature, and then decreases again, but still remains above the, than at room temperature.
Deformed aluminum alloys, It is most often used for operation at low temperatures, are alloys 1100, 2014, 2024, 2090, 2219, 3003, 5083, 5456, 6061, 7005, 7039 and 7075. Their cryogenic properties and applications are considered separately: The use of aluminum alloys at low temperatures.