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Age hardnening


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age hardening of Al-Cu alloys

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Age hardnening

  1. 1. 1 Abstract: This report contains a brief introduction about an important heat treatment technique for aluminum-copper alloy called “precipitate hardening”, objective of this technique, different types of aging, how to perform this treatment and the results obtained. Objective: To study the effect of Age Hardening in Al-Cu alloy Material and Equipment:  Al-Cu alloy (4% CU)  Heating furnace  Quenching medium (water)  Rockwell Hardness Testing machine Introduction: The term “heat treating” for aluminum alloys is frequently restricted to the specific operations employed to increase their strength and hardness. These usually are referred to as the “heat-treatable” alloys to distinguish them from those alloys in which no significant strengthening can be achieved by heating and cooling. The process is called “Precipitate hardening or age hardening”. Heat treatment to increase strength of aluminum alloys is a three-step process:  Solution heat treatment: dissolution of soluble phases  Quenching: development of supersaturation  Age hardening: precipitation of solute atoms either at room temperature (natural aging) or elevated temperature (artificial aging or precipitation heat treatment).
  2. 2. 2 The mayor aluminum alloy systems with precipitation hardening include:  Aluminum-copper systems with strengthening from CuAl2  Aluminum-copper-magnesium systems (magnesium intensifies precipitation)  Aluminum-magnesium-silicon systems with strengthening from Mg2Si  Aluminum-zinc-magnesium systems with strengthening from MgZn2 Solution Heat Treating: To take advantage of the precipitation hardening reaction, it is necessary first to produce a solid solution. The process by which this is accomplished is called solution heat treating and its objective is to take into solid solution the maximum practical amounts of the soluble hardening elements in the alloy. The process consists of soaking the alloy at a temperature sufficiently high and for a time long enough to achieve a nearly homogeneous solid solution. The equilibrium solid solubility of copper in aluminum increases as temperature increases--from about 0.20% at 250 °C (480 °F) to a maximum of 5.65% at the eutectic melting temperature of 548 °C (1018 °F). The time requirement can vary from less than a minute for thin sheet to as much as 20 h for large sand or plaster-mold castings. Fig 1. Aluminum-Copper Binary Phase Diagram
  3. 3. 3 Quenching: Quenching is in many ways the most critical step in the sequence of heat- treating operations. The objective of quenching is to preserve the solid solution formed at the solution heat-treating temperature, by rapidly cooling to some lower temperature, usually near room temperature; to produce supersaturated solution at room temperature - the optimum condition for precipitation hardening. Most frequently, parts are quenched by immersion in cold water, or in continuous heat treating of sheet, plate, or extrusions in primary fabricating mills, by progressive flooding or high-velocity spraying with cold water. Age Hardening: After solution treatment and quenching, hardening is achieved either at room temperature (natural aging) or with a precipitation heat treatment (artificial aging). In some alloys, sufficient precipitation occurs in a few days at room temperature to yield stable products with properties that are adequate for many applications. These alloys sometimes are precipitation heat treated to provide increased strength and hardness in wrought or cast products. Other alloys with slow precipitations reactions at room temperature are always precipitation heat treated before being used. Natural Aging: The more highly alloyed members of the 6xxx wrought series, the copper- containing alloys of the 7xxx group, and all of the 2xxx alloys are almost always solution heat treated and quenched. For some of these alloys, particularly the 2xxx alloys, the precipitation hardening that results from natural aging alone produces useful tempers (T3 and T4 types) that are characterized by high ratios of tensile to yield strength and high fracture toughness and resistance to fatigue. For the alloys that are used in these tempers, the relatively high supersaturation of atoms and vacancies retained by rapid quenching causes rapid formation of GP zones (solute rich microstructural domains) and strength increases rapidly, attaining nearly maximum stable values in four or five days.
  4. 4. 4 Precipitation Heat Treatment: Generally are low-temperature, long-term processes. Temperatures range from 115 to 190°C; times vary from 5 to 48 h. Choice of time- temperature cycles for precipitation heat treatment should receive careful consideration. Larger particles of precipitate result from longer times and higher temperatures. Precipitation heat treatment following solution heat treatment and quenching produces T6- and T7-type tempers. Alloys in T6-type tempers generally have the highest strengths. Precipitation heat treatment following solution heat treatment and quenching produces T6- and T7-type tempers. Alloys in T6-type tempers generally have the highest strengths. Procedure:  First of all, given 6 sample of Al-Cu alloy were solution heat treated at 550°C for 1h  After that, all the samples were quenched in water bath  After quenching, 2 samples were aged at 150°C, other 2 samples were aged at 190°C and remaining 2 samples were aged at 230°C for 24h  After cooling, their hardness were measured Observations and Calculations: Specimen No. Solution Treating Temp. (°C) Solution Treating Time (h) Aging Temp. (°C) Aging Time (h) HRc 1 550 1 150 24 54 2 550 1 150 24 65 3 550 1 190 24 71 4 550 1 190 24 82 5 550 1 230 24 61 6 550 1 230 24 68
  5. 5. 5 Fig 1. Graph representing relation b/w aging temperature and hardness References:  ASM Handbook Volume 4  “Introduction to Physical Metallurgy” by Sydney H. Avner  “Heat Treatment Principles And Techniques” by T.V Rajan, C.P Sharma and Ashok Sharma 0 10 20 30 40 50 60 70 80 90 0 50 100 150 200 250 HRc Aging Temperature Aging Temperature Vs Hardness