Standards for Welding Structural Aluminum
The National Aeronautic and Space Administration sets standards for welding structural aluminum framing. These standards are compiled and published as the Unified Facilities Guide Specifications. Printed standards address and detail aluminum preparation, inspection procedures, levels of inspector examinations and classifications used for specific welding joints.
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Classifying Fabrications
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Types of joint welding are referred to as fabrications. Classifications are A, B, C and D with D representing the kind of weld that is liable to cause the least damage upon joint failure. Class D welds are plug and slot joints that weld overlapping metals. Penetration groove and fillet weld joints which are not subjected to a lot of stress are Class C fabrications. Class B grooves and joints are the same as those of Class C except their malfunction can result in the loss of structural functionality. Class A fabrications are complete penetration weld joints that upon failure can cause hazards to welders. If Class A welds cripple under stress, there is no redistribution of the stress.
Aluminum Temperature Preparation Standards
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Aluminum must be completely dry and pre-heated before welding is commenced. Aluminum and base metals surrounding the joint must be 50 degrees Fahrenheit or 10 degrees Centigrade. If temperatures are verified to be lower, welders should pre-heat all areas that are at least three inches or 75 millimeters from the joint to be welded to 100 degrees Fahrenheit, or 38 degrees Centigrade. Welds of cross sections of metal are referred to in the industry as interpass welds. Interpass welds require that the interpass temperature, or base temperature of the aluminum, does not fall below 100 degrees Farenheit or 38 degrees Centrigrade during a cross-sectional weld. The base metal temperature and the welded metal temperatures must ideally be the same.
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Inspection Standards
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Structural aluminum welding requires inspection of the metal before and after welding with a magnifying lens of [5X] and [10X] power, radiography, liquid penetrant and ultrasonography. Pre-performance inspections guarantee that the metals meet minimum contractual requirements. Post-welding inspections ensure that workmanship meets minimum contractual requirements. The size and contours of the welds are examined. Unacceptable welds must be repaired and re-inspected until found to be satisfactory. After passing inspections, weld joints have to be cleaned, brightened and smoothed to remove pits and scars in the metal.
Post-Welding Examination Levels
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Level one examinations require a 100 percent visual inspection and a 100 percent radiographic inspection of each joint in an aluminum structure. If radiography is impractical, a liquid penetrant inspection should be performed on the root pass of each weld and the weld joint's surface. Level two examinations require a 100 percent visual inspection and a 100 percent liquid penetrant inspection of each joint's surface. A level three examination are less stringent requiring only a 100 percent visual inspection of each joint in an aluminum structure.
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References
- Unified Engineering Inc.: Welding Structural Aluminum Framing: 1.2 Definitions
- Welding Innovation Vol. XV, No. 1, 1998: R. Scott Funderburk: Key Concepts in Welding Engineering
- Unified Engineering Inc.: Welding Structural Aluminum Framing: 3.5.1 Inspection
- Unified Engineering Inc.: Welding Structural Aluminum Framing: 3.3 Heat Input Requirements
Resources
- Photo Credit welding image by glgec from Fotolia.com
