Influence Of Scanning Strategy Research Articles

Influence of Scanning Strategy and Post-Treatment on Cracks and Mechanical Properties of Selective-Laser-Melted K438 Superalloy

The feasibility of manufacturing high-performance components with complex structures is limited due to cracks in some superalloys fabricated by selective laser melting (SLM). By controlling the main process parameters such as scanning strategy, the adverse effects of cracks can be effectively reduced. In this paper, the effects of two different SLM scanning strategies with island and ‘back-and-forth’ and post-heat treatment on the cracks and mechanical properties of selective-laser-melted (SLMed) K438 alloy were investigated. The results show that the SLM method of the ‘back-and-forth’ scanning strategy had better lap and interlayer rotation angles and a more uniform distribution of laser energy compared with the island scanning strategy. The residual stress accumulation was reduced and crack formation was inhibited under this scanning strategy owing to the cooling and shrinkage process. In addition, the dislocation motion was hindered by the formation of uniformly dispersed MC carbides and γ’ phases during the SLM K438 alloy process, which resulted in the density of the as-built SLMed K438 alloy being up to 99.34%, the hardness up to 9.6 Gpa, and the tensile strength up to 1309 MPa. After post-heat treatment, the fine secondary γ’ phases were precipitated and dispersed uniformly in the Ni matrix, which effectively improved the Young’s modulus and tensile strength of the alloy by dispersing the stress-concentrated area.

Influence of scanning strategy on magnetic properties of 630 stainless steel fabricated by directed energy deposition method

We investigated how the scanning strategy (SS) influences the magnetic properties of ferromagnetic metal specimens additively manufactured using 630 stainless steel powder and a directed energy deposition machine. The magnetization curves were measured by using B and H coils and a magnetic flux meter. The specific saturation magnetization, coercivity, and specific magnetic susceptibility were calculated from the magnetization curves obtained. Moreover, the martensite fraction, magnetization structure, and martensite structure were measured and visualized using X-ray diffraction, the Bitter method, and electron backscatter diffraction, respectively. The magnetic properties and martensite fraction depended on the SS, and magnetic poles were generated in each sub-block structure. Furthermore, a numerical analysis using a magnetic micromechanics model indicated that the variation in the martensite structure due to the SS produced differences in the magnetic properties.

Effect of scanning strategy on microstructure and mechanical properties of a biocompatible Ti–35Nb–7Zr–5Ta alloy processed by laser-powder bed fusion

The influence of scanning strategy (SS) on microstructure and mechanical properties of a Ti–35Nb–7Zr–5Ta alloy processed by laser-powder bed fusion (L-PBF) is investigated for the first time. Three SSs are considered: unidirectional-Y; bi-directional with 79° rotation (R79); and chessboard (CHB). The SSs affect the type and distribution of pores. The highest relative densities and more homogeneous distribution of pores are obtained with R79 and CHB scanning strategies, whereas aligned pores are formed in the unidirectional-Y. The SSs show direct influence on the crystallographic texture with unidirectional-Y strategy showing fiber texture. The R79 strategy results in a weak texture and the CHB scanning strategy forms a randomly oriented heterogeneous grain structure. The lowest Young modulus is obtained with the unidirectional-Y strategy, whereas the R79 strategy results in the highest yield strength. It is shown that the SSs may be used for tuning the microstructure of a beta-Ti alloy in L-PBF. Graphical abstract

Corrosion behavior of in situ-synthesized (TiC+TiB)/Ti composites by laser powder-bed fusion: Role of scan strategy

The in-situ TiC and TiB reinforced titanium matrix composites were fabricated by laser powder fusion (LPBF) of B4C/Ti powder mixture, and the influence of scan strategy on the microstructure morphologies and corrosion resistance in NaCl solution were also investigated. The experimental results reveal that anodic dissolution of titanium matrix is accelerated and thus facilitates the formation of the TiO2 passive layer on the surface of the LPBF-produced (TiC+TiB)/Ti composites owing to the in-situ TiC and TiB reinforcements acting as micro-cathode in NaCl aqueous solution. The island scan strategy tends to result in a net-shaped distribution of TiC and TiB reinforcements and pore defects, which causes the inhomogeneity of galvanic couples and resultant TiO2 passive layer. Fortunately, the scan strategies of XY+ 30° and XY+ 67° can alleviate the thermal accumulation and leads to the transformation of refinements distribution to radial-shaped arrangement, which favors for the homogeneity of galvanic couples and resultant TiO2 passive layer. Moreover, the scan strategy of XY+ 90° generates a refinement and increased volume fraction of reinforcements and micro-galvanic couples, which significantly facilitates the formation of a thicker and less-defective TiO2 passive layer, thus leading to better corrosion resistance with a larger value of Rf. Furthermore, the corrosion mechanism of LPBF-produced (TiC+TiB)/Ti composites has also been discussed in detail based on the Electrochemical impedance spectroscopy (EIS) and X-ray photoelectron spectroscopy results.

