Heat Treatment of Non-Ferrous Metals PDF

HEAT TREATING OF NON FERROUS ALLOY Advanced Engineering Materials Lecture Juliana Anggono, Mechanical Engineering Department Petra Christian University - INDONESIA 1 Aluminum Aluminum is light weight, but engineers want to improve the st...

HEAT TREATING OF NON FERROUS ALLOY Advanced Engineering Materials Lecture Juliana Anggono, Mechanical Engineering Department Petra Christian University - INDONESIA 1 Aluminum Aluminum is light weight, but engineers want to improve the strength for high performance applications in automobiles and aerospace. To improve strength, they use precipitation hardening. Age-hardening heat treatment phase diagram Quenching Quenching is the second step in the process. Its purpose is to retain the dissolved alloying elements in solution for subsequent precipitation hardening. Generally the more rapid the quench the better, from a properties standpoint, but this must be balanced against the concerns of part distortion and residual stress if the quench is non-uniform. Changes in Microstructure due to quenching Precipitation Hardening The strength and hardness of some metal alloys may be improved by the formation of extremely small, uniformly dispersed particles (precipitates) of a second phase within the original phase matrix. Alloys that can be precipitation hardened or age hardened:  Copper-beryllium (Cu-Be)  Copper-tin (Cu-Sn)  Magnesium-aluminum (Mg-Al)  Aluminum-copper (Al-Cu)  High-strength aluminum alloys  Phase Diagram for Precipitation Hardened Alloy c11f40 Criteria:  Maximum solubility of 1 component in the other (M);  Solubility limit that rapidly decreases with decrease in temperature (M→N). Process:  Solution Heat Treatment – first heat treatment where all solute atoms are dissolved to form a single-phase solid solution.  Heat to T0 and dissolve B phase.  Rapidly quench to T1  Non-equilibrium state (a phase solid solution  Precipitation Heat Treatment c11f43 The supersaturated a solid  solution is usually heated to an intermediate temperature T2 within the a+b region (diffusion rates increase).  The b precipitates (PPT) begin to form as finely dispersed particles. This process is referred to as aging.  After aging at T2, the alloy is cooled to room temperature.  Strength and hardness of the alloy depend on the ppt temperature (T2) and the aging time at this temperature. Solution Heat Treatment Heat treatable aluminum alloys gain strength from subjecting the material to a sequence of processing steps called solution heat treatment, quenching, and aging. The primary goal is to create sub-micron sized particles in the aluminum matrix, called precipitates that in turn influence the material properties. While simple in concept, the process variations required (depending on alloy, product form, desired final property combinations, etc.) make it sufficiently complex that heat treating has become a professional specialty. The first step in the heat treatment process is solution heat treatment. The objective of this process step is to place the elements into solution that will eventually be called upon for precipitation hardening. Developing solution heat treatment times and temperatures has typically involved extensive trial and error, partially due to the lack of accurate process models. Aging-microstructure The supersaturated solid solution is unstable and if, left alone, the excess q will precipitate out of the a phase. This process is called aging. Types of aging: Natural aging process occurs at room temperature Artificial aging If solution heat treated, requires heating to speed up the precipitation Overaging After solution heat treatment the material is ductile, since no precipitation has occurred. Therefore, it may be worked easily. After a time the solute material precipitates and hardening develops. As the composition reaches its saturated normal state, the material reaches its maximum hardness. The precipitates, however, continue to grow. The fine precipitates disappear. They have grown larger, and as a result the tensile strength of the material decreases. This is called overaging.  Precipitation Heat Treatment c11f43  PPT behavior is represented in the diagram:  With increasing time, the hardness increases, reaching a maximum (peak), then decreasing in strength.  The reduction in strength Small solute-enriched regions in a and hardness after long solid solution where the lattice is periods is overaging identical or somewhat perturbed from (continued particle that of the solid solution are called growth). Guinier-Preston zones. Guinier-Preston (GP) zones - Tiny clusters of atoms that precipitate from the matrix in the early stages of the age-hardening process. Hardness vs. Time The hardness and tensile strength vary during aging and overaging. Influence of Precipitation Heat Treatment on Tensile Strength (TS), %EL 2014 Al Alloy: TS peak with precipitation time. %EL reaches minimum Increasing T accelerates with precipitation time. process. ita ll cip ma “ag tes ” ed ion pre ny s so. era ita ge lid uil lut 30 “ov cip lar so n-eq ge tes ma tensile strength (MPa) %EL (2 in sample) pre wer d” no 400 fe 20 300 149°C 10 200 204°C 149°C 204°C 100 0 1min 1h 1day 1mo 1yr 1min 1h 1day 1mo 1yr precipitation heat treat time precipitation heat treat time  Effects of c11f45Temperature  Characteristics of a 2014 aluminum alloy (0.9 wt% Si, 4.4 wt% Cu, 0.8 wt% Mn, 0.5 wt% Mg) at 4 different aging temperatures. Effects of Aging Temperature and Time The properties of an age-hardenable alloy depend on both aging temperature and aging time. More beneficial to use the lower temperature: a. Max strength increases as the aging temperature decreases. b. The alloy maintains its max strength over a longer period of time. 11/12/2024 14 c. The properties are more uniform. Requirement for Age Hardening 1. The phase diagram must display decreasing solid solubility with decreasing temperature. 2. The matrix should be relatively soft and ductile and the precipitate should be hard and brittle. 3. The alloy must be quenchable. 4. A coherent precipitate must form. 11/12/2024 15  Aluminum rivets  Alloys that experience significant precipitation hardening at room temp, after short periods must be quenched to and stored under refrigerated conditions.  Several aluminum alloys that are used for rivets exhibit this behavior. They are driven while still soft, then allowed to age harden at the normal room temperature. c11f44 Several stages in the formation of the equilibrium PPT (q) phase. (a) supersaturated a solid solution; (b) transition (q”) PPT phase; (c) equilibrium q phase within the a matrix phase. Precipitation Hardening Particles impede dislocation motion. 700 Ex: Al-Cu system T(°C) L CuAl2 600 Procedure: a +L +L -- Pt A: solution heat treat A 500 q (get a solid solution) a+q 400 C -- Pt B: quench to room temp. (retain a solid solution) 300 0 B 10 20 30 40 50 -- Pt C: reheat to nucleate (Al) wt% Cu composition range small q particles within available for precipitation hardening a phase. Temp. At room temperature the stable state Pt A (solution heat treat) of an aluminum-copper alloy is an aluminum-rich solid solution (α) and an intermetallic phase with a tetragonal crystal structure having Pt C (precipitate ) nominal composition CuAl2 (θ). Time Pt B PRECIPITATION STRENGTHENING Hard precipitates are difficult to shear. Ex: Ceramics in metals (SiC in Iron or Aluminum). precipitate Large shear stress needed Side View to move dislocation toward precipitate and shear it. Unslipped part of slip plane Dislocation Top View “advances” but precipitates act as “pinning” sites with S spacing S. Slipped part of slip plane 1 Result:  y ~ S 24 Aging Aging either at room or moderately elevated temperature after the quenching process is used to produce the desired final product property combinations. The underlying metallurgical phenomenon in the aging process is precipitation hardening. Due to the small size of the precipitate particles, early understanding was hampered by the lack of sufficiently powerful microscopes to actually see them. With the availability of the transmission electron microscope (TEM) with nanometer-scale resolution, researchers were able to actually image many precipitate phases and build on this knowledge to develop improved aluminum alloy products.