1、Best Practices Entry: Best Practice Info:a71 Committee Approval Date: 2000-03-06a71 Center Point of Contact: MSFCa71 Submitted by: Wil HarkinsSubject: Welding Practices for 2219 Aluminum and Inconel 718 Practice: Gas Tungsten Arc Welding and Variable Polarity Plasma Arc Welding are preferred for joi
2、ning 2219 Aluminum, and Electron Beam Welding is preferred for joining Inconel 718 in critical aerospace flight applications.Programs that Certify Usage: Programs That Certified Usage: Saturn I, Saturn V, Space Shuttle External Tank, and Space Shuttle Main Engine.Center to Contact for Information: M
3、SFCImplementation Method: This Lessons Learned is based on Reliability Practice No. PD-ED-1205; from NASA Technical Memorandum 4322A, NASA Reliability Preferred Practices for Design and Test.Benefit:Adhering to proven design practices and processing techniques for 2219 Aluminum and Inconel 718 will
4、result in high performance joints, reduced weld defects, reduced weld repair costs, and reduced inspection costs. These practices, if conscientiously applied, will reduce the potential for galvanic corrosion, hot cracking, imperfect bead shape, inclusions, lack of fusion, lack of penetration, microf
5、issuring, mismatch, peaking, porosity, residual stresses, start/stop defects, and stress corrosion Provided by IHSNot for ResaleNo reproduction or networking permitted without license from IHS-,-,-cracking.Implementation Method:Three types of welding for two commonly used materials have been used pr
6、edominantly at NASA/MSFC for the welding of aerospace hardware: (1) Gas Tungsten Arc Welding (GTAW) for 2219 aluminum; (2) Variable Polarity Plasma Arc Welding (VPPAW) for 2219 aluminum; and (3) Electron Beam Welding (EBW) for Inconel 718. In Gas Tungsten Arc Welding for 2219 aluminum, heat required
7、 to join the aluminum is generated through an electrical arc applied at the joint. An inert atmosphere of helium surrounds the arc to prevent oxidation during the welding process. The type of GTAW covered in this practice is direct current, straight polarity (DCSP) in which the torch serves as the n
8、egative electrode (cathode) and the work piece as the positive electrode (anode). In Variable Polarity Plasma Arc Welding for 2219 aluminum, an arc gas (argon) is constricted by an orifice in the torch so that it forms a narrow column of high density gas that carries the arc current. Current is reve
9、rsed up to 20 percent of the time in a 20 to 25 microsecond cycle to provide a cleaning action to the work piece. Electron Beam Welding is performed in a vacuum and generates heat for fusing adjoining metals by impacting high kinetic energy electrons upon the work piece surface. Definitions of selec
10、ted welding terms applicable to the three types of welding are provided at the end of this section.The three types of welding, along with distinguishing recommended practices for each, are shown in Table 1. Among the important practices that will aid in ensuring high reliability welds are welding in
11、 the proper position, use of high purity shield and plasma gasses, proper cleaning of the joint prior to welding, operator certification, and computer control of the welding process. The use of 2-percent thoriated tungsten electrodes for GTAW and VPPAW provides arc stability and increases electrode
12、service life over that of standard tungsten electrodes. Important additional precautions for all three types of welding are use of rigid tooling to reduce weld joint deformation, nonmagnetic tooling to prevent skewing of the arc due to magnetic deflection, and correct and properly marked weld rod an
13、d wire. Automation is highly desirable for all three methods to maintain weld uniformity. If tack welds are used to temporarily hold adjoining parts in place, they should be consumed during the welding process. Specific characteristics, parameters, precautions, and criteria for each type of welding
14、are described in the following three paragraphs:1. Gas Tungsten Arc Welding (GTAW) (DCSP) for 2219 AluminumAs shown in Table 1, the flat (downhand) position is preferred for the best weld uniformity and penetration. Welding in the direct current, straight polarity (DCSP) mode while using high purity
15、 helium shield gas provides deep penetration without oxidation or contamination. (Welding in the alternating current mode is particularly suitable for welding thin aluminum as it produces less heat and provides good cathodic cleaning.) Pulsing of the weld current can be used to provide better contro
16、l for some out-of-position welds. Minimizing the number of weld passes will decrease the tendency for distortion. A pointed tip electrode is used in GTAW. A positive torch “lead angle“ is Provided by IHSNot for ResaleNo reproduction or networking permitted without license from IHS-,-,-desirable for