Influence of scanning strategies on microstructure, residual stress, and corrosion behavior of 17-4 PH stainless steel fabricated by selective laser melting

In the present study, the effect of hexagonal (inside out and outside in), rectangular (horizontal, vertical, alternate, and both), and strip (horizontal, vertical, alternate, and both) scan strategies on the microstructure, residual stresses, and the corrosion behavior of 17-4 PH stainless steel samples fabricated by selective laser melting were investigated. The results indicate that through hexagonal inside-out scanning strategy, the sample density and hardness are double than that of wrought and cast alloys. Microstructure consists of small and large molten pools on the side and front surface of each sample generated using a different scan strategy. Austenite phase and ferrite peak at the highest intensity were observed by X-ray diffraction for hexagonal inside-out sample. Electron back scattered diffraction (EBSD) orientation maps showed a high-density of ɣ-austenite phase (43%) in the finer grain size area of the hexagonal inside-out scan strategy sample. Low residual stresses and high corrosion resistance (<0.5 mpy) were obtained for the hexagonal inside-out scan strategy sample compared to other two strategies because of lower defects, lower thermal gradient, and smaller scan tracks.

Scanning Manufacturing Parameters Determining the Residual Stress State in LPBF IN718 Small Parts

The influence of scan strategy on the residual stress (RS) state of an as‐built IN718 alloy produced by means of laser powder bed fusion (LPBF) is investigated. Two scan vector rotations (90°‐alternation and 67°‐rotation), each produced following two different scan vector lengths (long and short), are used to manufacture four rectangular prisms. Neutron diffraction (ND) and laboratory X‐ray diffraction (XRD) techniques are used to map the bulk and surface RS state, respectively. The distortion induced upon removal from the baseplate is measured via profilometry. XRD measurements show that the two long scan vector strategies lead to higher RS when compared with the equivalent short scan vector strategies. Also, the 67°‐rotation strategies generate lower RS than their 90°‐alternation counterparts. Due to the lack of reliable stress‐free references, the ND results are analyzed using von Mises stress. In general, ND results show significant RS spatial non‐uniformity. A comparison between ND and distortion results indicates that the RS component parallel to the building direction (Z‐axis) has a predominant role in theZ‐displacement. The use of a stress balance scheme allows to discuss the variability along the length of the specimens, as well as examine the absolute RS state.

Critical assessment of the impact of process parameters on vertical roughness and hardness of thin walls of AlSi10Mg processed by laser powder bed fusion

This work deals with the impact of prominent process parameters in laser powder bed fusion (L-PBF) technique, also called selective laser melting (SLM), on relevant properties of AlSi10Mg such as surface roughness and hardness. More particularly, the influence of scanning strategies, track energy density (TED), and of the offset between the contours and the bulk hatching were scrutinized. Basic thin structures such as single wall tracks (SWT) and ultra thin walls (UTW) were used to isolate the effect of the TED and of the offset between parallel melt pool tracks on the vertical roughness, respectively. It is shown that the TED reduces the level of roughness mostly due to a geometrical effect. The optimum range of TED is shown to be at the beginning of the keyhole conditions, at the expense of the amount of sub-surface porosities and of hardness. Increasing the offset also allows reducing the roughness. Thicker structures with contours and bulk hatching were also used and brought the same conclusions. In this work, low levels of roughness (about 4 μm) have been obtained on vertical walls without any post-treatment but at the expense of the mechanical properties.

Influence of Scanning Strategy on the Performances of GO-Reinforced Ti6Al4V Nanocomposites Manufactured by SLM

In this work, the effects of line (L-scanning strategy), stripe (S-scanning strategy), hollow square (H-scanning strategy) and chess board partition (C-scanning strategy) on the performances of graphene oxide reinforced Ti6Al4V matrix nanocomposites (GO/TC4) as fabricated by selective laser melting (SLM) were investigated. Numerical temperature field simulation of four different scanning strategies was utilized to investigate the effects of thermal concentration on SLM-processed GO/TC4 nanocomposites, linking to its micro-voids, surface roughness, porosity, microhardness and tribological properties. The proposed simulation scheme is validated by comparing the simulated thermal analysis with experimental results. Simulation results show that the thermal concentration effects of a part during SLM process is distinctive under different scanning strategies, with the slowest cooling rate of 64,977.5 °C/s that is achieved by C-scanning strategy specimen. The experimental results indicate that the performances of the L-scanning strategy or S-scanning strategy sample are seriously affected by the thermal concentration, causing a large number of micro-voids and defects. All the experimental results suggest that the sample using C-scanning strategy exhibits the optimal performance of all investigated specimens, which closely correlates with its lowest temperature gradients. This study highlights the importance of using a partition scanning strategy during SLM process, which can be easily extended to other powder bed fusion process.

