Gas pore formation in tungsten arc weld in Cu-Ni 90-10 alloy

Copper and nickel are soluble in each other in all proportions to give a range of cupro-nickels which are ductile and can be hot and cold worked. Copper base alloys are frequently used for heat exchanger plants and especially for seawater piping systems. F.c.c. alloys are easily used and all classical processes can be employed to weld these alloys if the residual elements content is low enough to avoid hot cracking. Control of metallurgical elaboration and development of suitable filler metal prevent from hot cracking in sensible alloys like Cu-Ni. Although, porosities are frequently found in these alloys. Origins of these defects are usually assigned to the dissolved gases in the melted metal during the welding. Yet, no direct experimental evidence has been shown. In this present work, the mechanisms of porosity formation in weld metal are reported. The welding process is the orbital tungsten inert gas (T.I.G.) on a cupro-nickel 90-10 pipe (external diameter = 14 mm, infernal diameter = I I mm). The alloy used in this study has a nominal composition of Cu 90 wt %, Ni 10 wt %, the main other element are Fe (1.6 wt %) and Mn (0.6 wt %). The iron addition increases corrosion resistance and the manganese addition traps free sulfur to form MnS precipitates. The filler wire is cupro-nickel 90-10 alloys (0.8 mm in diameter) enriched with titanium (0.39 wt %). This filler metal reduces the formation of porosity and possibility of oxygen embrittlement in either the melted metal or the heat affected zone (HAZ). No preheating, post heating or thermal treatment are needed. The shielding gase is mixed argon-helium (Inarc 6). Transversal sections of the welds were thoroughly cleaned, mechanically polished with abrasive paper from grade 400 to 4000 and finished with diamond paste 3 mu m, cleaned with acetone in an ultrasonic bath and then etched with ammonium persulfate at la % volume. Cross-section were examined with a standard optical microscope to localize porosity. A scanning electron microscope (SEM) equipped with an energy dispersive spectrometer is used to reveal the solidification substructures, to examine the surface morphology and for semi-quantitative microanalysis. Size and shape of each pore were analysed using a semi-quantitative image analysis system attached to the SEM. The research of gazeous emission during the metal melting was studied by coupled differential scanning calorimetry thermogravimetric analysis and mass spectrometry All these experiments were carried out under an argon controlled atmosphere. Metallographic observations exhibit three types of porosity. They can be classified then according to their size, localization in the bead, chemical composition around porosity and reactivity with chemical agents. First type of porosity has the biggest size. They are more than 50 mu m. They are neither especially spherical nor deep (< 25 pm) and have a rough cast intern surface. The composition of porosity surface is the same than the base material: copper, nickel and some iron and manganese. The porosities are found on the top of the melted metal and it seems that they evolve with time. These bubbles are formed by gas entrapment during solidification. Some emerging bubbles can be found. The bigger size can be explain by effect of hydrostatic pressure when bubble come to the surface. Second type has an intermediate size, Porosity diameters included between 2 and 50 mu m. The surface composition around these porosities is : copper and nickel, titanium, magnesium, aluminum and silicium which are associated with oxygene. These porosities are located along the weld metal. There is no natural evolution for these porosities. The zone around some porosities may present a sensitivity with the reagent used for the chemical attack. Actually we cannot explain the formation mecanism of these bubbles. However as pores are coated with elements which have a high affinity with oxygen, we think that the formation of pore may result of oxidizing reactions in the melting bath. Third type is the smaller. Their size is less than 2 mu m in diameter. These porosities are randomly distributed in the weld metal. A chemical analysis of their surfaces shows the presence of sulfur, titanium and magnesium in addition to copper and nickel. They do not seem to evolve with time. These pores are formed by a reaction beetween manganese sulfur (MnS) precipitates contained in base metal and magnesium and titanium of the filler metal in the melting bath. It's well known that TiS and MgS precipitates are more stable than MnS precipitate. So it is the sulfur vaporisation which result of the heat reaction which form porosities.