Effect of scanning speed during PTA remelting treatment on the microstructure and wear resistance of nodular cast iron

The surface of nodular cast iron （NCI） with a ferrite substrate was rapidly remelted and solidified by plasma transferred arc （PTA） to induce a chilled structure with high hardness and favorable wear resistance. The effect of scanning speed on the microstructure, micro-hardness distribution, and wear properties of PTA-remelted specimens was systematically investigated. Microstructural characterization in-dicated that the PTA remelting treatment could dissolve most graphite nodules and that the crystallized primary austenite dendrites were transformed into cementite, martensite, an interdendritic network of ledeburite eutectic, and certain residual austenite during rapid solidifica-tion. The dimensions of the remelted zone and its dendrites increase with decreased scanning speed. The microhardness of the remelted zone varied in the range of 650 HV0.2 to 820 HV0.2, which is approximately 2.3-3.1 times higher than the hardness of the substrate. The wear re-sistance of NCI was also significantly improved after the PTA remelting treatment.

Effects of scanning speed on microstructure in laser surface-melted single crystal superalloy and theoretical analysis

Scanning speed is a critical parameter for laser process, which can play a key role in the microstruc- ture evolution of laser melting. In the laser melting of single crystal superalloy, the effects of scanning speed were investigated by experimental analysis and computational simulation. The laser was scanning along [710] direction on （001） surface in different speeds. Solidification microstructures of dendrites growth direction and the primary dendritic spacing were analyzed by metallograph. Besides, a planar interface during solidification was taken into attention, Experiment results indicated that the primary dendritic spacing and thickness of planar interface decrease with the increase of speed. Through simu- lation, distribution of dendrites growth velocity and thermal gradient along dendrite growth direction were calculated, and the simulation of dendrites growth direction agreed with the experiment results. Additionally, a constant value was acquired which can be used to predict the primary dendritic spacing. Moreover, according to curve-fitting method and inequality relation, a model was proposed to predict the thickness of planar interface.

Keyhole-induced Porosity in Laser Manufacturing Processes: Formation Mechanism and Dependence on Scan Speed

Laser welding and laser-based powder-bed fusion additive manufacturing in the deep penetration(keyhole)mode are promising technologies for the synthesis of metal components.The significant potential of these technologies remains latent because of structural defects(porosity),which significantly degrade the structural integrity and performance of the end products.Practical strategies for reducing those defects are addressed through fundamental understanding of their formation.In this study,pore formation of hydrodynamic origin is investigated,including the dynamics and mechanisms of the formation based on the above mentioned technologies.The pore volume and frequency of pore appearance,depending on the amplitude and frequency of capillary vibrations,are considered.Physical analysis is performed to obtain the scanning velocity values for the maximum and zero amplitudes and the frequency of capillary waves.A comparison between calculated curves and experimental data confirms both the capillary origin of the pores and the estimated scanning speeds at which the parameters of the pores exhibit their maximum values or vanish.The results obtained may facilitate in the selection of the optimal scanning speed when designing a pore-free technology.