Selecting Controlled Expansion Alloys
Controlled Expansion Alloys
Alloys with controlled expansion characteristics are used for a broad variety of applications where thermal change of the size of the metal must be considered in a component or parts design. Generally, these controlled expansion alloys expand when heated and contract when cooled and their rate of expansion and contraction is matched with another material. The alloy composition and crystal structure together help to determine the thermal expansion behavior of each controlled expansion alloy. The rate of expansion of the alloy is expressed as an average or mean coefficient of thermal expansion of the alloy. The thermal expansion behaviour of some controlled expansion alloys is not linear. Hence, it is important to specify the upper and lower limits of the temperature when describing the thermal expansion coefficient that is needed in controlled expansion material.
In general there are three types of controlled expansion alloys:
- Low Thermal Expansion - Low thermal expansion alloys are predominantly iron-nickel alloys having very low coefficients of thermal expansion within certain specific temperature ranges. They are generally used in electronic devices, instrumentation and thermostats.
- Matching Thermal Expansion - Matching thermal expansion alloys possess approximately the same coefficients of thermal expansion as that of glasses and ceramics commonly used in the electronics and other industry applications.
- High Thermal Expansion - High thermal expansion alloys are alloys which exhibit high coefficients of thermal expansion. Higher than those for stainless steels and carbon steels and others. These alloys are primarily used in thermostat applications.
Low Expansion Alloys
Low thermal expansion alloys have very low coefficients of thermal expansion - less than 1.8 to approximately 9 x 10-6 per °C - within certain specific temperature ranges. These alloys exhibit unusual expansion behavior. This unique thermal expansion characteristic of this family of alloys is related to ferromagnetism exhibited by the alloy. Each alloy in the family exhibits very low thermal expansivity below its Curie temperature - the temperature below which it is ferromagnetic. This low thermal expansivity anomaly referred to as the "Invar Effect" is related to spontaneous volume magnetostriction where lattice distortion counteracts the normal lattice thermal expansivity. Above the Curie temperature, these alloys have a high rate of thermal expansion as they are no longer ferromagnetic.
Many theories give insight to the Invar Effect in these alloys phenomenon, the mechanism is not sufficiently understood. In addition, with certain nickel-iron compositions, it is possible to obtain a very high magnetic permeability in these alloys. Consequently, the alloys in this family are used in applications requiring high magnetic permeability such as transformers, cores, laminations for efficient motors, relays and solenoids.
All the alloys in this group are iron-nickel or iron-nickel-cobalt alloys with face-centered cubic crystal structure. As the nickel content in these iron-nickel alloys increases from 36%, thermal expansivity and Curie temperature of the alloy also increases. Curie temperature increases from 280°C for 36% nickel to greater than 510°C for 50% nickel. When selecting an alloy from this family, consideration must also be given to the useful temperature range as this might be limited by the Curie temperature of the alloy.
Low Expansion Alloy Types
The uses of low-expansion alloys are divided into two general categories.
- The first category is that in which material size change due to temperature variations must be minimized. These applications include structural components for measurement and control instruments in which excessive expansion and contraction due to temperature changes would seriously impair accuracy. Typical applications include aircraft and missile control components, laser and optical systems and wave guide tubes.
- The second category includes temperature controls utilizing a bimetallic strip. This simple type of control consists of a low expansion alloy metallurgically bonded to a high expansion alloy to produce a bimetallic element. When the strip is heated, the difference in thermal expansion rates between the two alloys causes the element to bend in curvature. This change in curvature is directly proportional to the difference in coefficient of thermal expansion and the temperature change of the strip components; and inversely proportional to the thickness of the combined components. The amount of bending is also affected by the ratio of the moduli of elasticity of the two components and by their thickness ratio.
One of the best known low expansion alloys is Alloy 36 (UNS K93601), a 36% nickel-balance iron alloy. It is used in many applications such as radio and electronic devices where dimensional changes due to temperature must be minimal, for structural members in precision optical laser measuring devices and as the low expansion side in bimetal thermostats. Alloys in this family are suitable for unique low expansion requirements. Alloy 36 (UNS K93602), with a slight increase in expansion properties, offers improved machinability for applications where high parts productivity is important. This alloy has been used for aircraft controls and a variety of electronic devices.
