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Laser based directed energy deposition (L-DED) is a promising type of additive manufacturing technology. The non-destructive testing technology for the quality monitoring of L-DED processed parts is becoming more and more demanding in terms of accuracy, real-time, and ease of operation. This paper introduces a new image recognition system based on a deep convolutional neural network, which uses multiple lightweight architectures to reduce detection time. In order to eliminate the interference better, it improves the penalty function, which effectively improves the accuracy. Judging from the detection results of the data set, the accuracy of the model training reaches 94.71%, which achieves a very good image segmentation effect and solves the technical problem of in-situ monitoring of the L-DED process. This system realizes the positioning of the spatters for the first time, and at the same time, the number of spatters and area of molten pool are correlated to the laser scanning speed and the laser power. [ABSTRACT FROM AUTHOR]

Pores in the AlSi10Mg products fabricated by laser powder bed fusion (LPBF) decrease the density of components. To study the mechanism of the formation and evolution of hydrogen pores, a model based on molecular dynamics (MD) is built. The influence of initial hydrogen content, heating time, and cooling time on the porosity is analyzed. Low initial hydrogen content, long cooling time, and appropriate heating time are beneficial to decrease the porosity of LPBF-fabricated products. When the heating time is 40 ps, the cooling time is 200 ps and initial hydrogen content is 0.081%; porosity can be reduced to 3.01%. [ABSTRACT FROM AUTHOR]

Featured Application: This paper presents the in-situ laser polishing of four typical alloys in the selective laser melting (SLM) process. Selective laser melting (SLM) is a layer by layer process of melting and solidifying of metal powders. The surface quality of the previous layer directly affects the uniformity of the next layer. If the surface roughness value of the previous layer is large, there is the possibility of not being able to complete the layering process such that the entire process has to be abandoned. At least, it may result in long term durability problem and the inhomogeneity, may even make the processed structure not be able to be predicted. In the present study, the ability of a fiber laser to in-situ polish the rough surfaces of four typical additive-manufactured alloys, namely, Ti6Al4V, AlSi10Mg, 316L and IN718 was demonstrated. The results revealed that the surface roughness of the as-received alloys could be reduced to about 3 μm through the application of the laser-polishing process, and the initial surfaces had roughness values of 8.80–16.64 μm. Meanwhile, for a given energy density, a higher laser power produced a laser-polishing effect that was often more obvious, with the surface roughness decreasing with an increase in the laser power. Further, the polishing strategy will be optimized by simulation in our following study. [ABSTRACT FROM AUTHOR]

Heat-assisted magnetic recording (HAMR) technology is a great way to achieve higher storage density. In HAMR, dynamic performance of the head–disk interface (HDI) is critically important for the read/write stability. Lubricant bridge formation and lubricant transfer can significantly deteriorate HDI performance. In this paper, a 3-D full-atom model of a slider-lubricant-disk substrate system is established to study the dynamic performance of HDI in HAMR. The overlayers of the slider and disk substrate are made of diamondlike carbon, and the main lubricating ingredient is perfluoropolyether (PFPE). The formation and breaking of the lubricant bridge are investigated with the slider moving up and down. It is found that during the moving down process, PFPE molecules escape from the lubricant layer, and a lubricant bridge between the slider and disk is formed. During the moving up process, the lubricant bridge becomes thin and eventually breaks. The stable value of PFPE weight percentage on the slider during the moving up process is always larger than that during the moving down process. This is because of easier transfer of lubricant molecules to the slider in the low flying height region. The formation height is smaller than the breaking height, but these values are closer when the operational temperature is higher. [ABSTRACT FROM AUTHOR]

Currently, heat-assisted magnetic recording (HAMR) technology has been proposed as a method for enabling higher storage density. In this paper, the molecular dynamic simulation method was used to investigate the consumption and recovery of the lubricant perfluoropolyether (PFPE) layer under rapid heating and cooling through HAMR. A full-atom model for the 3-D slider-lubricants-disk substrate system was built for this paper. The lubricant adheres to the diamond-like carbon substrate, and the space between the slider and the lubricant is filled with helium atoms. It is found that HAMR laser heating results in the gradual consumption of the PFPE lubricant, and that the target operational temperature T0 significantly affects the consumption of the PFPE lubricant on the disk substrate. The extent of lubricant consumption increases sharply when T0 is above 600 K. The lubricant’s weight percentage on the disk substrate rises up again when cooling occurs. But it cannot be completely recovered. On the other hand, the bearing pressure reduces the degree of consumption of the PFPE lubricant by laser heating. However, the effect of bearing pressure on the amount of remaining PFPE lubricant becomes weak when T0 is above 800 K. [ABSTRACT FROM AUTHOR]

