The mismatch between the thermal expansion coefficients of ceramics and metals and the differential stresses it causes at the interface create problems in metal to ceramic joining. Research has been conducted to solve this problem in thermal barrier coating technology. Previous studies have considered metal-ceramic multilayers or graded-coatings, which include a metallic bond coat. In this study, a graded plasma-sprayed metal-ceramic coating is developed using the deposition of premixed metal and ceramic powders without the conventional metallic bond coat. Influences of thickness variations, number, and composition of the layers are investigated. Coatings are prepared by atmospheric plasma-spraying on Incone1718 superalloy substrates. Ni-Cr-AI and ZrO2 -8 % Y203 powders are used for plasma spraying. Adhesive and cohesive strength of the coatings are determined. The concentration profile of the elements is determined by x-ray energy-dispersive analysis. The microstructure and morphology of the coatings are investigated by optical and scanning electron microscopy (SEM). Results show that the mixed metal-ceramic coating obtained with the deposition of premixed powders is homogeneous. The morphology and microstrncture of the coatings are considered satisfactory. l

Stress relaxation of welded joints can increase the safety of structures. Although it is not absolutely clear why it happens, several researches have proved the phenomenon. This work presents the application of the ultrasonic technique to evaluate the stress relaxation in welds of 5052 aluminum. A special geometry was developed, so each welded plate had uni-axial stresses in the measurement region. Twenty-two plates were tested, eleven in the rolling direction and the remaining in the perpendicular direction. The plates were stress relief using a heat treatment to set an initial stress free reference state. After that, they were welded to create the stress field in the area of investigation. A final step was to cut each of the samples after different periods of time from the welding. The stress was measured in the uni-axial stress region. A calendar of the process was established to record precisely the time for each step. Longitudinal critically refracted waves were used to measure the stresses. The results showed a clear effect of relaxation. The magnitude was not as pronounced as in steel, but kept the same relation with the ultimate strength of material.

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Uso del ensayo de implante y la técnica de emisión acústica para estudiar el agrietamiento asistido por hidrógeno en la zona fundida en la soldadura de un acero HSLA-80 (#) H.C. Fals<*) y R.E. Trevisan^**)Resumen Se estudió el agrietamiento asistido por hidrógeno en la zona fundida, usando dos alambres tubulares (E 70T-5 y E 120 T5-K4) y una mezcla gaseosa de C0 2 + 5 % H 2 , para inducir altos niveles de hidrógeno difusible en la soldadura de un acero HSLA-80. Un sistema de medición de emisión acús-tica basado en un voltímetro RMS (Root Mean Sqüare), acoplado a la instalación de ensayo de implante (NF 89-100), permitió determinar la energía, amplitud RMS y número de ciclos de la señal. Se observó por microscopía óptica y electrónica de barrido, que en todos los ensayos las grietas se formaron en la zona parcialmente fundida. Cuando se usó el alambre E 70T-5, las grietas se propagaron a través de la región de crecimiento de grano de la zona térmicamente afectada, predominando el modo de fractura cuasiclivage. En los ensayos del alambre E 120 T5-K4, las grietas crecieron verticalmente a través de la zona fundida, verificándose mayor incidencia de un modo de fractura mixto. Se encontró una relación importante entre los parámetros de la señal de EA y los modos de fractura que actúan en el mecanismo de agrietamiento.Palabras clave: Aceros microaleados. Soldadura. Agrietamiento asistido por hidrógeno. Modos de fractura. Emisión acústica.Implant test and acoustic emisssion technique used to investígate hydrogen assisted cracking in the melted zone of a welded HSLA-80 steel AbstractWeld metal hydrogen assisted cracking was studied using two flux cored wire (AWS E 70T-5 and AWS E 120 T5-K4) and a mixture gas of C0 2 + 5 % H 2 to induce high valúes of diffusible hydrogen in high strength low alloy steel (HSLA-80) weldments. An Acoustical Emission Measurement System (AEMS) RMS voltmeter was coupled to the implant test (NF 89-100) apparatus to determine energy, amplitude and event numbers of signal. All cracks were initiated in the partially melted zone and propagated into the coarse-grained región of the heat affected zone when E 70 T5 consumable was used, and the quasi-cleavage fracture mode was predominant. When E 120 T5 K4 consumable was used the cracks propagated vertically across the fusión zone, and a mixed fracture mode was the most important. A significant relationship between acoustic emission parameters and fracture modes was found.

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