Laser Welding of Dissimilar Metals - Lab Report Example

Laser Welding of Dissimilar Metals In performing laser welding we made use of 316L Austenitic Stainless Steel and Low Carbon Steel plates as the basematerials in this experiment. The chemical composition of both the base plate materials is shown in the table below as a comparison of composition of both metals, in Tables (5.1). Table (5.1) shows the Chemical composition of the materials (wt. %) These metals were received in the form of plates of 350×10×3 mm from the vendors and were precisely machined on the Milling machine in the mating areas of both the metals to avoid the gap between the metals which lead to misalignment. Both the metals were mounted in butt joint geometry as shown the figure (5.1) . Figure (5.1) the butt joint geometry of the base metal Laser welding of thee metals was performed by making use of RTM machine which has the capacity: CO2 6kw, with 4D (X, Y, Z, A`) work table geometry having dimensions of 1m×1.5m. The joints are positioned with respect to the high laser using power machine. The table is scanned along the (Z) direction to form continuous weld , three sets of continuous welds are carried out with varying a laser power , laser scan speed , the focus position , all the welding process were in constant flow rate of gas and Nitrogen gas was used as shielding gas. The first set the power is varied (1kw, 1.5kw, 2kw, 2.5kw, 3kw) as listed in table (5.2) Table (5.2) The best value of power was (2kw). The second set the laser scan speed is varied (0.8, 1, 1.2, 1.4, 1.6, 1.8,) as shown in table (5.3) Table (5.3) The best value of power was (1.8 m/min). The third set the focus position is varied (-1mm, -2mm) as shown in table (5.4) Table (5.4) The best value of focus position was (-1.5 mm). Figure (5.2) The focus position of laser beam on the work surface (surface is zero point). Radiographic Test (X-ray) The process were taken place of all samples after welding by using x-ray machine, the radiographic equivalent factors were selected according to metal and the thickness of the samples. The results show some welding defects in some samples and rest of the samples were acceptable. The machine parameters used in the X-ray test was 145 volt, 3mlampair, and 25 second. Tensile Test Tensile test of welding joint was done on Zwick Roell universal test machine controlled by computer, the test were performed at the room temperature to determine the tensile strength and the yield strength. The samples whose thickness and gauge length are (3mm) and (300 mm) were prepared to the test according the AWS standards; the figure (5.3) illustrated the dimensions of the samples. Figure (5.3) The dimensions of the samples (the unit in mm) Macrostructure and Microstructure The samples were cross-sectioned, ground by using (80, 120, 320, 600, 1200) grinding paper and polished by Diamond sucbention for metallurgical examination. The mounted specimens were etched in a solution comprised of 50% Hydraulic acid + 25% Nitric acid + 25% water and ethyl alcohol for 20–60 s until the microstructure was revealed. The microstructure and microhardnees distribution were characterized by optical microscope. Results and Discussion:- Nondestructive Tests:- Visual Examination:- Visual inspection was performed for all samples and showed some defects as the laser parameters varied, the results and the defect causes are listed in the table (6.1) below. Table (6.1) shows the Results of Visual Examination Effect of laser power The effect of heat input as a function of laser power, H, = P/S, was clarified using type 316L and low carbon steels. Both welding speed and focus position were kept constant (1.2mm/min) and (-1.5 mm) respectively. The penetration depth increased with increasing laser power. Complete penetration for the 3 mm base metal was obtained at laser power equal to or greater than 2kw. Laser power has a less influence on both weld profile and (HAZ) width. Effect of welding speed:- The effect of welding speed was investigated at the optimum laser power (2 kW) and (-1.5) focus position. The relationship between welding speed and heat affected zone (HAZ) for both base metals. The (HAZ) increased sharply with the decrease in welding speed from (1.8 to 0.8) m/mm. Effect of the focus position Focus position, is the distance between specimen surface and the optical focal point. In order to study its effect on both penetration depth and weld profile, complete penetration butt welds were made using previously obtained optimum laser power (2 kW) and optimum welding speed (1.8 m/min). The values has been changed from (-1 to -2 mm) include (-1.5 mm). The penetration depth decreased on changing the defocus position from (-1.5) to (-1) mm. Then, the penetration decreased on changing the defocus position to (-2 mm). Radiographic Test Results (X-ray) Table (6.2) shows the Results of X-ray test Effect of laser power The effect of laser power welding is clearly leads to undercut as the increasing power values; samples which is (2.5 and 3 kw) were used. Effect of laser speed:- According to the heat input equation the heat input value has inversely property with the speed value, this is explains the defect (undercut) which is shows up in the small values of speed, as speed were (0.8 and 1 m/min). The incorrect procedure in laser welding process in the sample No (9) lead to misalignment which produce a gap between the