Any one of four other low expansion alloys may be suitable for applications requiring higher temperature ranges. Low Expansion Alloy 39 (ASTM B-753) has a useful low thermal expansivity extending to approximately 340°C. Its used for tunable capacitors and as the low expansion element in thermostat bimetallic products. Low Expansion Alloy 42 (ASTM B-753) has a virtually constant low rate of thermal expansion at temperatures up to about 380°C, while Low Expansion Alloy 45 (ASTM B-753) has a relatively constant rate of thermal expansion to about 440°C. Both alloys are used in thermostats and thermoswitches. The thermal expansivity of higher-nickel containing alloys approximates the thermal expansivity of some alumina ceramics over certain temperature ranges. The alloys in this family with the highest nickel content - Low Expansion Alloy 49 - is used for glass sealing of fiber optics.
Fabrication of Low Expansion Alloys
Machining of low expansion alloys is similar to but not as good as a Type 316 austenitic stainless steel. These alloys are readily machinable about produce gummy chips. Hence large, sharp and rigidly supported tooling is required for their machining with slower speeds. All of the alloys in this family are very ductile and can be readily cold headed and formed. Stamping from cold rolled strip of these alloys is easily accomplished. Parts may be deep drawn from properly annealed strip of these alloys. Fabrication of these alloys does add stresses which if unrelieved can change the thermal expansion behavior of the material. After severe forming, bending and machining, stresses introduced by these operations can removed by annealing for sufficient time to thoroughly heat through the section. However, the nickel-irons oxidize readily at these high temperatures hence non-oxidising atmosphere is required for annealing. When annealing cannot be done in a non-oxidising environment like vacuum, dry hydrogen or dissociated ammonia, then sufficient material must be allowed on the pieces to clean with grinding or pickling after annealing.
Matching Expansion Alloys
Matching expansion family of alloys possesses thermal expansion characteristics that are compatible with those of certain glass and ceramic materials. These specialty controlled expansion alloys are designed specifically to match the rate at which glasses and ceramics cool from various elevated temperature ranges. This characteristic of these alloys allows a lasting metal-to-glass/ceramic fusion/sealing in hermetically-sealed devices. The glass/metal interface is heated until the glass becomes molten and wet the oxide layer on the surface of the alloy. As cooling happens, it is important that the glass and metal contraction behavior is similar below the glass strain point. When there are excessive contraction differences between the materials being sealed together, the stresses that develop will cause glass breakage.
Hermetic seals have been used for years and with great success to protect vacuum tubes, transistors and semiconductors from the environment. With the evolution of the integrated circuit, these matching expansion alloys have played a critical part in sealing the silicon chip within the ceramic substrate. Alloys used for glass-to-metal seals tend to form a surface oxide which is readily wetted or chemically bonded to certain types of glasses. This property in all iron-chromium alloys and iron-nickel sealing alloys assures high quality seals of good strength and hermeticity. The metal parts are pre-oxidize in a special, controlled-atmosphere furnace before the actual glass sealing. The use of ceramic-to-metal seals for all types of electronic devices has increased substantially. These seals consist of a metallized alumina or beryllia substrate brazed to a controlled expansion alloy member. Ceramic parts are often produced with a metallized surface in order to attach the controlled expansion alloy. In the case of alumina, the moly-manganese process is widely used to metallize the surface for sealing. After plating with nickel or nickel and gold, the controlled expansion alloy parts are then soldered or brazed onto the metallized ceramic in a continuous or batch-type furnace with a controlled reducing atmosphere. Well brazed joints can be obtained consistently by proper adjustment of temperature, time, atmosphere and fixturing.
Sealing Alloy Choices
For specific applications, any of the glass and ceramic sealing alloys that match the thermal expansion characteristics of the part or assembly to be protected can be selected.
Alloy K (ASTM F-15) is a low expansion alloy used extensively for both glass-to-metal and ceramic-to-metal sealing applications. It provides a strong hermetic seal to hard borosilicate glasses and ceramic materials such as those used in power tubes, microwave tubes, transistors and diodes, and high-end electronic products such as miniaturised integrated circuits. This is one of only two alloys in the glass and ceramic sealing family containing a large amount of cobalt. The cobalt lowers the thermal expansivity of low expansion iron-nickel alloys.
Alloy C (ASTM F-1466) is another cobalt-containing alloy that is considered for applications where Alloy K may not provide the service needed. This iron-nickel-cobalt alloy is specially designed for ceramic-to-metal sealing applications. It has expansion characteristics more closely matching those of alumina ceramics for high temperature brazing applications. Other iron-nickel alloys offer a range of thermal expansion characteristics suitable for glass sealing applications. As nickel content changes so do the essential properties such as thermal expansivity and Curie temperature of these alloys.
