Laser Engineered Net Shaping Research Articles

Online in situ detection of deposited height deviation during additive manufacturing

Deposited height deviation (DHD) of printed layers is a common surface defect that restricts vertical printing accuracy during the additive manufacturing process. The accumulation of DHD layer by layer inevitably leads to the failure of subsequent additive manufacturing tasks. Therefore, accurate online measurement of DHD is crucial. This study proposed a novel amplification computer-vision measurement (ACVM) method that effectively utilizes both melt pool images and temperature information, achieving a DHD detection sensitivity of approximately 9.96 μm. Theoretical connections between image features and DHD, as well as the theoretical associations between instantaneous temperature characteristic and DHD, have been systematically deduced. Based on these two theoretical relationships, DHD can be accurately and synchronously detected directly through the positions of image features and temperature. A single-camera dual-channel multi-signal detection (SDMD) system was developed and implemented within a laser-engineered net shaping (LENS) additive manufacturing system. Subsequently, an online measuring and verification experiment was designed to assess the height deviation of thin-walled structural parts. The experimental results demonstrated that the ACVM method provided an early response to DHD. The method exhibits significant technical application value in quality control in future.

Evaluating productivity characteristics of laser engineered net shaping titanium alloy

Advances in Additive Manufacturing (AM) technologies have made it possible to reduce the design and prototyping costs to a minimum especially for a low-productivity material like titanium. Titanium alloys are commonly and widely used alloys in the aerospace and biomedical sector due to their advantageous material properties. This paper is an evaluation study of factors affecting the productivity characteristics of Laser Engineered Net Shaping (LENS) titanium alloy (Ti-6Al-4 V) using face milling. Some of the productivity challenges associated with titanium such as rapid tool wear, poor surface finish, and high-power consumption are explored in this paper. All materials processed using AM face the same critical problem that the manufactured part requires a post machining since AM produces relatively poor surface finish. Machining trials are conducted using the combinations of machining parameters such as spindle speed of 800 and 1600 rev/min; feed rate of 50 and 100 mm/min; and a constant depth of cut of 1 mm, respectively. Titanium being a poor thermal conductivity material, the effect of coolant was investigated using wet/dry machining. Data related to the productivity factors and material behavior under a milling trial was recorded and analyzed. The obtained data from the trials include productivity factors such as Metal Removal Rate (MRR), power consumed, and the surface finish for each plate/trial. The power consumed in dry milling was observed to be lower than that in wet milling which is contrary to the observations from conventional wet milling. The paper concludes the trends observed for LENS titanium are opposed to the trends in conventional machining such as increasing cutting speed will result in lower cutting force and power consumed.

The Chemistry-Process-Structure Relationships of a Functionally Graded Ti-6Al-4V/Ti-1B Alloy Processed with Laser-Engineered Net Shaping Creates Borlite.

We quantify the chemistry-process-structure-property relationships of a Ti-6Al-4V alloy in which titanium-boron alloy (Ti-B) was added in a functionally graded assembly through a laser-engineered net shaping (LENS) process. The material gradient was made by pre-alloyed powder additions to form an in situ melt of the prescribed alloy concentration. The complex heterogeneous structures arising from the LENS thermal history are completely discussed for the first time, and we introduce a new term called "Borlite", a eutectic structure containing orthorhombic titanium monoboride (TiB) and titanium. The β-titanium grain size decreased nonlinearly until reaching the minimum when the boron weight fraction reached 0.25%. Similarly, the transformed α-titanium grain size decreased nonlinearly until reaching the minimum level, but the grain size was approximately 2 μm when the boron weight fraction reached 0.6%. Alternatively, the α-titanium grain size increased nonlinearly from 1 to 5 μm as a function of the aluminum concentration increasing from 0% to 6% aluminum by weight and vanadium increasing from 0% to 4% by weight. Finally, the cause-effect relationships related to the creation of unwanted porosity were quantified, which helps in further developing additively manufactured metal alloys.

Влияние технологических параметров процесса прямого лазерного выращивания на качество формируемого объекта из титанового сплава ВТ23

