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Influence of Shielding Gas on Properties of High-Brightness Laser

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Influence of Shielding Gas on Properties of High-Brightness Laser sept amp features_am&p master template new QX6.qxt 8/22/2012 9:41 PM Page 24 Influence of Shielding Gas on Properties of Brad Nagy ver the past decade, major technolog- turing cost implications for the two gases. Edison Welding ical advancements in the area of high- Argon is completely inert, while nitrogen po- Institute Opower, high-brightness lasers (disk and tentially can react with elements in base mate- Columbus, Ohio fiber lasers) brought laser processing to the rials, which could affect mechanical properties. forefront of many manufacturing industries in- The same composition was used for the pri- Shielding-gas cluding automotive, aerospace, medical, and mary shield gas (top surface of the weld), purge composition heavy manufacturing. The potential productiv- gas (root of the weld), and trail gas (top surface ity benefits resulting from laser processing are of the weld). The primary shield gas was deliv- can affect dramatic. Traditional multipass, high heat- ered through a copper nozzle that had a hole weld properties input welding procedures can potentially be re- for the laser beam to pass through (Fig. 2). A in high-power, placed by a single-pass, low heat-input weld diffuser was used to encourage laminar flow of high-brightness (Fig. 1). Lasers for material processing are cur- the shield gas. laser welds, rently available at power levels up to 20 kW, and development of even higher-power lasers is un- Welding and testing approach depending on derway. Commercially available laser power has A 15-kW ytterbium fiber laser (IPG Pho- the base been increasing more rapidly in recent years tonics Corp., Oxford, Mass.) was used to make material than industry’s ability to explore the potential full-penetration, 12-mm thick, square-butt chemistry. applications for these systems. welds required for this research. Four base ma- While the nuclear industry has been using terials investigated were ASTM A508 Gr. 2 steel laser welding on fuel assemblies for years, the forging, 316L stainless steel, Inconel 690, and lack of large-scale construction projects some- solution-annealed Inconel 718. Depending on what prevented the U.S. commercial nuclear the material being welded, the laser power was power industry from exploring other applica- set between 10 and 14 kW. A travel speed of 2 tions for laser welding. With plans to produce m/min was used for all welds. Visual inspec- new nuclear facilities in the U.S. over the next tion, radiographic testing (RT), and transverse several years, it is time for the nuclear industry cross sections were used to analyze weld qual- to reexamine potential applications for high- ity. RT was performed per ASME Section V, power, high-brightness lasers in the fabrication while weldment mechanical properties and of these facilities. soundness were analyzed using microindenta- This article summarizes EWI’s ongoing ef- tion-hardness tests, tensile tests (ASTM E8), fort to better understand the effect of shielding Charpy V-notch tests (ASTM E23), and bend gas composition on visual and mechanical tests (ASTM E290 and E190). properties of high-power, high-brightness laser Visual inspection revealed that shielding gas welds. Argon and nitrogen shielding gas were composition affects the appearance of the high- examined because they are both commonly brightness laser welds. For each of the base ma- used in laser welding (depending on the alloy terials tested, argon shielding gas produced being welded) and there are different manufac- better visual weld quality than nitrogen. Argon- Fig. 1 — Autogenous laser weld on 316L stainless steel made using 15-k IPG fiber laser (2- Inlet for root shield Trail shield Primary shield m/min travel speed). Fig. 2 —Shielding gas set-up. 24 ADVANCED MATERIALS & PROCESSES • SEPTEMBER 2012 sept amp features_am&p master template new QX6.qxt 8/23/2012 2:39 PM Page 25 High-Brightness Laser Welds shielded welds have an appealing convex weld profile at the cap and root (likely due to the high surface tension of liquid steel in an argon environment), while nitrogen-shielded welds have undercut and root concavity. The difference in visual quality between argon- and nitrogen-shielded welds was most apparent in the two nickel-base alloys (Fig. 3). The difference in visual quality was less apparent on 316L stain- less steel and A508 steel forging. Argon would be the pre- ferred shielding gas if visual appearance was the only criteria for judging weld quality. Radiographic testing revealed that shielding gas com- Fig. 3 — Effect of shielding gas composition on Inconel 690 laser welds. position also affects the amount of porosity resulting from laser welding. Gross porosity was detected in each of the three argon-shielded A508 steel welds, and would be con- sidered reject welds by most welding standards. Nitrogen- shielded A508 steel welds had no detectable porosity. The exact mechanism by which