J Adv Prosthodont.  2015 Feb;7(1):39-46. 10.4047/jap.2015.7.1.39.

Mechanical and interfacial characterization of laser welded Co-Cr alloy with different joint configurations

• Kokolis J
• Chakmakchi M
• Theocharopoulos A
• Prombonas
• Zinelis S

• KMID: 1974889
• DOI: http://doi.org/10.4047/jap.2015.7.1.39

Abstract

PURPOSE
The mechanical and interfacial characterization of laser welded Co-Cr alloy with two different joint designs.
MATERIALS AND METHODS
Dumbbell cast specimens (n=30) were divided into 3 groups (R, I, K, n=10). Group R consisted of intact specimens, group I of specimens sectioned with a straight cut, and group K of specimens with a 45degrees bevel made at the one welding edge. The microstructure and the elemental distributions of alloy and welding regions were examined by an SEM/EDX analysis and then specimens were loaded in tension up to fracture. The tensile strength (TS) and elongation (epsilon) were determined and statistically compared among groups employing 1-way ANOVA, SNK multiple comparison test (alpha=.05) and Weibull analysis where Weibull modulus m and characteristic strength sigmaomicron were identified. Fractured surfaces were imaged by a SEM.
RESULTS
SEM/EDX analysis showed that cast alloy consists of two phases with differences in mean atomic number contrast, while no mean atomic number was identified for welded regions. EDX analysis revealed an increased Cr and Mo content at the alloy-joint interface. All mechanical properties of group I (TS, epsilon, m and sigmaomicron) were found inferior to R while group K showed intermediated values without significant differences to R and I, apart from elongation with group R. The fractured surfaces of all groups showed extensive dendritic pattern although with a finer structure in the case of welded groups.
CONCLUSION
The K shape joint configuration should be preferred over the I, as it demonstrates improved mechanical strength and survival probability.

Keyword

Laser welding; Co-Cr alloys; SEM/EDX; Mechanical properties

MeSH Terms

Alloys*
Joints*
Tensile Strength
Welding
Alloys

Figure

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  Fig. 1 (A) Representative BE image from the microstructure of R group showing characteristic dendritic microstructure with the distribution of a heavier phase in the interdendritic spaces. (B) The interface between cast alloy (left) and laser welded region (right) for I and K groups. The interface between the cast alloy and the welding is easily distinguished due to the difference in microstructure (nominal magnification 300×). (C) A representative EDX spectrum analysis of the cast alloy and the welded region.

• 

  Fig. 2 BE image (A) from the cast alloy-welded region interface and corresponding EDX elemental maps for Co (B), Cr (C), Mo (D), Si (E) and Mn/O (F). The horizontal white line in BE image indicates the line scan analysis (bar: 20 µm).

• 

  Fig. 3 Line scan EDX analysis for Co, Cr and Mo of the cast alloy-welded region interface (I). The small picture at the bottom shows part of the BE image of Fig. 2A where the line scan was taken.

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  Fig. 4 Failure probability versus tensile loading after Weibull fitting for all groups tested.

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  Fig. 5 Representative SEI from the surfaces of all groups tested. (A) Mutual fractured surfaces from group R (Magnification 30×). (B) Higher magnification of Fig. A with intense dendritic structure. (Magnification 2000×). (C) Representative image of fracture surfaces from groups I and K with gross internal porosity (Magnification 30×). (D) Fracture surface at the vicinity of an internal pore with a finer dendritic structure compared to cast material (magnification 2000×). Inset: Higher magnification where the dendritic structure is clearly shown.

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