As far as mechanical properties are concerned, a laser weld is expected to be as strong, or stronger, than the parent material. Many different testing procedures are used to assess the performance of laser welded metallic materials. Generally, the choice of test depends on the service requirements of the weld.
For the shipbuilding industry, European classification societies have prepared 'Guidelines for the approval of CO2 laser welding'. A document to this effect The Classification Societies' Requirements for approval of CO2 Laser Welding Procedures has been produced and is used in practice. This document is quite exhaustive.
EURACS: 1996 Guidelines
More guidelines are being produced as an outcome of the Brite Euram projects SHILWACQ, BE 97-4223 and SHIPYAG, G3RD-CT-2000-00251.
1. Non-destructive testing of laser welds in metallic materials
Visual examination All butt and T joints can be examined for external imperfections such as: undercut, sagging, suck-back, crater cracking, surface cracking, surface porosity, blow holes, mismatch, excess weld metal, excessive penetration and spatter.
There are two established standards to evaluate the above mentioned imperfections -
- Welding and allied processes - Electron and laser beam welded joints - Guidance on quality levels for imperfections:
- Part 1: Steels
- EN ISO 13919-1: 1992
- ISO 13919-1: 1997
- Part 2: Aluminium and its weldable alloys
- EN ISO 13919-2: 2001
- ISO 13919-2: 2001
Radiography Internal imperfections such as cracking, porosity, inclusions, lack of penetration and lack of fusion are identified using this technique. All imperfections are evaluated in accordance with the standards given above.
Ultrasonic testing
To a certain extent, ultrasonic testing of laser welds is also carried out.
2. Destructive testing of laser welds in metallic materials
Largely, the destructive testing procedures used for evaluating arc welds are also applied to laser welds:
- Tensile testing (shear or peel for lap joints, break tests for fillet welds, all weld tensiles)
- Bend tests (root and face bend, longitudinal bend)
- Charpy tests and fracture toughness tests (e.g. KIc )**
- Fatigue testing
- Hardness evaluation (both macro and micro hardnesses)
- Corrosion testing (general corrosion, stress corrosion and intergranular corrosion tests based on the alloy)
- ** Please be aware that for very narrow, hard laser welds, a phenomenon known as fracture path deviation (FPD) can cause difficulties in the interpretation of Charpy and fracture toughness test results. Further information on FPD can be found in 'What is the significance of Fracture Path Deviation during Charpy testing of laser welds and how can it be prevented?'
The reader is advised to refer to the following documents for more detailed information:
- Welding procedures test:
- Electron and laser beam welded joints
- EN ISO 15614-11: 2002
- ISO 15614-11: 2002
- EN 895: 1995, Destructive tests on welds in metallic materials. Transverse tensile test
- EN 910: 1996, Destructive tests on welds in metallic materials. Bend tests
- EN 1320: 1997, Destructive tests on welds in metallic materials. Fracture tests
- EN 1321: 1997, Destructive tests on welds in metallic materials. Macroscopic and microscopic examination of welds
- ASME Boiler and Pressure Vessel Code, Section IX, Qualification standard for welding and brazing procedures, welders, brazers, and welding and brazing operators
- ASTM E 8M-99, Standard test methods for tension testing of metallic materials [metric]
- ASTM E 604-83, Standard test method for dynamic tear testing of metallic materials.
- ANSI/AWS B4.0-98, Standard methods for Mechanical testing of welds
- Hardness test method of laser weld metal (ISO 22826. Currently a pre ISO draft).
In spite of all the above, there is still a shortage of standards specific to laser welds in metallic materials. This has lead to a study by GKSS in Germany which is in the process of producing publications on recommendations for tensile and fracture toughness testing procedures for power beam welds. TWI is also involved in the formulation of standard test procedures for qualification of laser welds - more information for specific applications can be obtained from TWI's laser centre.