nitrogen produces cleaner welds is not fully understood. Higher solubility of nitrogen (compared to argon) in molten steel is a likely contributor, but further research is required to determine if this is the only reason. All other base material/shielding gas combina- tions were free of detectable porosity. Fig. 4 — Effect of shielding gas composition on A508 steel Weld properties and soundness laser welds. Examination of transverse cross sections of welds from base material and not different base material/shielding-gas combinations con- the fusion zone or heat firmed the main findings from visual testing and radiogra- affected zone. phy; i.e., argon resulted in visually acceptable cap and root Failure of 316L ten- profiles, but caused gross porosity in the A508 welds (Fig. sile specimens occurred 4). In addition, cross sections revealed that microporosity in the weld fusion zone. (fractions of a millimeter in diameter) was present in the There was negligible dif- other argon-shielded welds. The total volume of microp- ference between the ulti- orosity was low, but it is significant that it was present in all mate strength of the argon-shielded welds, but not in nitrogen-shielded welds. nitrogen-shielded welds Higher solubility of nitrogen is likely the reason for this versus the argon- phenomenon. shielded welds (both Fig. 5 — Microporosity in the fracture Hardness testing revealed that shielding gas composi- failed at 91% of the base surface of argon-shielded 316L tensile tion did not significantly impact weld hardness. All laser material strength). How- specimen. welds on the A508 steel forging exhibited high hardness in ever, there was a notice- the fusion zone regardless of the shielding gas composi- able drop in the 0.2% yield strength of the tion. Hardness of 316L welds was similar to base material nitrogen-shielded welds. While yield strength of argon- hardness. Both nickel-base alloys exhibited hardening of shielded welds matched that of base material, nitrogen- the weld regardless of the shielding gas composition (likely shielded welds yielded at only 82% of the base material a strain-induced hardening phenomenon). yield strength. Further analysis is required to determine Cross-weld tensile tests showed that both shielding gas the cause for the reduction in yield strength. compositions resulted in welds that were stronger than the Microporosity was present in the fracture face of base material (tensile specimens failed in the base metal) argon-shielded tensile specimens and could have con- for the A508 material. Therefore, it is difficult to conclude tributed to the fusion zone failure (Fig. 5). Microporosity if shielding gas composition affected weld strength; any af- was not observed in nitrogen-shielded welds. Changes in fect was not enough to degrade the weld properties to the grain size and orientation (weld centerline solidifica- below base material strength. tion pattern) are other potential causes for failure in the There is a direct relationship between tensile strength weld fusion zone. It should be noted that tensile proper- and hardness in carbon steels; tensile strength increases ties of welds produced using both gas compositions ex- with increasing hardness. High weld hardness explains why ceeded ASTM A240 standard minimum values 316L. the failure of the A508 tensile specimens occurred in the Failure of the Inconel 690 tensile specimens occurred ADVANCED MATERIALS & PROCESSES • SEPTEMBER 2012 25 sept amp features_am&p master template new QX6.qxt 8/22/2012 10:02 PM Page 26 welds performed slightly better, absorb- ing 60% of the energy. High-hardness equates to good weld strength in carbon steels, but it typically has a negative im- pact on toughness. Low toughness values obtained in WCL tests of A508 welds confirm this. Porosity in argon-shielded welds likely contributed to lower WCL toughness of these welds compared to ni- trogen-shielded welds. Therefore, it can be concluded that shielding gas composi- tion has an indirect impact on toughness Fig. 6 — Weld centerline solidification of A508 welds through the formation of pattern typical of high-speed, low heat-input welds. porosity. Low heat input and high cooling rate in the weld fusion zone. Nearly identical of autogenous laser welding contributed to values were obtained for 0.2% yield high hardness and corresponding low strength and ultimate strength of the toughness of A508 welds. Preheating the argon- and nitrogen-shielded welds. Weld base material prior to welding would slow ultimate strength was approximately 96% the cooling rate and potentially lower the of base material strength, and weld yield hardness while improving weld toughness. strength was only 75% of base material WCL toughness of argon- and nitro- strength. Weld centerline-solidification gen-shielded 316L welds was slightly pattern typical of high-speed, low heat- lower than base material toughness input welds was a likely contributor to fu- (-160ºC test temperature). Argon- sion zone failures at less than base shielded welds absorbed 71% of the en- material strength (Fig.
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