Introduction. Laser engineered net shaping (LENS) or Direct metal deposition (DMD) is considered as a promising method for manufacturing products of complex configurations from titanium-based alloys, as it allows minimizing the use of machining and loss of material to waste. Currently, neither the LENS technological process of titanium alloy VT23 has not been developed, nor the structural features of the alloy after LENS have not been studied, which will make it possible to determine the scope of application of the material after LENS. The purpose of this study is to determine optimal modes of the LENS process for manufacturing of quality parts from titanium alloy VT23. Methodology. The alloy specimens obtained with laser power 700÷1300 W in increments of 100 W and scanning speed 600÷1,000 mm/min in increments of 200 mm/min and distance between adjacent laser tracks 0.5–0.9L (L — track width) in increments of 0.2L were analyzed in the study. The elemental composition of the powder material was studied by X-ray fluorescence analysis and reducing combustion in a gas analyzer, the structure of the objects obtained by LENS was analyzed by metallographic and X-ray phase analysis methods as well as microhardness was determined. Results and discussion. It is established that high-quality objects without cracks, with low porosity can be synthesized from VT23 alloy by LENS method using the following modes: laser power 700÷1100 W, scanning speed 800–1,000 mm/min, track spacing 0.5–0.7 of the individual track width L. It is shown that after all investigated LENS modes, the VT23 alloy had a dispersed (α+β) structure of the “basket weave” type. It is revealed that regardless of LENS mode the amount of β-phase in the alloy structure is about 30 %. It is shown that the microhardness of the deposited material does not depend on LENS modes and is 460 HV.

Optimization of functional performance of additively manufactured cobalt‑chromium‑molybdenum alloy for dental implant applications

Improper selection of Laser Engineered Net Shaping (LENS) process parameters for the fabrication of cobalt‑chromium‑molybdenum (CoCrMo) alloy may result in poor quality components for dental implant applications. To solve the LENS fabrication challenge, this work utilizes response surface methodology (RSM) to investigate the influence of LENS deposition variables on the microstructural and mechanical performance of the manufactured samples for dental implant applications. Analysis of statistical data demonstrated that microhardness and porosity were both significantly influenced by laser scan speed and powder feed rate, while surface roughness is impacted substantially by laser scan speed and laser power. The analysis of the responses revealed that the optimum factors were at a scan speed of 5.3 mms−1, powder feed rate of 4.748 gmin−1 and laser power of 386.896 W to give surface roughness, porosity, and microhardness responses of 8.7775 μm, 0.06 %, and 387.4286 HV, respectively. The models revealed a strong interaction between the actual experimental data and RSM-predicted responses. The results of this research can serve as a guide for determining suitable LENS input factors for the manufacture of dental implants.

Additive manufacturing of WE43 and modified AZ91D magnesium alloys using the laser engineered net shaping process

Magnesium alloys WE43 and AZ91D offer weight savings for structural applications. WE43 contains rare-earth elements such as yttrium and neodymium, whereas AZ91D and the AZ class of alloys are lower-cost containing aluminium and zinc. This work uses a modified version of the latter, identified as AZ91D*, where calcium and yttrium have been added. Laser Engineered Net Shaping (LENS) equipment was used to investigate the 3D printing behaviour of gas-atomized WE43 and AZ91D* material. Process evaluations were first done using single track laser scans across wrought substrates of WE43 and AZ31. Process parameters were then developed that enabled deposition of build coupons with less than 1% porosity. These were used for microstructural and mechanical property assessments. The energy densities used were in the range of 40 – 60 J/mm² for AZ91D*, while for WE43 a value of 50 J/mm² was selected. A focus of the work was managing the tendency of magnesium materials to fume heavily during deposition, while achieving a deposit with close to nominal density, i.e. having a low incidence of voids or other micro- or macro-structural defects. The porosity of AZ91D* builds did not seem to correlate with increasing deposition energy density, however WE43 builds did, up to a practical limit imposed by generation of excessive fume. WE43 generated much less fume than AZ91D*. Analysis of the input powder materials indicate the presence of oxides on the particle surfaces. The WE43 powders had a noticeable shell of yttrium oxide. The microstructures of WE43 builds show the presence of arc-shaped structures that are likely the same yttrium oxides, partially, or fully ‘unfolded’. Tensile testing was conducted on WE43 build coupons to assess the effect of a T6 heat treatment. The effect on the ultimate tensile and yield strengths was negligible, however the % elongation at break almost doubled. Inspection of the fracture surfaces suggests this was due to a change in mechanism from fracture initiation at sites of lack-of-fusion/un-melted particles, to a more ductile micro-void coalescence. Furthermore, a not previously described microstructure anomaly was observed: the presence of pure elemental magnesium crystallites and films on the fracture surface. These are attributed to condensation of magnesium vapour on previously built layers and are a possible reason for the existence of lack-of-fusion defects. The in-depth study of process fundamentals will enable future work on process fume inhibition based on process optimization and alloy composition with the goal of producing parts for biomedical and lightweighting applications.

Comparison of the Microstructural, Mechanical and Corrosion Resistance Properties of Ti6Al4V Samples Manufactured by LENS and Subjected to Various Heat Treatments.

