Laser cladding is one of the available technologies for repairing and strengthening material surfaces, especially for improving corrosion and wear resistance. However, the trade-off between the hardness and toughness properties of the coating induces cracking effects, which remains the biggest challenge limiting the practical applications of high hardness coatings fabricated through laser cladding. Due to its dependence on toughness and elongation properties of materials, the crack resistance can be evaluated using different methods like impact, bending and tensile tests. Although these conventional tests are effective, they are not suitable for assessing crack resistance of high hardness coatings due to the limitation of coating size, making it difficult to obtain an appropriate test sample with full coating.
These limitations can be addressed by using novel tests like miniatured tensile tests, which are not practically feasible because they require costly special equipment. Besides, the outcomes of these methods are significantly affected by the material thickness and microstructural uniformity. To date, there is a lack of particular test methods for evaluating the crack resistance of high hardness coatings, which has since attracted an increasing resistance owing to its practical importance.
The crack resistance of welded joints is currently evaluated using various special methods, among them being the self-restraint test, which is simple and effective. Previous results show that the stress concentration is too high to accurately assess the crack resistance of high hardness coatings fabricated by laser cladding. Moreover, the restraint strength is dependent on the size of the weld specimen and can be modified by adjusting the groove size to broaden the capability of the conventional restraint test method.
Inspired by the previous findings, Dr. Yulei Feng, Dr. Xiaotong Pang, Professor Kai Feng, Dr. Yueqiao Feng and led by Professor Zhuguo Li from Shanghai Jiao Tong University developed a novel test method (named Y-groove test) to evaluate the crack resistance of high hardness coatings fabricated using laser cladding. This crack test was based on improved self-restraint tests for welding processes. The relationship between the crack resistance of the coating and the product of plasticity and strength was evaluated. Finally, its feasibility was validated by using it to select and predict the preheating temperature of cladding processes. A total of four different high hardness and crack Fe-based coatings prepared via laser cladding, T1, T2, T3 and T4, were investigated. The work is currently published in the journal, Surface and Coatings Technology.
The researchers showed that the self-restraining crack rate reflected the crack resistance of the coating. The relationship between the restraint, crack resistance, width and depth of the Y-groove were demonstrated. An increase in the depth of the Y-groove increased the restraint of the specimen. Consequently, an increase in the width of the Y-groove decreased the specimen restraint. For the different coatings, the crack resistance order was T1>T3>T2>T4. The high residual stress and the interfacial bond strength reported at the matric and M2B interface was conductive to crack nucleation and propagation, leading to worse crack resistance for T4 coatings compared to other coatings.
Preheating temperature obtained from the test were used to prepared high hardness crack-free coatings. The resulting coating exhibited heavy thickness and high hardness values of 7 mm and 800 HV, respectively. Preheating decreased the temperature gradient to reduce the restraint stress. Furthermore, an increase in the preheating temperature was beneficial in reducing not only the differences between the room and solidification temperatures but also the residual stresses surrounding the reinforcements.
In summary, Shanghai Jiao Tong University scientists successfully evaluated the crack resistance of different high hardness coatings prepared using laser cladding using a newly proposed Y-groove test method. The empirical formulations were proposed to qualitatively evaluate the crack resistance, providing a clear understanding of the relationship between the crack rate, product strength and plasticity, and the width and depth of the Y-groove. This method can be used to select systems and predict preheating temperatures in laser cladding. In a statement to Advances in Engineering, Professor Zhuguo Li, the lead and corresponding author stated that the new proposed crack resistance test provides a general useful method for fabricating high hardness crack-free coatings for various metals and alloys.
Feng, Y., Pang, X., Feng, K., Feng, Y., & Li, Z. (2022). A method for evaluating the crack resistance and predicting the preheating temperature of high hardness coating prepared by laser cladding. Surface and Coatings Technology, 432, 128076.