stainless steel and the low carbon steel and that is produce to undercut. Mechanical Tests Tensile Test Results Mechanical property of the first set welded samples is shown in Table (6.3). Table (6.3) The Tensile Test Result for the first set. Figure (6.1) The relationship between the stress in (N/mm2) and strain in (%) in first set Figure (6.2) The relationship between the power and tensile strength Mechanical property of the second set welded samples is shown in Table (6.4). Table (6.4) The Tensile Test Result for the Second set. Figure (6.3) The relationship between the stress in (N/mm2) and strain in (%) in second set Figure (6.4)The relationship between the welding speed and tensile strength Mechanical property of the third set welded samples is shown in Table (6.5). Table (6.5) The Tensile Test Result for the third set. Figure (6.5) The relationship between the stress in (N/mm2) and strain in (%) in third set Figure (6.6) The relationship between the focus position and tensile strength In the figure (6.7) shows the result of the welded samples were less than the typical values of stainless steel and higher than the steel values, These results indicate that: the Tensile strength has the tendency to increase at first and then decrease when the welding heat input is increased, and the values of Tensile strength, With one exaption in the first sample (1kw,1.2m/min and -1.5 mm) ,the broken was in weld metal the rest of the samples brakes in low carbon said , the reason of that exaption is incorrect procedures which lead to misalignment. Figure (6.7) The relationship between the stress in (N/mm2) and strain in (%) Figure (6.8) The location of break in the specimen The microhardnees Test Hardness tests were carried out in samples welded, the samples were prepared by grinding, polishing and etching, the test was by applied (50gf) load for (15 sec) as indication time, and the samples were tested in weld metal and base metal. In general the hardness profiles show an expected high hardness in weld metal, the hardness values of weld metal were around (400 HV) which straightly high than the hardness values of low carbon steel and stainless steel, the reason is using the Nitrogen gas as shielding gas in laser welding process, The Investigations shows using Nitrogen as shielding gas in welding increases the hardness value [22]. The table (6.6) shows the results of micro hardness test. The table (6.6) shows the results of microhardnees test in weld metal area Figure (6-9) The relationship between the power and Micro hardness Figure (6-10) The relationship between the welding speed and Micro hardness Figure (6-11) The relationship between the focus position and Micro hardness Macrostructure and Microstructure Macrostructure of weld joint Fig. (6.12) shows the overall optical cross sections of laser welded 316L Stainless Steel and 1009 Low Carbon Steel joints under the conditions of different heat input. From the figure, it can be seen that there exists some influence of heat input on the joint geometry, which mainly reflects the change of weld penetration and weld width. When the heat input is very low, obvious incomplete penetration appears. As the heat input is increased, the weld penetration is increased and the weld appearance becomes better. At the same time, an increase in heat input results in slightly broadening the top side of the weld. However, the bottom width of the weld becomes obviously larger when the heat input is higher. That is to say, the weld width difference between the top and the bottom becomes smaller as the heat input is increased. The main reason may be that the cooling rate decreases with increasing the heat input [22]. Therefore, when the heat input is higher, more volume of the base metal will melt and the welding heat has more time to conduct into the bottom from the top. And the different of cooling rate in between the base metal (316L and 1009) produce a crack in the low carbon steel. In Fig. (6.12), some typical defects also appear in the welded samples. When the heat input is high , and using (N2) as shielding gas lead to produce porosity , low focus position lead to lack penetration , (The magnification =50X) Figure (6.12) a) the section area of specimen (1) Figure (6.12) b) the section area of specimen (2) Figure (6.12) c) the section area of specimen (3) Figure (6.12) d) the section area of specimen (4) Figure (6.12) e) the section area of specimen (5) Figure (6.12) f) the section area of specimen (6) Figure (6.12) g) the section area of specimen (7) Figure (6.12) h) the section area of specimen (8) Figure (6.12) I) the section area of specimen (9) Figure (6.13) j) the section area of specimen (10) Figure (6.12) k) the section area of specimen (11) Figure (6.12) l) the section area of specimen (12) Microstructure of weld joint According to the results of Macrostructure, the best and worst welded samples were chosen to observe the microstructures. Fig (6.13) illustrates the typical microstructure of welded 316L and 1009 joint. As shown in Fig. (6.13) (a), (b), 1009 and (c), (d) 316L base metal far from the weld centre presents the same typical rolled structure as in the wrought alloy. The grain size is non-uniform and some fine recrystallized grains distribute around the large elongated crystals. As shown in Fig. (6.13) the transition zone between base metal and fusion bead is excellently joined. magnification = 200X Read More