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Week 4-5Heat Treatment of Nonferrous Alloys

MME444 Heat Treatment Sessional

Prof. A.K.M.B. RashidDepartment of MMEBUET, Dhaka

2 Classes of Non-ferrous Alloys:

1. Heat Treatable AlloysSubject to age or precipitation hardening

2. Non-heat Treatable AlloysCold temper (Work hardening)Anneal temper (Annealing and recrystallisation)

Heat Treatment of Non-ferrous Alloys

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Age or Precipitation Hardening

Age hardening - A special dispersion-strengthening heat treatment. By solution treatment, quenching, and aging, a coherent precipitate forms that provides a substantial strengthening effect.

Also known as precipitation hardening, it is a form of dispersion strengthening.

A treatment used on non-optimum alloy structures to produce a uniform dispersion of fine, hard, coherent precipitates in a softer, more ductile matrix.

Requirements for Age Hardening

The alloy system must display decreasing solid solubility with decreasing temperature, i.e., the phase diagram should exhibit a change from a single solid phase to two solid phases (

The matrix should be relatively soft and ductile, and the precipitate should be hard and brittle.

The alloy must be quenchable.

A coherent precipitate must form.

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Equilibrium Cooling of Al-4% Cu Alloy

The aluminum-copper phase diagram and the microstructures that may develop

during equilibrium cooling of an Al-4% Cu alloy.

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Steps in Age or Precipitation Hardening

Step 1: Solution treatment

Heat the material at a temperature between the solvus and the eutectic to obtain the single phase structure and to dissolve all second phase particles.

Don’t exceed the eutectic temperature !!

Step 2: Quenching

Quench to room temperature fast enough to prevent the precipitate phase from forming.

This results a supersaturated non-equilibrium structure.

Step 3: Ageing

Reheat at a temperature below the solvus to form a fine dispersion of the second phase throughout the primary matrix.

This can be done either naturally (at room temperature) orartificially (at higher temperature).

The aluminum-rich end of the Al-Cu phase diagram showing the three steps in the age-hardening heat treatment and the microstructures that are produced

Microstructural Evolution in Age Hardening

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Zones, ”, ’ and are CuAl2 precipitates with different size and shape having a variable degrees of lattice distortion.

During plastic deformation, dislocation motions are impeded by the lattice distortion caused by the precipitates and, consequently, the alloy becomes harder and stronger.

During “over” ageing, lattice distortion is minimised and the alloy becomes softer and weaker.

Coherent PrecipitateWith Lattice Distortion

Non-coherent PrecipitateWithout Lattice Distortion

Microstructural Evolution in Age Hardening

Al-Cu Solid Solution

A non-coherent precipitate has no

relationship with the crystal

structure of the surrounding matrix

A coherent precipitate forms so that

there is a definite relationship

between the precipitate’s and the

matrix’s crystal structure

Microstructural Evolution in Age Hardening

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Effects of Aging Temperature and Time

The effect of aging temperature and time on the yield strength of an Al-4% Cu alloy

Design of an Age-Hardening Treatment

Suppose a Mg-10% Al alloy is responsive to an age-hardening heat treatment. Design a heat treatment for the alloy.

Portion of the aluminum-magnesium phase diagram

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Design of an Age-Hardening Treatment

Step 1: Solution-treat at a temperature between the solvus and the eutectic to avoid hot shortness. Thus, heat between 370oC and 470oC.

Step 2: Quench to room temperature fast enough to prevent the precipitate phase β from forming.

Step 3: Age at a temperature below the solvus, that is, below 370oC, to form a fine dispersion of β phase.

Portion of the aluminum-magnesium phase diagram

The operator of a furnace left for his hour lunch break without removing the Al-4% Cu alloy from the furnace used for the aging treatment.

Compare the effect on the yield strength of the extra hour of aging for the aging temperatures of 190oC and 260oC.

Example:Effect of Aging Heat Treatment Time on the

Strength of Aluminum Alloys

The effect of aging temperature and time on the

yield strength of an Al-4% Cu alloy

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Example:Effect of Aging Heat Treatment Time on the

Strength of Aluminum Alloys

At 190oC, the peak strength of 400 MPaoccurs at 2 h. After 3 h, the strength is essentially the same.

At 260oC, the peak strength of 340 MPaoccurs at 0.06 h. However, after 1 h, the strength decreases to 250 MPa .

Thus, the higher aging temperature gives lower peak strength and makes the strength more sensitive to aging time.

The effect of aging temperature and time on the

yield strength of an Al-4% Cu alloy

Microstructuralchanges that occur in age-hardened alloys during fusion welding:

(a) microstructure in the weld at the peak temperature

(b) microstructure in the weld after slowly cooling to room temperature.

Use of Age-Hardenable Alloys at High Temperatures

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Part 1: Age hardening of Al-4.5Cu Alloys

Part 2: Analyse structure and properties of heat treated samples

1. Collect your sample and solution treat it at 540 ±2C for 4 hours.

2. Quench the sample in water.

3. Artificially age the samples at the temperature and time mentioned.

1. After heat treatment, prepare a metallographic sample from your heat treated steel sample and obtain micrographs in different magnifications.

2. Measure Brinell hardness (500 kg, 10 mm) of your heat treated sample.

Week 4-5: Heat Treatment of Nonferrous Alloys

Work Schedule

Student Group

Ageing Temperature Ageing TimeEffective Ageing Time at 170 C

1As-cast sample Solution treated, no ageing (or, natural ageing) 0 h

2 160±2 C 1 h 0.5 h3

170±2 C1 h 1 h

2 2 h 2 h5 4 h 4 h6

200±2 C1 h 25 m 10 h

7 6 h 15 m 50 h8

230±2 C1 h 35 m 100 h

9 16 h 1000 h10 260±2 C 19 h 30 m 10000 h

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Any Question ?