Laser polishing is a widely used technology to improve the surface quality of the products. However, the investigation on the physical mechanism is still lacking. In this paper, the established numerical transient model reveals the rough surface evolution mechanism during laser polishing. Mass transfer driven by Marangoni force, surface tension and gravity appears in the laser-induced molten pool so that the polished surface topography tends to be smoother. The AlSi10Mg samples fabricated by laser-based powder bed fusion were polished at different laser hatching spaces, passes and directions to gain insight into the variation of the surface morphologies, roughness and microhardness in this paper. The experimental results show that after laser polishing, the surface roughness of Ra and Sa of the upper surface can be reduced from 12.5 μm to 3.7 μm and from to 29.3 μm to 8.4 μm, respectively, due to sufficient wetting in the molten pool. The microhardness of the upper surface can be elevated from 112.3 HV to 176.9 HV under the combined influence of the grain refinement, elements distribution change and surface defects elimination. Better surface quality can be gained by decreasing the hatching space, increasing polishing pass or choosing apposite laser direction. [ABSTRACT FROM AUTHOR]

During metal additive manufacturing, porosity is one of the main quality defects of products. Laser post-processing is an effective method of reducing porosity of the metal surface and sub-surface. Herein, the evolution mechanism of multi pores during this process is studied. Two samples for each alloy, Ti6Al4V and AlSi10Mg, were fabricated by laser powder bed fusion (LPBF), and the upper surface of one sample of each alloy was scanned using a laser beam for comparison. Then, the inner layer morphology and pores distribution of these four samples were measured by micro-computed tomography (micro-CT) technology. It was found that the porosity reduced from 0.7961% to 0.6034% for Ti6Al4V and from 3.7105% to 3.6287% for AlSi10Mg after laser post-processing and the phenomenon of combination of pores was obvious for AlSi10Mg alloy. Next, the laser post-processing mechanisms of Ti6Al4V and AlSi10Mg alloys, including heat transfer, Marangoni flow, mass transfer related to recoil pressure and other phenomena, were numerically simulated, and the evolution of pores was tracked by the level-set method. Moreover, the effects of preheating temperature on porosity reduction were discussed. • The porosity of Ti6Al4V alloy and AlSi10Mg alloy with and without laser post-processing was detected by micro-computed tomography. • The evolution of multi pores during laser post-processing was discussed. • The differences between Ti6Al4V alloy and AlSi10Mg alloy during laser post-progressing were analyzed. • The effects of preheating temperature on porosity reduction during laser post-processing was studied. [ABSTRACT FROM AUTHOR]

• High laser energy density lead to the spatter connected with the molten pool. • An in situ monitoring method for acquiring spatter images in SLM. • A novel image processing algorithm for spatter signature eliminated three errors. • Spatter signature variation with changing laser energy density. Although selective laser melting (SLM) has the advancements of fabricating complex geometric, multi-material and multi-functional structures, several defects still affect the process stability, in particular spatter. The formation of spatter in SLM depends on the process parameters, and these can potentially be used to tune the process to obtain better product quality. However, there is still a lack of efficient methods for processing spatter images to allow in situ detection of the onset of spatter. In this paper, an in situ monitoring method for acquiring spatter images in SLM was presented. A maximum-entropy double-threshold image processing algorithm based on genetic algorithm (MEDTIA-GA) was proposed to recognize spatter from images, and its results were compared with three conventional threshold segmentation methods: Otsu's method, Triangle threshold segmentation algorithm, and K -means clustering algorithm. Results show that MEDTIA-GA method was able to eliminate three types of errors: noise sensitivity, spatter conglutination, and spatter omission. In addition, the average processing time of 37 ms for MEDTIA-GA method was far shorter than those for other three conventional threshold segmentation methods. Finally, the relationship between the spatter area as well as the number and the laser energy density were analyzed [ABSTRACT FROM AUTHOR]