Microstructural evolution and hardness of as-cast Be-Al-Sc-Zr alloy processed by laser surface remelting

As-cast beryllium-aluminum(Be-Al)alloy exhibits a coarse microstructure with pore defects due to a large solidification interval,greatly limiting its mechanical properties.In this research,the relationship between laser surface remelting process and microstructure and hardness of as-cast Be-Al-Sc-Zr alloy was established.The experimental results demonstrated that a pore-free refined microstructure of remelted layer was obtained by controlling the parameter of effective laser energy input.The microstructure of as-cast Be-Al-Sc-Zr alloy consisted of equiaxed grains with Al phase forming a continuous frame wrapping Be phase,which was significantly refined in the remelted zone(from 25μm to 2μm).The Vickers hardness in the remelted zone(approximately 210 HV)was approximately 3 times that of as-cast Be-Al-Sc-Zr alloy.Analysis of the Vickers hardness and the Be phase size showed a good agreement with a Hall-Petch equation.In addition,transmission electron microscopy(TEM),auger electron spectroscopy(AES)and X-ray diffraction(XRD)analysis evidenced that Sc and Zr elements formed a single blocky phase Be13(Scx,Zr1-x),which was also greatly refined from 8μm to 1.5μm in the remelted zone.The results obtained in this study indicate that the laser surface remelting allowed refining the microstructure and further strengthening the Vickers hardness of Be-Al-Sc-Zr alloy.

Influence of laser surface remelting on microstructure and degradation mechanism in simulated body fluid of Zn-0.5Zr alloy

In this study,the Zn-0.5 wt%Zr(Zn-Zr)alloy was treated by laser surface remelting(LSR),and then the microstructure and degradation mechanism of the remelting layer were investigated and compared with the original as-cast alloy.The results reveal that after LSR,the bulky Zn(22)Zr phase in the original Zn-Zr alloy is dissolved and the coarse equiaxed grains transform into fine dendrites with a secondary dendrite arm space of about 100 nm.During the degradation process in simulated body fluid(SBF),the corrosion products usually concentrate at some certain areas in the original alloy,while the corrosion products distribute uniformly and loosely in the LSR-treated surface.After removing the corrosion products,it was found that the former suffers obvious pitting corrosion and then localized corrosion.The proposed mechanism is that corrosion initiates at grain boundaries and develops into the depth at some locations,and then leads to localized corrosion.For the LSR-treated sample,corrosion initiates at some active sites and propagates in all directions,corrosion takes place in the whole surface with distinctly uniform thickness reduction,while the localized corrosion and peeling of bulky Zn(22)Zr particles were eliminated.The electrochemical results also suggest the uniform corrosion of LSR-treated sample and localized corrosion of original sample.Based on the results,a new approach to regulate the corrosion mode of the biodegradable Zn alloy is proposed.

Prediction of Dendrite Orientation and Stray Grain Distribution in Laser Surface-melted Single Crystal Superalloy

A vectorization analysis technique for crystal growth and microstructure development in single-crystal weld was developed in our previous work. Based on the vectorization method, crystal growth and stray grain distribution in laser surface remelting of single crystal superalloy CMSX-4 were investigated in com- bination of simulations with experimental observations. The energy distribution of laser was taken into consideration in this research. The experimental results demonstrate that the simulation model applies well in the prediction of dendrite growth direction. Moreover, the prediction of stray grain distribution works well except for the region of dendrites growing along the [100] direction.

Effects of Substrate Crystallographic Orientations on Microstructure in Laser Surface-Melted Single-Crystal Superalloy:Theoretical Analysis

A geometric analysis technique for crystal growth and microstructure development in single-crystal welds had been previously developed.And the effect of welding conditions on the tendency of stray grains formation during solidification was researched.In the present work,these analytical methods were further extended.Combined with an original vectorization method,a 3D Rosenthal solution was used to determine thermal conditions of the welds.Afterward,the dendrite growth orientation,the dendrite growth velocity and the thermal gradient along dendrite direction were calculated and lively plotted.Finally,the tendency of stray grains formation in the solidification front was forecasted and its distribution was presented with a 3D plot.The results indicate that substrate orientation has some impacts on the crystal growth pattern,dendrite growth velocity,distribution of thermal gradient and stray grain.Based on the research methods proposed in this work,any substrate crystallographic orientation can be studied,and predicted stray grains distribution can be visualized.

Rapid Directional Solidification with Ultra-High Temperature Gradient and Cellular Spacing Selection of Cu-Mn Alloy

The detailed laser surface remelting experiments of Cu-31.4 wt pct Mn and Cu-26.6 wt pct Mn alloys on a 5 kW CO2 laser were carried out to study the effects of processing parameters (scanning velocity, output power of laser) on the growth direction of microstructure in the molten pool and cellular spacing selection under the condition of ultra-high temperature gradient and rapid directional solidification. The experimental results show that the growth direction of microstructure is strongly affected by laser processing parameters. The ultra-high temperature gradient directional solidification can be realized on the surface of samples during laser surface remelting by controlling laser processing parameters, the temperature gradient and growth velocity can reach 106 K/m and 24.1 mm/s, respectively, and the solidification microstructure in the center of the molten pool grows along the laser beam scanning direction. There exists a distribution range of cellular spacings under the laser rapid solidification conditions, and the average spacing decreases with increasing of growth rate. The maximum, λmax, minimum, λmin, and average primary spacing, A, as functions of growth rate, Vb, can be given by,λmax=12.54Vb-0.61, λmin=4.47 Vb-0.52, λ=9.09Vb-0.62, respectively. The experimental results are compared with the current Hunt-Lu model for rapid cellular/dendritic growth, and a good agreement is found.