Melting point and some physical properties of materials

Grafiche Bronca

The melting point

The melting point of a metal depends on the energy needed to separate its atoms. The melting temperature of a metal alloy can have a wide range, depending on its composition, and is different from that of a pure metal, which has a well-defined melting point. The temperature range within a component or structure is designed to work is an important consideration in material selection.

Plastics, for example, have the narrowest useful temperature range, while graphite and refractory metal alloys have the widest useful range.

The melting point of a metal has a number of indirect effects on production. Since the recrystallization temperature of a metal is related to its melting point, operations such as annealing, heat treatment, and hot processing require a knowledge of the melting points of the materials involved. These considerations are also important in the selection of tool materials and molds.

The melting point also plays an important role in the selection of machines and in the casting techniques used in casting operations. The higher the melting point of the material, the harder the operation becomes. In the electrical-discharge machining (EDM) process, the melting point of metals is related to the speed of material removal and electrode wear.

Physical properties of some materials. Clic to enlarge.

The specific heat

The specific heat of a material is the energy needed to increase the temperature of a unit of mass by 1 degree. Alloy elements have a relatively minor effect on the specific heat of metals. The increase in temperature in a workpiece, resulting from the forming or processing itself, is a function of the work done and the specific heat of the material of the workpiece.

Excessive temperature increase in a workpiece can reduce product quality by negatively affecting the surface finish and dimensional accuracy, can cause excessive tool and mold wear, and also bring unwanted metallurgical changes to the material.

Thermal conductivity

Thermal conductivity indicates the rate at which heat flows inside and through a certain material. Metallically bonded materials (metals) generally have high thermal conductivity, while ionic or covalently bonded materials (ceramics and plastic) have poor conductivity.

Alloy elements can have a significant effect on the thermal conductivity of alloys as can be seen by comparing metals with their alloys. In general, materials with high electrical conductivity also have high thermal conductivity.

This property is important in many applications. For example, high thermal conductivity is desirable in cooling fins, cutting tools and in die casting molds to extract heat. On the contrary, materials with low thermal conductivity are used, for example, in oven coatings, insulation, coffee cups, and handles for pots and pans.

Proprietà fisiche dei materiali
Some physical properties of material in descending order

Thermal expansion

The thermal expansion of materials can have several significant effects, in particular, the relative expansion or contraction of different materials in assemblies such as electronic and computer components, glass-metal seals, jet engine uprights, on cutting tools, coatings, and moving parts in machinery that requires some clearance for proper operation.

The use of ceramic components in cast-iron motors, for example, also requires considering their related expansions.

In general, the thermal expansion coefficient is inversely proportional to the melting point of the material. Alloy elements have a relatively minor effect on the thermal expansion of metals.

Shrink fits use thermal expansion and contraction. A shrink fit is a part, often a tube or hub, that must be installed on a shaft. The part is first heated and then slid over the shaft or spindle; when left to cool, the hub shrinks and the assembly becomes a unique component.

Thermal expansion in combination with thermal conductivity plays a very significant role in causing thermal stresses (due to temperature gradients), both in manufactured components and tools and dies, and in molds for casting operations.

This consideration is especially important, for example, in a forging operation during which hot pieces are repeatedly placed on a relatively cold die or shape, subjecting the mold surfaces to a thermal cycle. In order to reduce thermal stresses, the combination of high thermal conductivity and low thermal expansion is desirable.

Thermal stresses can also be caused by the anisotropy of thermal expansion; that is, the material expands differently in different directions, a property generally observed in some metals, ceramics, and composite materials.

Thermal expansion and contraction processes can lead to cracking, distortion, or loosening of components during their useful life, as well as cracking of ceramic parts, tools, and matrices made from relatively fragile materials.

Thermal fatigue is the result of a thermal cycle and causes a series of surface cracks, especially in tools and molds for casting and metalworking operations. The term Thermal Shock is generally used to describe the development of cracks after a single thermal cycle.

To minimize some of the problems caused by thermal expansion, a family of iron-nickel alloys with very low thermal expansion coefficients, called low-expansion alloys, has been developed. The low thermal expansion characteristic of these alloys is often referred to as the Invar effect, a name derived from the Invar metal alloy.

The expansion thermal coefficient is typically in the range 2x1O-6 to 9×10-6 for °C. Typical compositions are 64% Fe-36% Ni for Invar and 54% Fe-28% Ni-18% Co for Kovar.

Low expansion alloys also have good resistance to thermal fatigue and ductility; as a result, they can be easily printed in various shapes. Applications include bimetallic strips consisting of a low-expansion alloy metallurgically glued to a high-expansion alloy (the strip bends when subjected to temperature changes) and high-quality glass-metal seals in which equal thermal expansions are coupled.

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