Effect of ultrasonic intensity on microstructure and mechanical properties of steel alloy in direct energy deposition-Arc.

To study the effect of ultrasonic intensity on the microstructure and mechanical properties during the direct energy deposition-Arc (DED-Arc) of ER70S-6 steel alloy, an ultrasound assisted DED-Arc system was developed by coupling ultrasonic energy with the electric arc deposition process. The propagation and vibration distribution of ultrasound in the substrate were analyzed by numerical simulation method. Deposition layers were fabricated using different ultrasonic amplitudes, and the microstructure, microhardness and tensile properties of the fabricated parts were systematically investigated. The results show that as the ultrasonic intensity increased, the grain refinement area expanded from the center of the molten pool to the surrounding area, and the grain morphology transforms from coarse columnar grains to fine equiaxed grains. When the ultrasonic amplitude was 15 µm, the grain refinement area of the cross-section was 94.6%, the average grain size was significantly increased to about grade 6. The microhardness increased by 10.6%. Thousands of ultrasonic cavitation events not only enhance the supercooling and wettability of the melt pool to promote nucleation, but also break the columnar grains into small grains by intense shock waves, which significantly improve the microstructure homogeneity and mechanical properties. The research provides an alternative approach to overcoming the long-standing problem of coarse columnar grains in the field of DED-Arc.

Microstructure and Mechanical Properties of Gradient Interfaces in Wire Arc Additive Remanufacturing of Hot Forging Die Steel.

Hot forging dies are subjected to periodic thermal stress and often fail in the forms of thermal fatigue, wear, plastic deformation, and fracture. A gradient multi-material wire arc additive remanufacturing method for hot forging dies was proposed to extend the service life of hot forging dies and reduce total production costs. The properties of multi-material gradient interfaces play a critical role in determining the overall performance of the final products. In this study, the remanufacturing zone of a hot forging die was divided into three deposition layers: the transition layer, the intermediate layer, and the strengthening layer. Experiments of wire arc additive manufacturing with gradient material were conducted on a 5CrNiMo hot forging die steel. The microstructure, microhardness, bonding strength, and impact property of gradient interfaces were characterized and analyzed. The results revealed that the gradient additive layers and their interfaces were defect-free and that the gradient interfaces had obtained a high-strength metallurgical bonding. The microstructure of the gradient additive layers presented a gradient transformation process of bainite-to-martensite from the bottom to the top layer. The microhardness gradually increased from the substrate layer to the surface-strengthening layer, forming a three-level gradient in the range of 100 HV. The impact toughness values of the three interfaces were 46.15 J/cm2, 54.96 J/cm2, and 22.53 J/cm2, and the impact fracture morphology ranged from ductile fracture to quasi-cleavage fracture. The mechanical properties of the gradient interfaces showed a gradient increase in hardness and strength, and a gradient decrease in toughness. The practical application of hot forging die remanufactured by the proposed method had an increase of 37.5% in average lifespan, which provided scientific support for the engineering application of the gradient multi-material wire arc additive remanufacturing of hot forging dies.

Laser radius measurement method based on the amplitude jump of a thermal wave.

The laser radius should be calculated with high precision when analyzing the characteristics of a Lamb wave excited by a laser in a thin plate. However, traditional methods for measuring the radius of a laser beam are complex. The paper aims to propose a novel convenient method, to the best of our knowledge, for measuring the radius of a laser ultrasonic beam. A laser interferometer (receiving laser) is used to receive ultrasonic signals excited by the laser to be measured (exciting laser) on the surface of a test block. Considering the characteristics represented by the thermal wave, positions where the receiving beam contacts and separates from the exciting beam are determined, and the spot of the radius is calculated by corresponding geometric relations. Experiments show that the accuracy of the proposed method is in the order of magnitude of 0.01 mm, and the relative errors of experimental data are within 2%. This paper provides an alternative method for the measurement of the laser beam radius and has great significance for the application of laser ultrasonic detection in the thin plate and the calculation and analysis of the dispersion curve.