In this paper, the influences of two post-heat treatments on the structural, mechanical and corrosion resistance properties of additively manufactured Ti6Al4V alloys were discussed in detail. The materials were produced using the laser engineering net shaping (LENS) technique, and they were subjected to annealing without pressure and hot isostatic pressing (HIP) under a pressure of 300 MPa for 30 min at temperatures of 950 °C and 1050 °C. Annealing without pressure led to the formation of a thin plate structure, which was accompanied by decreasing mechanical properties and increasing elongation and corrosion resistance values. For the HIP process, the formation of a thick plate structure could be observed, resulting in the material exhibiting optimal mechanical properties and unusually high elongation. The best mechanical and corrosion resistance properties were obtained for the material subjected to HIP at 950 °C.

Fatigue behavior of Ti‐6Al‐4 V alloy manufactured by cold metal fusion

AbstractThis study investigates the fatigue behavior of small specimens of Ti‐6Al‐4 V produced using the additive manufacturing process cold metal fusion. As the process is still comparatively new, no data were available beforehand. Strain‐controlled fatigue tests were carried out on small specimens under alternating loads. The results are presented in the form of a stress–strain diagram according to Ramberg–Osgood and a strain–life curve according to Coffin–Manson. Comparison with results reported for similar specimens fabricated via the laser engineered net shaping (LENS) additive method shows comparable fatigue behavior. Analysis of the fracture surfaces shows a very mixed fracture pattern with an often mountainous surface, especially for specimens tested under high strain. This made the detection of crack initiation very difficult for those specimens. The analysis of specimens tested under low strain reveals large surface defects to be the crack initiators. Smaller secondary cracks form at internal crack initiation facets.

Experimental Investigation on a Novel Hybrid Composite Developed by Laser Engineering Net Shaping: Optimization and Ranking Analysis

Experimental Investigation on a Novel Hybrid Composite Developed by Laser Engineering Net Shaping: Optimization and Ranking Analysis

Tailoring microstructure and mechanical properties of β-solidifying TiAl alloy fabricated by laser-engineered net shaping through heat treatment

Tailoring microstructure and mechanical properties of β-solidifying TiAl alloy fabricated by laser-engineered net shaping through heat treatment

Microstructure and Mechanical Properties of NiTi Shape Memory Alloys by Laser Engineered Net Shaping

Herein, NiTi alloys are prepared by laser engineered net shaping (LENS) technology. By exploring the mechanical properties and microstructures of NiTi samples under different laser powers, it is concluded that NiTi alloys with a laser power of 350 W have better tensile strength (783 MPa) and strain recovery rate (59.01%). In addition, the mechanical properties and microstructure of NiTi samples with a laser power of 350 W at different aging temperatures (400–800 °C) are also explored. It is found that NiTi alloys aged at 600 °C had better tensile strength (729 MPa) and strain recovery rate (92.85%). In addition, the phase‐transition temperature and endothermic enthalpy and exothermic enthalpy of the NiTi samples after thermal treatment increased, which is attributed to the formation of stable martensite (B19′) inside the NiTi samples after thermal treatment, and the type of internal NiTi phases structure change significantly. Thermal treatment can generate Ni4Ti3, NiTi2 precipitates, and martensite (B19′) with high density of dislocations inside the NiTi samples. Herein, it is showed that LENS can prepare NiTi alloys with excellent mechanical and structural properties by controlling the equipment parameters and aging temperature.

Microstructural characterization of additively manufactured Ti-6Al-4V with the addition of β-stabilizer, niobium

This study investigated the influence of beta stabilizer niobium on the microstructure and hardness of Ti-6Al-4V. The relationship between increased the concentration of alloying elements into Ti-6Al-4V is generally associated with an increase in hardness due to solid solution strengthening. The research involved the fabrication of a series of laser engineering net shaping (LENS™) Ti-6Al-4V alloys with varying concentrations of niobium (13%, 20%, and 28%) to determine whether increasing the Nb content has an influence on the concentration of Nb dissolved into Ti-6Al-4V. The microstructures of these alloys were analysed the hardness examined. the results revealed that the solid solution strengthening did not lead to a proportional increase in hardness as expected. Further microstructural examination showed that the alloy with 13% Nb exhibited a distinct microstructure with a higher concentration of un-melted Nb particles, whereas the alloy with 28% Nb had a higher dissolution of Nb into the Ti-6Al-4V. The least percentage of Nb dissolved into the Ti-6Al-4V compared to the addition of 28% Nb. Increased Nb content resulted in a higher percentage of Nb dissolved in Ti-6Al-4V but the lower hardness value. The undissolved Nb did not contribute solid solution strengthening but rather the presence of defects, and features of microstructure are responsible for hindering the ease of plastic deformation.