Influence of scanning strategy and parameter on microstructural feature, residual stress and performance of Sc and Zr modified Al–Mg alloy produced by selective laser melting

Selective laser melting (SLM), due to its track-by-track and layer by layer localized additive manufacturing process nature, has significant requirements for the optimization of laser scanning strategy and parameter. In this work, rare earth element Sc- and Zr- modified Al–Mg alloy was processed by SLM using an “island” scanning strategy with different island sizes. The influence of island size on surface quality, microstructural feature, residual stress, tensile property and corrosion resistance of SLM-processed Al–Mg-Sc-Zr alloy was systematically investigated. Experimental results indicated that the surface smoothness was enhanced as the island size decreased, but the influence of island size on microstructural development and tensile properties was not distinct. Considerably high tensile properties above 500 MPa combined with elongations of 10%–12% were generally obtained for Al–Mg-Sc-Zr specimens processed under different island sizes. Minimum residual stress was generated at a moderate island size of 2.4 mm and the residual stress increased both at a small and a large island size. A similar tendency was also observed for the corrosion resistance of SLM processed Al–Mg-Sc-Zr specimens, where a superior corrosion resistance (icorr = 6.82 × 10−6 A cm−2) was generated at the middle island size. The different temperature gradient and thermal behaviors under different island sizes contribute to the variation of surface conditions and residual stresses. The change in corrosion behaviors under different island sizes was related to the different molten pool configurations. These findings provided the knowledge of the relationship between laser scanning strategy and properties of SLM processed Al–Mg-Sc-Zr alloy, which further facilitated process optimization and engineering applications of laser additive manufacturing of high-performance Al–Mg-Sc-Zr alloy.

Influence of scan strategy on porosity and microstructure of Ti-6Al-4V fabricated by electron beam powder bed fusion

Ti-6Al-4V is of high value in biomedical, aerospace, and industrial sectors and is ideal for additive manufacturing (AM) because of its poor machinability. In electron beam powder bed fusion of Ti-6Al-4V, a raster scan is commonly used and often results in porosity that is detrimental to the fatigue strength. In this paper, we demonstrate that spot melting can significantly minimize porosity but produces a coarser microstructure compared to raster melting. Process parameters can be optimized to achieve minimal porosity with tailored microstructures. A finite element method was used to rationalize these observations.

The Effect of a Scanning Strategy on the Residual Stress of 316L Steel Parts Fabricated by Selective Laser Melting (SLM).

The laser scanning strategy has an important influence on the surface quality, residual stress, and deformation of the molten metal (deformation behavior). A divisional scanning strategy is an effective means used to reduce the internal stress of the selective laser melting (SLM) metal part. In order to understand and optimize the divisional scanning strategy, three divisional scanning strategies and an S-shaped orthogonal scanning strategy are used to produce 316L steel parts in this study. The influence of scanning strategy on the produced parts is verified from the aspects of densification, residual stress distribution and deformation. Experiments show that the 316L steel alloy parts adopted spiral divisional scanning strategy can not only obtain the densification of 99.37%, but they also effectively improve the distribution of residual stress and control the deformation degree of the produced parts. Among them, the spiral divisional scanning sample has the smallest residual stress in plane direction, and its σx and σy stress are controlled within 204 MPa and 103 MPa. The above results show that the spiral divisional scanning is the most conducive strategy to obtain higher residual stress performance of SLM 316L steel parts.

A Study on the Influence of Scanning Strategies on the Levelness of the Melt Track in Selective Laser Melting Process of Stainless Steel Powder

By using a numerical model for a selective laser melting process, this study investigates the melt pool behaviours and the resulting defects due to different scanning strategies. Two common scanning patterns were tested: sequential and boustrophedon. The study found that sequential scanning strategy leads to a non-flat printed surface, while the boustrophedon strategy produces a flat printed surface. At the beginning of the track, the molten material is pulled from the hot laser spot to the cold tail by the Marangoni force leading to an increase in the melt pool height of the sequential and alternated strategies. On the other hand, due to the deep keyhole and the high cooling rate when the laser is switched off, a recess is formed at the end of the tracks. Similar phenomena can be observed in the boustrophedon scanning strategy at the start and ending of the track.

