Conductivity of Aluminum Alloys. During a failure analysis investigation of a heat treated aluminum alloy, conductivity testing may be performed to evaluate proper heat treat condition, to assess areas for heat damage, or to estimate tensile rength. Conductivity is the reciprocal of electrical resistivity and is directly proportional to the mean free path of an electron in the crystal structure of the material. The mean free path is affected by the microstructure of the material, which is affected by heat treat condition Solution-annealed structures have soluble constituents that have precipitated out of solution, providing for a material with high conductivity. Aging causes fine precipitation of a second phase and thus decreases the conductivity of the material Natural aging yields the lowest conductivity and intermediate strength, while artificial aging results in intermediate conductivity and highest hardness/strength. Overaging of the alloy causes the conductivity and hardness values te approach those of the solution-annealed condition The manufacturer does not typically specify conductivity values of an aluminum alloy component. However, with proper design and material selection, the alloy and temper of the component are specified, and corresponding values of conductivity may be found in reference data such as specification SAE-MIL-H-6088. Conductivity measurements must be paired with hardness measurements to determine if the failed component meets alloy and heat treatment requirements. An aluminum alloy and its temper can be determined by measuring hardness and conductivity and verifying against reference Chemical Analysis In a failure investigation, routine analysis of the material is usually recommended. Often it is done last because an analysis usually involves destroying a certain amount of material. There are instances where the wrong material was used, under which conditions the material might be the major cause of failure. In many cases, however, the difficulties are caused by factors other than material composition In most instances, slight deviations from specified compositions are not likely to be of major importance in failure analysis. However, small deviations in aluminum content can lead to strain aging in steel, and small quantities of impurities can lead to temper embrittlement. In specific investigations, particularly where corrosion and stress corrosion are involved, chemical analysis of any deposit, scale, or corrosion product, or a substance with which the affected material has been in contact, is required to assist in establishing the primary cause of failure here analysis shows that the content of a particular element is slightly greater than that required in the specifications, it should not be inferred that such deviation is responsible for the failure. Often, it is doubtful whether such a deviation has played even a contributory part in the failure. For example, sulfur and phosphorus in structural steels are limited to 0.04% in many specifications, but rarely can a failure in service be attributed to sulfur content slightly in excess of 0.04% Within limits, the distribution of the microstructural constituents in a material is of more importance than their exact proportions. An analysis(except a spectrographic analysis restricted to a limited region of the surface)is usually made on drillings representing a considerable volume of material and therefore provides no indication of possible local deviation due to segregation and similar effects Also, certain gaseous elements, or interstitials, normally not reported in a chemical analysis, have profound effects on the mechanical properties of metals. In steel, for example, the effects of oxygen, nitrogen, and hydrogen are of major ortance.Oxygen and nitrogen may give rise to strain aging and quench aging. Hydrogen may induce brittleness icularly when absorbed during welding, cathodic cleaning, electroplating, or pickling. Hydrogen is also responsible for the characteristic halos or fisheyes on the fracture surfaces of welds in steels, in which instance the presence of hydrogen often is due to the use of damp electrodes. These halos are indications of local rupture that has taken place under the bursting microstresses induced by the molecular hydrogen, which diffuses through the metal in the atomic state and collects under pressure in pores and other discontinuities. Various effects due to gas absorption are found in other metals and alloys. For example, excessive levels of nitrogen in superalloys can lead to brittle nitride phases that cause failures of highly stressed parts Various analytical techniques can be used to determine elemental concentrations and to identify compounds in alloys bulky deposits, and samples of environmental fluids, lubricants, and suspensions. Semiquantitative emission petrography spectrophotometry, and atomic-absorption spectroscopy can be used to determine dissolved metals( as in nalysis of an alloy with wet chemical methods used where greater accuracy is needed to determine the concentration of metals. Combustion methods ordinarily are used for determining the concentration of carbon, sulfur, nitrogen, hydrogen and oxygen Wet chemical analysis methods are employed for determining the presence and concentration of anions such as Cl, NO and S. These methods are very sensitive X-ray diffraction identifies crystalline compounds either on the metal surface or as a mass of particles and can be used to analyze corrosion products and other surface deposits. Minor and trace elements capable of being dissolved can be determined by atomic-absorption spectroscopy of the solution X-ray fluorescence spectrography can be used to analyze both crystalline and amorphous solids, as well as liquids andConductivity of Aluminum