17、GTAW (DCSP) to provide preheating and more uniform melting.Lower energy inputs generally increase weld strength. GTAW (DCSP) is a preferred process for tack welding of aluminum. GTAW (DCSP) also can be used for welding steels (including stainless steel), titanium, magnesium (with care), and refracto
18、ry metals. The 2219 aluminum is an excellent alloy for maximum strength in cryogenic applications with good weldability.Table 1. Weld Characteristics/Parameter/Criteria for Three Types of Welding CHARACTERISTIC OR PARAMETER RECOMMEND OR STANDARD PRACTICES GAS TUNGSTEN ARC WELDING (2219 Al) VARIABLE
19、POLARITY PLASMA ARC WELDING (2219 Al) ELECTRON BEAM WELDING (INCONEL 718) Preferred Position Flat Vertical Flat Shield GasHelium (99.999% Purity) Helium (99.999% Purity) Vacuum Plasma Gas N/A Argon (99.999% Purity) N/A Backing Required No No In Some Instances (1) Preferred Electrode2% Thoriated Tung
20、sten 2% Thoriated Tungsten (Tungsten Filament) Appropriate for Repair Yes Not Usually (2) Yes Cleaning RequirementsMechanical Removal of Oxide, Free of Hydrocarbons Mechanical Cleaning Not Required, Degrease Only Special Cleaning for Vacuum Requirements Used for Tack Welding Yes No Yes Computer Cont
21、rol Desirable Essential Desirable Most Prominent Potential DefectsOxide and Tungsten Porosity, Lack of Penetration or Fusion Undercut, Lack of Fusion Improper Seam Tracking, Microfissuring Provided by IHSNot for ResaleNo reproduction or networking permitted without license from IHS-,-,-N/A: Not Appl
22、icableNotes: 1. Backing bars are used for EBW in some instances to ensure full penentration without overshooting or to eliminate excessive spatter.2. VPPAW usually is employed in an initial manufacturing process since it is the most sophisticated method and the most adaptable to automation. Repair o
23、f larger seam welds may be appropriate for jobs of sufficient size warranting setup.refer to D descriptionProvided by IHSNot for ResaleNo reproduction or networking permitted without license from IHS-,-,-D 2. Variable Polarity Plasma Arc Welding (VPPAW) for 2219 AluminumFigure 1 shows three variatio
24、ns of plasma arc welding: (1) Straight Polarity; (2) Reverse Polarity; and (3) Variable Polarity. Variable Polarity Plasma Arc Welding retains the high heating capacity of the straight polarity process while offering the part cleaning feature of reverse polarity. A flat-tipped electrode is used in V
25、PPAW. A minimum of reverse cycle time is required to keep electrode erosion low. As in the case of GTAW (DCSP), lower energy inputs generally increase weld strength and minimize distortion. Axial torch rotation to align the arc with the weld joint, and torch designs incorporating self-centering elec
26、trodes will compensate for electrode tip erosion and will provide greater accuracy in seam tracking and weld bead uniformity. A positive torch “lead angle“ of 0 to 3 degrees is desirable for automated welding with VPPAW. Electromagnetic interference caused by the welding process will require shieldi
27、ng or distance separation of computer monitoring and control devices. High purity grades of both the plasma gas and the shield gas are required. VPPAW also can be used for welding steels, Inconel, and other metals.3. Electron Beam Welding (EBW) of Inconel 718EBW produces deep, narrow welds with para
28、llel sides and a narrow heat-affected zone. It is performed in a vacuum in most aerospace applications, and a near-zero joint gap is required to ensure fusion of the parts. EBW provides minimum distortion because of low total heat input whether used on thick or thin sections; however, full penetrati
29、on welds may result in excessive spatter. The vacuum environment requires special cleaning. The size of work to be welded is limited by the size of the vacuum chamber and configuration of the manipulator. While EBW can be used to weld almost any metal, it often is used on high melting point metals i
30、ncluding the refractory metals requiring stringent control of oxides. EBW is suitable for welding dissimilar metals and parts of dissimilar mass.DESIGN CONSIDERATIONS FOR WELDED COMPONENTS As will be seen in “Impact of Nonpractice,“ the concepts of a workable welded part design and a producible and
31、inspectable weld joint configuration are very important to successful welding using any of the three processes described. The design of weld joints and of the special processes, tooling, and equipment needed to weld each configuration is best accomplished through use of a team approach in which the
32、designer consults with materials, stress analysis, weld engineering, Provided by IHSNot for ResaleNo reproduction or networking permitted without license from IHS-,-,-manufacturing, and inspection personnel as the welded design evolves. The process to be used to create the weld; the welds fracture m
33、echanics and fatigue properties in its planned environment; compatibility of the materials to be welded; the desired strength of the completed weld joint; and the method to be used to inspect the weld are among the many important factors that must be considered in weld joint design. Manufacturing, q