Influence of scan strategy and molten pool configuration on microstructures and tensile properties of selective laser melting additive manufactured aluminum based parts

Selective laser melting additive manufacturing of the AlSi12 material parts through the re-melting of the previously solidified layer using the continuous two layers 90° rotate scan strategy was conducted. The influence of the re-melting behavior and scan strategy on the formation of the “track-track” and “layer-layer” molten pool boundaries (MPBs), dimensional accuracy, microstructure feature, tensile properties, microscopic sliding behavior and the fracture mechanism as loaded a tensile force has been studied. It showed that the defects, such as the part distortion, delamination and cracks, were significantly eliminated with the deformation rate less than 1%. The microstructure of a homogeneous distribution of the Si phase, no apparent grain orientation on both sides of the MPBs, was produced in the as-fabricated part, promoting the efficient transition of the load stress. Cracks preferentially initiate at the “track-track” MPBs when the tensile stress increases to a certain value, resulting in the formation of the cleavage steps along the tensile loading direction. The cracks propagate along the “layer-layer” MPBs, generating the fine dimples. The mechanical behavior of the SLM-processed AlSi12 parts can be significantly enhanced with the ultimate tensile strength, yield strength and elongation of 476.3MPa, 315.5MPa and 6.7%, respectively.

Influence of scanning strategies on measuring and surface digitising by means of a conoscopic holography sensor integrated in a machining centre

A conoscopic holography sensor (CH) integrated in a machining centre was used to analyse how the scanning strategy affects distance measurements between planes and to the reconstruction of a digitised surface. Distance measurement tests were conducted on a stepped specimen whereas the surface reconstruction was studied on a cylindrical surface. Two different scanning strategies were compared. In the first, the height position of the sensor kept constant along the surface scanning so that all the points were located at different positions within the sensor depth of field (DOF). In the second strategy, the sensor was continuously adjusted describing a trajectory equidistant to the surface so that all the points were acquired at a same distance within the DOF. Both strategies were compared by the discrepancies between measurements taken by the CH sensor and those obtained by a touch probe. Some recommendations are provided for distance measurement and surface reconstruction.

Influence of scanning strategies on measuring and surface digitising by means of a conoscopic holography sensor integrated in a machining centre

Influence of scanning strategies on measuring and surface digitising by means of a conoscopic holography sensor integrated in a machining centre

Influence of scan strategy and process parameters on microstructure and its optimization in additively manufactured nickel alloy 625 via laser powder bed fusion.

Laser powder bed fusion (L-PBF) as an additive manufacturing process produces nearly fully dense nickel alloy 625 (IN625) parts with complex features. L-PBF generates surfaces and microstructure through directional solidification that can be controlled by scan strategies and selection of process parameters. This study provides experimental investigations on microstructure formation including sizes of cellular grains and growth directions of columnar grains on the nickel alloy 625 test coupons. The effects of process parameters including laser power, scan velocity, hatch distance, and scan strategy that produce various solidification cooling rates and thermal gradients during the process, which also contribute to resultant microstructure, have been analyzed. Optimization studies are conducted on several objectives to improve the productivity while controlling the process effects on the resultant microstructure using response surface regression, desirability functions, and multi-objective genetic algorithm optimization.

Influence of scanning strategies on the accuracy of digital intraoral scanning systems.

The digital intraoral impression is a central part in today's CAD/CAM dentistry. With its possibilities, new treatment options for the patient is provided and the prosthetic workflow is accelerated. Nowadays, the major issue with intraoral scanning systems is to gain more accuracy especially for larger scan areas and to simplify clinical handling for the dentist. The aim of this study was to investigate different scanning strategies regardingtheir accuracy with full arch scans in an in-vitro study design. A reference master model was used for the digital impressions with the Lava COS, the Cerec Bluecam and a powderfree intraoral scanning system, Cadent iTero. The trueness and precision of each scanning protocol was measured. Lava COS provides the a trueness of 45.8 microm with the scanning protocol recommended from the manufacturer. A different scanning protocol shows significantly lower accuracy (trueness +/- 90.2 microm). Cerec Bluecam also benefits from an optimal scanning protocol with a trueness of +/- 23.3 microm compared to +/- 52.5 microm with a standard protocol. The powderfree impression system Cadent iTero shows also a high accurate full-arch scan with a trueness of +/- 35.0 microm and a precision of +/- 30.9 microm. With the current intraoral scanning systems, full arch dental impressions are possible with a high accuracy, if adequate scan strategies are used. The powderfree scanning system provides the same level of accuracy compared to scanning systems with surface pretreatment.