Alloys. During a failure analysis investigation of a heat treated aluminum alloy, conductivity testing may be performed to evaluate proper heat treat condition, to assess areas for heat damage, or to estimate tensile strength. Conductivity is the reciprocal of electrical resistivity and is directly proportional to the mean free path of an electron in the crystal structure of the material. The mean free path is affected by the microstructure of the material, which is affected by heat treat condition. Solution-annealed structures have soluble constituents that have precipitated out of solution, providing for a material with high conductivity. Aging causes fine precipitation of a second phase and thus decreases the conductivity of the material. Natural aging yields the lowest conductivity and intermediate strength, while artificial aging results in intermediate conductivity and highest hardness/strength. Overaging of the alloy causes the conductivity and hardness values to approach those of the solution-annealed condition. The manufacturer does not typically specify conductivity values of an aluminum alloy component. However, with proper design and material selection, the alloy and temper of the component are specified, and corresponding values of conductivity may be found in reference data such as specification SAE-MIL-H-6088. Conductivity measurements must be paired with hardness measurements to determine if the failed component meets alloy and heat treatment requirements. An aluminum alloy and its temper can be determined by measuring hardness and conductivity and verifying against reference data. Chemical Analysis In a failure investigation, routine analysis of the material is usually recommended. Often it is done last because an analysis usually involves destroying a certain amount of material. There are instances where the wrong material was used, under which conditions the material might be the major cause of failure. In many cases, however, the difficulties are caused by factors other than material composition. In most instances, slight deviations from specified compositions are not likely to be of major importance in failure analysis. However, small deviations in aluminum content can lead to strain aging in steel, and small quantities of impurities can lead to temper embrittlement. In specific investigations, particularly where corrosion and stress corrosion are involved, chemical analysis of any deposit, scale, or corrosion product, or a substance with which the affected material has been in contact, is required to assist in establishing the primary cause of failure. Where analysis shows that the content of a particular element is slightly greater than that required in the specifications, it should not be inferred that such deviation is responsible for the failure. Often, it is doubtful whether such a deviation has played even a contributory part in the failure. For example, sulfur and phosphorus in structural steels are limited to 0.04% in many specifications, but rarely can a failure in service be attributed to sulfur content slightly in excess of 0.04%. Within limits, the distribution of the microstructural constituents in a material is of more importance than their exact proportions. An analysis (except a spectrographic analysis restricted to a limited region of the surface) is usually made on drillings representing a considerable volume of material and therefore provides no indication of possible local deviation due to segregation and similar effects. Also, certain gaseous elements, or interstitials, normally not reported in a chemical analysis, have profound effects on the mechanical properties of metals. In steel, for example, the effects of oxygen, nitrogen, and hydrogen are of major importance. Oxygen and nitrogen may give rise to strain aging and quench aging. Hydrogen may induce brittleness, particularly when absorbed during welding, cathodic cleaning, electroplating, or pickling. Hydrogen is also responsible for the characteristic halos or fisheyes on the fracture surfaces of welds in steels, in which instance the presence of hydrogen often is due to the use of damp electrodes. These halos are indications of local rupture that has taken place under the bursting microstresses induced by the molecular hydrogen, which diffuses through the metal in the atomic state and collects under pressure in pores and other discontinuities. Various effects due to gas absorption are found in other metals and alloys. For example, excessive levels of nitrogen in superalloys can lead to brittle nitride phases that cause failures of highly stressed parts. Various analytical techniques can be used to determine elemental concentrations and to identify compounds in alloys, bulky deposits, and samples of environmental fluids, lubricants, and suspensions. Semiquantitative emission spectrography, spectrophotometry, and atomic-absorption spectroscopy can be used to determine dissolved metals (as in analysis of an alloy) with wet chemical methods used where greater accuracy is needed to determine the concentration of metals. Combustion methods ordinarily are used for determining the concentration of carbon, sulfur, nitrogen, hydrogen, and oxygen. Wet chemical analysis methods are employed for determining the presence and concentration of anions such as Cl- , NO3 - , and S- . These methods are very sensitive. X-ray diffraction identifies crystalline compounds either on the metal surface or as a mass of particles and can be used to analyze corrosion products and other surface deposits. Minor and trace elements capable of being dissolved can be determined by atomic-absorption spectroscopy of the solution. X-ray fluorescence spectrography can be used to analyze both crystalline and amorphous solids, as well as liquids and gases