WELD DEFECTS AND MATERIAL PROPERTIESBrett Leary

ME 296R Spring 2015

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CONTENTS

Objective

Relevance

Type of Defect

Inclusions (Porosity)

Conclusions

Questions

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OBJECTIVE

To describe the causes, effects, prevention methods, and detection techniques for a common weld defect

Inclusions (Porosity)

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RELEVANCE

No perfect materials

No flawless weld processes

Understanding defects is important

For choosing weld processes

For design limits

For failure investigations

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POROSITY - TYPES

Gas holes or voids found within a weld

Four main categories of porosity [1]:

Uniformly scattered

Clustered

Linear

Worm

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POROSITY - CAUSES

Caused by gases, lighter than the molten metal, that do not escape from the weld and are trapped within the weld as it solidifies

Root causes for porosity

Dirt and moisture on the base metal, in the electrodes and gases, and in the weld equipment [1]

Oxide layer on surfaces of aluminum alloys

Manual metal arc weld processes with coated electrodes contain moisture in the coating and thicker oxide layers in the weld wire [2]

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POROSITY - EFFECTS

Act as initiators for cracks that lead to fatigue failures

“Excessive concentration near one surface of the bead (most likely the top) can impose a bending component which amplifies the stress” [4]

Porosity affects “fillet welds far more adversely than butt welds,” reducing tensile strength by up to 60% [2].

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POROSITY - EFFECTS

“The single pore (is the) least detrimental in fatigue followed by co-linear porosity, and uniform porosity. Cluster porosity is predicted to be most detrimental” [3]

Clustered pores can grow together to behave like a single larger pore

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POROSITY - EFFECTS

Yield strength of aluminum is insignificantly affected by pores in the weld

Ultimate tensile strength is reduced as porosity grows [5]

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POROSITY - EFFECTS

“Ductility of the aluminiumweld metal is sharply reduced by the presence of small amounts of porosity” [5]

Affect occurs within the first 15% porosity loss in area

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POROSITY - EFFECTS

Fatigue life is severely shortened by the increased presence of porosity in aluminum welds [5]

This can be very significant to the product design

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POROSITY - PREVENTION

Cleanliness is critical

Base metal

Shielding gas

Filler metal

Etch or scrape to remove surface oxides

Preheat the weld zone

Maintain proper shielding gas flow rate [5]

Store filler metal to prevent contamination by dust and dirt, at the same temperature as the weld cell to prevent condensation [7]

Welder and fitter should wear gloves [7]

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POROSITY - PREVENTION

Greater likelihood to find porosity in manual GTAW over an automated electron beam or laser weld

Aluminum GTAWs contain many small pores

Aluminum EB welds have few pores, but tend to be larger

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POROSITY - DETECTION

Surface porosity

Visual inspection

Liquid penetrant

Eddy current

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POROSITY - DETECTION

Subsurface porosity

X-ray, CT

Gamma

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CONCLUSIONS

Root causes of porosity are cleanliness related

Detrimental to tensile strength, ductility, and life cycle fatigue

No effect to yield strength

Preventable with proper weld prep

Automated weld process result in less porosity

Easily detected by non-destructive tests

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REFERENCES

[1] Welding inspection (2nd ed., pp. 44, 47-48, 50-53, 56, 113-121). (1980). Miami, Florida: American Welding Society.

[2] Thielsch, H. (1967). The Sense and Nonsense of Weld Defects (1st ed., pp. 17-18, 21, 23, 25, 37, 46). Lake Zurich, Illinois: Monticello Books.

[3] Walsh, W., Leis, B., & Yung, J. (1989). Study to Determine the Influence of Weld Porosity on the Integrity of Marine Structures. SSC-334. Retrieved April 24, 2014, from http://www.shipstructure.org/pdf/334.pdf

[4] Rudy, J., & Rupert, E. (1970, July). Effects of Porosity on Mechanical Properties of Aluminium Welds. Welding Journal, 322-S. Retrieved April 24, 2015, from https://app.aws.org/wj/supplement/WJ_1970_07_s322.pdf

[5] Shore, R. & McCaulay R. (1970, June). Effects of Porosity on High Strength Aluminium 7039. Welding Research Supplement, 311-s. Retrieved April 24, 2015, from https://app.aws.org/wj/supplement/WJ_1970_07_s311.pdf

[6] FAQ: What are the typical defects in laser welds? (n.d.). Retrieved April 24, 2015, from http://www.twi-global.com/technical-knowledge/faqs/structural-integrity-faqs/faq-what-are-the-typical-defects-in-laser-welds/

[7] 6 Key Ways to Prevent Weld Failures. (2012, March 7). Retrieved April 24, 2015, from http://www.hobartbrothers.com/news/111/523/6-Key-Ways-to-Prevent-Weld-Failures.html

[8] Dilthey, U. (2005, January 1). Welding Technology 2 - Welding Metallurgy. Retrieved April 24, 2015, from http://www4.hcmut.edu.vn/~dantn/course/Pdf/WeldingTechnology2-English.pdf

[9] Understanding Weld Cracking, Its Causes, Consequences and Remedies. (2009, September 9). Retrieved April 24, 2015, from http://www.hobartbrothers.com/news/48/523/Understanding-Weld-Cracking-Its-Causes-Consequences-and-Remedies.html

[10] Gooch, T. (1974, July). Stress Corrosion Cracking of Welded Joints in High Strength Steels. Welding Research Supplement, 287-s. Retrieved April 24, 2015, from https://app.aws.org/wj/supplement/WJ_1974_07_s287.pdf

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QUESTIONSDiscussion of slag and tungsten inclusions, weld cracks, and incomplete fusion/penetration were not included here, but may be found the companion research paper

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APPENDIX

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POROSITY - CAUSES 20

POROSITY - CAUSES 21

POROSITY - EFFECTS 22

POROSITY - EFFECTS 23

POROSITY - PREVENTION 24