Alloy 42 (ASTM F-30; Alloy No. 42) is used for integrated circuit lead frames and other glass-to-metal sealing applications. A modification of this material is designed for bubble-free glass-to-metal seals. The composition of this alloy is carefully controlled to prevent formation of carbon dioxide or carbon monoxide bubbles at the seal interface. This characteristic makes the alloy useful for terminals for enameled resistors, industrial lamps and automotive headlamp ferrules which accept electrical contact. A special thermal treatment for Alloy 42 (ASTM F-31) develops a tightly adherent oxide layer beneficial to obtaining a durable hermetic seal. Metal parts are pre-oxidized by thermal treatment before the glass sealing. Alloy 46 (ASTM F-30; Alloy No. 46) is used for terminal bands in enameled resistors and for enameling without degasification. Alloy 51 (ASTM F-30; Alloy No. 51) with still more nickel content, is used in dry reed switches and hermetic feedthroughs. Alloy 52 (ASTM F-30; Alloy No. 52) is used exclusively for pin feed-throughs for semiconductor devices. This alloy successfully seals to soft glasses like potash soda lime glass. Soft glasses have higher thermal expansion coefficients - in the range of 7 to 11 x 10-6 per °C - than hard borosilicate glasses with thermal expansion coefficients of approximately 3 to 5 x 10-6 per °C which are better sealed with Alloy K. Two ductile iron-chromium alloys - Alloy 27 (ASTM F-256; Type II) and Alloy 18 (ASTM F-256; Type I) - are used for high volume glass-to-metal sealing applications such as fluorescent tube feed-throughs and as stud pins and other support members inside television monitors. Alloy 27 is used extensively for seals in electronic and vacuum tubes, and fluorescent lamps. Alloy 18 is used in television monitors and fluorescent lamps. The chemistry of this alloy is balanced to prevent phase transformation through the high temperature glass sealing cycle.
Matching Expansion Alloy Fabrication
The iron-chromium glass sealing alloys do not exhibit the Invar Effect; their rate of thermal expansion is nearly linear. These alloys are ductile and can be stamped, deep drawn into parts on a mass production scale and used in annealed condition. Both of iron-chromium alloys are mechanically harder than the iron-nickel alloys and the iron-nickel-cobalt grades and easier to machine. With their good ductility, they are readily formed and deep drawn for high volume part production. Manufacture of these controlled expansion alloys can be controlled within limits to provide the properties best suited for a particular fabrication operation. Factors that contribute to fabrication characteristics include composition, mechanical properties, internal quality of the metal, surface quality, shape and dimensional tolerances.
High Expansion Alloys
High expansion alloys are often iron-based alloys usually containing nickel and chromium. Sometimes certain common stainless steels are also used. Three high expansion alloys are used almost exclusively as the high expansion component in thermostat bimetallic applications.
Alloy 22-3 (ASTM B-753) - with 22% nickel, 3% chromium and balance iron - has thermal expansion properties higher, equal to or greater than 19.8 x 10-6 per °C - than any of the alloys in the Type 300 stainless steel series. A companion grade, Alloy 19-2 (ASTM B-753) - containing 19% nickel, 2% chromium and balance iron - has a thermal coefficient of expansion similar to that for Alloy 22-3. The Alloy 19-2, however, can be manufactured to higher tensile strengths for bimetal thermostats that must be stronger and/or springier. Alloy 72 (ASTM B-753) is a unique non-ferrous alloy which has a significantly higher thermal expansivity than the ferrous high expansion alloys. This alloy contains a nominal 72 percent manganese, 18 percent copper and 10 percent nickel. It has a thermal coefficient of expansion greater than 27 x 10-6 per °C. When combined with very low expansion alloys, this alloy promotes greater flexing due to the greater difference between the rates of thermal expansion.
In bimetallic strip thermostats, two strips of metals with different coefficients of thermal expansion are bonded together. The strips are joined by pressure, heat treatment or welding. The unequal rates of expansion of the two metals during temperature changes causes the strip to bend into an arc. This mechanical bending controls the movement thus making or breaking electrical contacts. These are made for any function or process which is temperature controlled like thermostats to prevent overheating of electric motors, thermostats for circuit breakers in homes and industry, safety devices in home appliances, furnace controls, flow control devices, etc. These devices act both as sensing elements and as active control components.
Differential expansion thermostats require the right match of high expansion and low expansion alloys. Knowing how much movement is desired within an anticipated temperature range, the design for proper degree of deflection is done by considering two alloy expansion rates. Bimetallic devices generally have specific thickness and cross-section requirements of each material. The final component is composed of thin gage materials to allow for heat-responsive flexing when in contact with a circulating gas or liquid. The iron-nickel-chromium high expansion alloys have good fabricability. They are compatible with the low expansion iron-nickel alloys when heat treating, welding and bonding.
Article courtesy of ReserachGate Publication