Copper and its alloys
In the beginning, we note the following technical characteristics of copper and its alloys, such as high resistance to the effects of various chemicals, maintenance of high mechanical properties in the deep cold, high thermal conductivity and electrical conductivity. Technical copper depending on the brand may have different amounts of impurities: Bi, Sb, As, Fe, Ni, Pb, Sn, S, Zn, P, O. In the most pure copper grade M00 impurities can be up to 0.01%, grade M4 - 1%. Alloys of copper-based, depending on the composition of the alloying elements are brass, bronze, copper-nickel alloys.
Brass. Brass called copper-zinc alloys (brass for short) zinc content can reach 42%. If, in addition to zinc, the alloy contains other alloying elements (Al, Fe, Ni, Si), referred to as a complex fusion of brass. Brass has higher strength than pure copper (up to 50 sigmav kgf/mm2) (or the endurance limit of up to 470 MPa). However, if they contain more than 20% Zn alloy are prone to stress-corrosion cracking and the formation of cracks at the local heating. Brass is widely used as a structural material with high corrosion stand-bone and more durable than copper. copper-based alloys in which zinc is not a major alloying element, called bronzes. The name specified for the bronze main alloying element, by which bronze acquires certain properties. Are widely used tin bronze (2-10% Sn), aluminum (A1 4-11,5%), siliceous (0,5-3,5% Si), manganese (4.5 - 5.5% Mn) Beryllium (1,9-2,2% Be), chromium (0.4-1% Cr). tin bronze has good corrosion resistance and antifriction properties. Therefore, they are widely used in the manufacture of corrosion-resistant fittings for different pipes, bearings, etc. Bronze aluminum and silica have high mechanical properties and good corrosion resistance. They are cheaper. If the permit conditions, they are widely used instead of tin. Manganese bronze, in addition to good corrosion resistance, have high heat resistance. Beryllium bronze has high corrosion resistance and after heat treatment are nonmagnetic with very high strength, corresponding to the strength of steel. Of these bronzes are produced a range of flexible, durable items in the various instruments and devices, Copper-nickel alloys may contain up to 30% Ni, as well as iron and manganese. Alloy MNZH 5-1, durable and corrosion-resistant, widely used as a construction for the manufacture of pipes and vessels operating in hostile environments (sea water, salt solutions, organic acids). The complex composition of copper-based alloys, the presence of various components in the form of copper impurities in the technical difficulties are responsible for the welding of these metals. should consider the following features of copper and its alloys, affecting the welding technology.
Features of copper
1. Due to the high temperature and thermal conductivity, hindering local heating requires more concentrated sources of heat and increased welding modes. However, due to the tendency of copper to grain growth during welding of multilayer metal joints of each passage for grain prokovyvayut at temperatures 550-800 deg. C.
2. Ease of oxidation of copper at high temperatures leads to clogging of the weld metal refractory oxides. Cuprous oxide is soluble in the liquid metal and limited - in the solid. With copper oxide forms a fusible eutectic Cu-Cu2O (melting point of 1064 deg. C), which is concentrated along the grain boundaries and reduces the ductility of copper, which can lead to hot cracking. As follows from the phase diagram of the copper - oxygen, a small concentration of oxygen decreases the temperature melting point of copper, with an oxygen content of 0.38% (which corresponds to 3.4% Su2O) forms a eutectic with a melting point of 1064 degrees. C. In connection with this, and because of the limited time the possibility of steel metal weld pool (short lifetime due to the high thermal conductivity of copper) to the introduction of energetic deoxidizers - phosphorus, manganese, silicon, and others with limited oxygen content up to 0.03% and in critical constructions (eg, marine pipelines, vessels, etc.), the oxygen content of no more than 0.01%. For the destruction of refractory oxides, which form a film on the surface of the weld pool, use of borax-based fluxes (95% Na2B4O7 and 5% Mg), which contribute to chemical cleaning, translating the refractory oxides in the low-melting complexes. However, the use of phosphorus for the purpose of deoxidation should be limited, since it also gives a low-melting eutectic. Reducing agent, participating in the steel during the welding process, not only deoxidized metal, but at the same time and alloying it, which could reduce its corrosion resistance and electrical conductivity.
3.Some impurities may contribute to addiction weld cracking. For example, bismuth, forming a series of oxides of BiO, Bi2O3, Bi2O4, Bi2O5, gives low-melting eutectics with a melting point 270 deg. C, and the lead, forming oxides of PbO, RO2, R2O3 gives a fusible eutectic with a melting point of 326 deg. C. For this reason, should be sharply limited by the content of these impurities (Bi <0,002%; Pb <0,005%), or they must be related to the introduction of refractory compounds in the molten pool of elements such as cerium, zirconium, while playing the role of modifiers. In welding of aluminum bronzes is readily formed a refractory oxide Al2O3, littering the weld pool, worsening the fusion of metal and welded joint properties. For its destruction fluxes used, consisting of fluorides and chlorides, alkali and other metals.
4. When welding brass zinc vaporization (boiling point 907 deg. C, ie below the melting point of copper) can occur. The resulting zinc oxide is toxic, so welding is needed ventilation. Evaporation of zinc can cause weld metal porosity. This complication can be overcome pre-heated metal to a temperature of 200 -300 degrees. C and higher welding speed, which reduces the spreading of liquid metal and the evaporation of zinc. high coefficient of linear expansion (1.5 times greater than that of steel) can cause elevated temperatures during welding and the welding residual stresses and strains. The combination of high thermal stresses with a decrease in mechanical properties may contribute to the formation of cracks. To reduce the welding deformation of the structure are hard to consolidate, to tack. With increased thickness of the metal regulate the magnitude of the gap.
5. Copper in the molten state absorbs significant amounts of hydrogen. During the crystallization of the metal weld pool at high speed due to the high thermal conductivity of copper and a sharp decrease in solubility of hydrogen in the metal atomic hydrogen does not have time to leave the metal due to desorption. Cuprous oxide reduced by hydrogen to form water vapor, which leads to the formation of pores and cracks the seam. in the weld zone of diffusion-mobile hydrogen interacts with Su2O, located along the grain boundaries, forming water vapor, which are not soluble in copper, and can not get out of it create significant stress in the metal, leading to the formation of a large number of microcracks. This phenomenon is called hydrogen illness copper. To prevent disease of the hydrogen copper should reduce the amount of hydrogen in the weld zone (calcination of electrodes and fluxes, the use of dried-GOVERNMENTAL shielding gas). Carbon monoxide can also participate in the deoxidation of copper, which also contributes to the formation of pores. The affinity of copper to nitrogen is very small, so the nitrogen can be used for welding copper as shielding gas.
6. The increased fluidity of the molten copper and its alloys (especially brass) makes it difficult to weld in vertical and overhead positions, so most are welded to the lower position. To form the root of the weld defect-free pads are needed. For copper and alloys based on it can be used by all major methods of fusion welding.
See also:
Welding of Aluminum
Titanium
Welding Magnesium
Welding of Titanium
Copper
Welding of Copper
Refractory Metals
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