The yield strength of CP titanium welds was measured for welds cooled at rates o
ID: 3156058 • Letter: T
Question
The yield strength of CP titanium welds was measured for welds cooled at rates of 10 °C/s, 15 °C/s, and 28 °C/s. The results are presented in the following table. (Based on the article "Advances in Oxygen Equivalence Equations for Predicting the Properties of Titanium Welds," D. Harwig, W. Ittiwattana, and H. Castner, The Welding Journal, 2001:1 26s- 136s.)
a. Construct an ANOYA table. You may give a range for the ? -value.
b. Can you conclude that the yield strength of CP titanium welds varies with the cooling rate?
Explanation / Answer
strength of CP titanium
demonstrates the mechanical properties of laser welded Co-Cr alloys along with a brief description of the experimental parameters tested in the current dental literature. Despite the diversity of experimental parameters, the general trend is that the welded groups have similar or significantly lower fracture strength compared to the reference groups. This finding is easily attributed to pores, gaps and other flaws developed at the joint during the welding procedure. 10,11,21,23 These multiple flaw type populations can be responsible for the relatively poorer agreement of the welded groups' failure probability curves in comparison to that of the cast group R (Fig. 4).31 The characteristic strength signifies the tensile stress below which 63.2% of the specimen population are expected to fail and thus the higher the better for the longevity of each group. Based on the results of this study, Weibull analysis indicated a significantly higher probability of failure for group I compared to R (Table 2,Fig. 4). The Weibull modulus m is a measure of the strength variability (the lower the m the higher the variability of strength). In a clinical scenario group I would therefore be expected to be more likely to fracture in low loading conditions as it is schematically presented in the sigmoid failure probability curves in Fig. 4. It seems that the I joint is more vulnerable in pore formation and unfused areas during welding.10,11,21,23 Conversely, it is suggested that the K joint might be less sensitive to pore formation as welding is initiated from a certain point and the material is added towards the surface of the framework spot by spot Fractography revealed internal pores (Fig. 5) in both I and K groups and thus internal porosity still remains a complication. Fractographic analysis of group R (Fig. 5A, B) is in accordance to previous studies, indicating a dendritic brittle fracture.11,21,23 This dendritic structure is also retained for the welded groups but with a much finer structure which can be identified only in a high magnification (Fig. 5D), a finding that might be explained by the rapid cooling rate of the welding spot.
The quality of laser welded joints is dependent on a variety of factors16 and the current knowledge is limited to only a few of them. Dental technicians and operators are provided with only a few general guidelines for laser welding procedures but these are far from a widely accepted consensus including framework dimensions, materials type, joint geometry and laser conditions to achieve more reliable joints. Further research is definitely required to deepen our knowledge in the field in order to optimize the laser welding procedure for dental alloys.
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CONCLUSION
While the microstructure of laser welded Co-Cr joints is dissimilar to that of the cast alloy, no significant difference in elemental composition exists. The I joint configuration is mechanically inferior to R. Within the limitations of this study, it is suggested that the K shape joint configuration should be preferred over the I, as it demonstrates improved in vitro mechanical strength and survival probability.
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References
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