ISSN:
1662-9752
Source:
Scientific.Net: Materials Science & Technology / Trans Tech Publications Archiv 1984-2008
Topics:
Mechanical Engineering, Materials Science, Production Engineering, Mining and Metallurgy, Traffic Engineering, Precision Mechanics
Notes:
Age hardening is one of the most important processes to strengthen aluminium alloys. Itusually consists of the steps solution annealing, quenching and aging. For heat treatmentsimulations as well as for the appropriate choice of quenching processes in heat treatment shops,knowledge of the temperature- and time-dependent precipitation behaviour during continuouscooling is required. Quenching should happen as fast as necessary to reach high strengths, but alsoas slow as possible, to reduce residual stresses and distortion. This optimal quenching rate of analuminium component depends on its chemical composition, initial microstructure and solutionannealing parameters as well as on its dimensions. Unfortunately continuous cooling transformation(CCT) diagrams of aluminium alloys do almost not exist. Instead isothermal transformation (IT)diagrams or given average quenching rates are used to estimate quenching processes, but they arenot satisfying neither for heat treatment simulations nor for heat treatment shops.Thermal analysis, especially Differential Scanning Calorimetry (DSC) provides an approach forCCT-diagrams of aluminium alloys, if the relevant quenching rates can be realized in the DSCequipment.The aluminium alloy Al-4.5Zn-1Mg (7020) is known for its relatively low quenchingsensitivity as well as for its technical importance. The complete CCT-diagram of 7020 with coolingrates from a few K/min to some 100 K/min has been recorded. Samples have been solutionannealed and quenched with different cooling rates in a high speed DSC. The resulting precipitationheat peaks during cooling have been evaluated for temperature and time of precipitation start, aswell as their areas as a measure for the precipitate amount. Quenched samples have been furtherinvestigated regarding their microstructure by light and electron microscopy, hardness after agingand precipitation behaviour during re-heating in DSC. The CCT-diagram correlated very well withthe microstructure, hardness and re-heating results. A critical cooling rate with no detectableprecipitation during continuous cooling 155 K/min could be determined for 7020. A model tointegrate the CCT-diagram in heat treatment simulation of aluminium alloys is under development
Type of Medium:
Electronic Resource
URL:
http://www.tib-hannover.de/fulltexts/2011/0528/02/13/transtech_doi~10.4028%252Fwww.scientific.net%252FMSF.519-521.1467.pdf
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