34、uality assurance, and design engineering personnel working hand-in-hand in this team approach will ensure reliable weld configurations. The need for this team approach cannot be overemphasized.Among the design practices for welding aerospace hardware are: (1) detailed consideration of fracture mecha
35、nics and fatigue effects, particularly in welding very thick materials to very thin materials; (2) designing, testing, and qualifying coupons of new weld configurations; (3) locating welds to avoid bending forces that concentrate stresses in the weld bead area; and (4) the design of joints that acco
36、mmodate adequate visibility, tool access, and inspectability. Conscientious adherence to the team approach to weld configuration and process design will account for these and many other factors that will result in defect-free, inspectable, and reliable welds.NONDESTRUCTIVE EVALUATION OF WELDS In add
37、ition to visual inspection, there are a number of methods to inspect welds. These include x-radiography, ultrasonics, eddy current, dye penetrant, and magnetic particle inspection. Real-time inspection can be performed while the welding is being performed in automated systems that incorporate weld b
38、ead profiling, infrared detection, and x-ray image display graphics. Although x-radiography is suitable for detecting voids or discontinuities in the weld or the parent material, fine surface cracks often go undetected. Double-walled inspections by this method should be avoided. This method is limit
39、ed by the welded part configuration because the film used to record the x-ray image must be placed to provide a suitable angle of incidence. Ultrasonics has some physical limitations due to thickness and angle of assembly of parts (angles less than 45 degrees cannot be effectively inspected using ul
40、trasonics). Eddy current inspection, which measures induced current in the weld and parent material in the presence of a magnetic field, requires highly skilled technicians, but is a favored method of weld inspection for many weld NDE situations. Both the ultrasonic and eddy current methods are sens
41、itive to most weld and parent material flaws. Real-time inspection and recording of NDE results, as is done with automated weld bead profiling, provides a permanent record of weld parameters as the weld bead is generated. Automated NDE techniques offer the advantage of on-site correction of welding
42、operations before detrimental effects of inaccurate welding become excessively costly.Technical Rationale:The design and processing recommendations in this practice have evolved over several decades of fabricating large aerospace vehicles and related ground support equipment. Rationale is documented
43、 in greater detail in the standards, specifications, technical memoranda, and reports listed as references. General processes are included in References 1, 2, and 7 through 9. Specific experience Provided by IHSNot for ResaleNo reproduction or networking permitted without license from IHS-,-,-on VPP
44、AW is described in References 5 and 6. The effect of impurities in plasma arc welding gasses was explored in an in-depth contracted effort by the University of Texas at El Paso (Reference 4). The causes of and remedies for microfissuring of welded Inconel 718 are described in Reference 3. Work is co
45、ntinuing to further explore and document effective practices for welding automation and nondestructive testing procedures. This continuing work will be the basis for additional practices that may be submitted for publication at a later date.References: Publications that contain additional informatio
46、n related to the practice are:1. Nunes, Jr., A. C., “A Comparison of the Physics of Gas Tungsten Arc Welding (GTAW), Electron Beam Welding (EBW), and Laser Beam Welding (LBW),“ NASA TM 86503, NASA/MSFC, August 1985.2. Yang, H. Q. and Przekwas, A. J., “A Mathematical Model to Investigate Undercutting
47、 and to Optimize Weld Quality,“ CFRDC Report 4095/2, CFD Research Corporation, June 1990.3. Nunes, Jr., A. C., “Interim Report on Microfissuring of Inconel 718.“ NASA TM-82531, NASA/MSFC, June 1983.4. McClure, John C., “The Effect of Impurity Gasses on Plasma Arc Welded 2219 Aluminum,“ NAS-8-37425,
48、The University of Texas at El Paso, August 1989.5. Nunes, Jr., A. C., “The Variable Polarity Plasma Arc Welding Process: Its Application to the Space Shuttle External Tank - First Interim Report,“ NASA TM-82532, NASA/MSFC, June 1983.6. Nunes, Jr., A. C., “The Variable Polarity Plasma Arc Welding Pro
49、cess: Its Application to the Space Shuttle External Tank - Second Interim Report,“ NASA TM-86482, NASA/MSFC, November 1984.7. Schuerer, Paul H., “Welding Aluminum Alloys,“ MSFC-SPEC-504C, NASA/ MSFC, November 1990.8. Schwinghamer, R. J., “Welding: The Fusion Welding of Steels, Corrosion and Heat Resistant Alloys,“ MSFC-SPEC-560A, NASA/MSFC, June 1988.9. “Welding, Electron Beam, Process for,“ MIL-W-46132, Depar
