Fig 1. Vacuum annealing of copper heat sink blanks
Different combinations of properties can be produced by varying the heat treatment of copper and its alloys—influencing strength, hardness, ductility, conductivity, impact resistance, and inelasticity.
Types of Heat Treatment
Common heat treatments applied to copper and its alloys are:
- Homogenizing to reduce chemical segregation and coring of cast structures, and create a more uniform structure in hot worked materials
- Annealing to soften work hardened (strain-hardened) materials
- Stress relief to stabilize properties and improve strength and dimensions particularly for cold worked parts, and to reduce residual stress
- Solution treating and precipitation (age) hardening to provide increased strength by precipitation of constituents from solid solution
- Quenching hardening by a martensitic-like transformation followed by tempering
Copper and copper alloys are supplied in the solution treated condition, in the solution treated and cold worked condition, and in the age-hardened condition. Their heat treatment falls into two general categories: hardening either by low temperature precipitation treatments or hardening by quenching from elevated temperature.
Hardening Mechanisms—Low Temperatures
Solution treating followed by precipitation hardening is applied to strengthen special types of copper alloys above the levels ordinarily obtained by cold working. All precipitation hardened copper alloys have similar metallurgical characteristics: they can be solution treated to a soft condition by high temperature heating and quenching and then precipitation hardened at a moderate temperature for a relatively short time (around 3 hours) since hardness reaches a peak then decreases with time. Copper alloys cannot be hardened by natural aging as aluminum alloys can.
The main advantages of these alloys are:
- Ease of fabrication in the soft solution annealed condition
- Precipitation hardening (usually) in air
- Wide variety of mechanical properties achievable;
Example of precipitation hardening copper alloys include beryllium copper (some of which contain nickel, cobalt or chromium), copper chromium, copper zirconium, copper-nickel-silicon and copper-nickel-phosphorous alloys.
What differentiates these alloys is that their hardening mechanism does not result in precipitation but results from a miscibility difference in the solid solution resulting in ultra fine chemical segregation of the alpha crystalline phase. Thus spinodal alloys exhibit excellent dimensional stability after hardening. Common spinodal hardening alloys include copper-nickel alloys with additions of chromium or tin.
When short range ordering of dissolved alloying elements occurs within the copper matrix, the alloys are called order hardening. These alloys nearly saturated with an alloying element dissolved in the alpha phase after significant cold working under respond by ordering when annealed at relatively low temperatures. Order hardening alloys typically exhibit improved stress relaxation, and the process is usually performed after the final fabrication step. Examples of order hardening alloys include some silicon and aluminum bronzes with silicon and copper-zinc-aluminum-cobalt alloys.
Hardening Mechanisms—High Temperatures
Transformation hardening mechanisms resulting from quenching at high temperatures induces internal changes producing harder, stronger phases. Tempering such structures improves toughness and reduces hardness in a similar manner to that of alloy steels. Quench hardening alloys include aluminum bronzes, nickel-aluminum bronzes, and a few copper zinc alloys.
Beryllium Copper Alloys
Heat treating is the key to the versatility in the beryllium copper alloy system, which varies considerably with form, alloy, and desired temper and is applicable to all beryllium copper cast and wrought alloys. These alloys are used when good electrical and thermal conductivities and high strength is required and are classified either as high conductivity or high strength alloys.
Heat treating of beryllium copper is a two-step process, which consists of solution annealing and age hardening (Table 2). Solution annealing is often done by the mill and involves a high temperature soak at 1450˚F (790˚C) for the high strength alloys and 1650˚F (900˚C) for the high conductivity alloys. Excess time or temperatures may cause undesirable grain growth. Solution annealing is followed by water quenching.
The age or precipitation hardening results from the precipitation of a beryllium containing phase from a supersaturated solid solution of mostly pure copper. The precipitation occurs during the slow cooling of the alloys because the solubility of beryllium in alpha copper decreases with decreasing temperature. Typically the alloys are rapidly cooled from the annealing treatment, so the beryllium remains in solid solution with the copper. Then the alloy is given a precipitation or age hardening treatment for an hour or more at a temperature between 400˚F (200˚C) and 860˚F (460˚C). Upon tempering, the beryllium containing phases, called beryllides, precipitate out of solution.
Common aging conditions include:
- Underaging—producing strength and hardness levels less than optimally attainable by using lower temperatures, less time, or a combination of the two, but can tailor a specific property or combination of properties. Underaging is used when better formability is needed rather than maximum strength.
- Peak Aging—producing the highest strength, hardness, and the highest electrical and thermal conductivity by using the optimal temperature and time combination.
- Overaging—producing strength and hardness properties less than optimally attainable by using a higher temperature, longer time at temperature, or both, but in different combinations. Overaging is used where greater toughness and/or impact strength is more important than absolute strength.
Precipitation hardening is cumulative. Multiple thermal treatments may be applied except for overaged material whose properties are beyond peak.
Unique Application—Shape Memory Alloys
A shape memory alloy (SMA) has the ability to return to some previously defined shape or size. These materials can be plastically deformed at relatively low temperature and upon exposure to some higher temperature, return to their original shape. Copper-based SMA’s with commercial interest include CuZnAl and CuAlNi alloys.
Copper SMA’s are metastable and require solution heat treatment in the parent (beta) phase region and subsequent controlled cooling to retain beta phase for shape memory effects. Prolonged solution heat treatment results in undesirable zinc evaporation and grain growth. Water quenching is widely used, but air cooling also may be sufficient for high aluminum content alloys. Post quench aging is needed to establish stable transformation temperatures. The thermal stability of these SMA’s requires avoidance of prolonged exposure above 300˚F (150˚C) for CuZnAl and 390˚F (200˚C) for CuAlNi alloys.
Dan Herring is president of THE HERRING GROUP Inc., which specializes in consulting services (heat treatment and metallurgy) and technical services (industrial education/training and process/equipment assistance. He is also a research associate professor at the Illinois Institute of Technology/Thermal Processing Technology Center.
- ASM Handbook, Volume 4, 1991, p 886
- Tech Briefs, Brush Wellman Inc. (www.brushwellman.com)
- Hodgson, Darel E., Wu, Ming H., and Biermann, Robert J., Shape Memory Alloys (www.immedical.com)