Laser Additive Manufacturing of Low‐Cost Ti–Mo Alloys: Microstructural Evolution, Phase Analysis, and Mechanical Properties

Design and fabrication of low‐cost biomedical Ti alloys has recently received considerable attention. Herein, additive manufacturing employing laser engineered net shaping (LENS) is used to fabricate low‐cost Ti–5wt%Mo and Ti–10wt%Mo alloys from mixed elemental powders, and the resulting features are studied and compared with those of LENS‐produced commercially pure Ti (CP‐Ti). Optimization of the processing parameters shows that, on increasing Mo, densification is slightly reduced. Phase analysis and microstructural investigations on the Ti–Mo alloys show strong dependency on the level of Mo additions. CP‐Ti sample exhibits an α phase microstructure with mainly coarse and some fine plate‐like features, whereas addition of 5% Mo results in a mixture of acicular α′ distributed within β phase grains. Addition of 10% Mo stabilizes the β phase, while minor peaks indicating the presence of some ω phase are detected by X‐ray phase analysis. Mechanical characterization reveals that Ti–10%Mo has the lowest elastic modulus due to its dominant β phase structure as well as the highest microhardness and yield strength due to solid‐solution strengthening effects from Mo and the presence of the nanoscale ω phase. However, the presence of the ω phase and the slightly lower densification also reduce the deformation strain in comparison to the Ti–5%Mo and CP‐Ti.

Sandwich structure printing of Ti-Ni-Ti by directed energy deposition

ABSTRACT In the current work, sandwich structure (Ti-6Al-4V)-Ni-(Ti-6Al-4V) is printed by Laser Engineered Net Shaping (LENS). This sandwich structure allows the general repair of broken parts with dissimilar materials. The chief objective of this research is to propose a new method to produce a sandwich structure comprising Ti-6Al-4V and Nickel by DED, which allows the investigation of the Ti-6Al-4V/Nickel and Nickel/Ti-6Al-4V interfaces. The results shed light on the production process and makes a proper roadmap for multi-material printing. The contributions of this paper are the detailed defect characterization of (Ti-6Al-4V)-Ni-(Ti-6Al-4V) sandwich structures, considering the rheological phenomena in the meltpool and thermophysical properties of the materials. This research also identifies how the interface quality and the overall bonding quality of the sandwich structures can be improved, enabling the exploration of the limitations of production, and the knowledge of how to potentially produce a defect-free sandwich structure of Ti-Ni-Ti. Results showed that cracks, pores, partial melting, keyholes and residual particles are the main problems during a build. These results indicate that the LENS is a promising method to produce sandwich structures for various applications by selecting the appropriate process parameters in such a way as to minimize the rheological instability.

Microstructures and mechanical behaviors of additive manufactured Inconel 625 alloys via selective laser melting and laser engineered net shaping

The microstructure and mechanical behaviors of as-built Inconel 625 prepared by selective laser melting (SLM) and laser engineered net shaping (LENS) technologies were investigated in this study. The results show that the microstructure of Inconel 625 alloy prepared by SLM process is composed of obvious columnar and cellular grains. Inconel 625 fabricated by LENS also has columnar and cellular structures, but the grain size is relatively large. Significant anisotropy of mechanical properties is observed in SLM and LENS printed Inconel 625. The mechanical properties in XY direction are higher than those in XZ direction, and the maximum tensile strength of SLM-XY sample is 1241.5 MPa. SLM printed Inconel 625 has a relatively higher dislocation density of 2.8 ± 1.2 × 10 14 m −2 . The difference in yield strength between SLM and LENS printed Inconel 625 is mainly caused by the different dislocation strengthening effects. • The microstructure and mechanical properties of Inconel 625 printed by SLM and LENS are systematically compared. • The anisotropy of mechanical properties is affected by grain morphology due to different printing directions. • Dislocation strengthening effects play a major role in mechanical properties.

Mechanical properties and microstructure evolution of additive manufactured 316L stainless steel under dynamic loading

Stainless steel 316L alloy with a complete austenite phase was manufactured using the Laser Engineered Net Shaping (LENS) technique. The microstructure of LENS-316L from the sub-millimeter to nanometer domains was analyzed using multi-scale characterization methods. The dynamic behavior of LENS-316L was studied by the Split Hopkinson Pressure Bar (SHPB) test. In this study, the dominating strengthening mechanism of the microstructure in LENS-316L was the dislocation strengthening mechanism, providing approximately 153 MPa. The molten pool deforms significantly after the material was dynamically compressed. When the strain rate reached 5000 s−1, the material exhibited an adiabatic shear phenomenon and formed adiabatic shear bands. The width of the shear band decreased with the increase in the strain rate. When the strain rate increases from 3000 s−1 to 4000 s−1, the dislocation density increases, the number of twins increases, and the twin thickness decreases. The effect of varying strain rates (3000, 4000, 5000, 6000 s−1) and varying temperatures (20, 150, 272, 450, 620 °C), on the stress–strain curves of the material were studied through the SHPB test. Additionally, the traditional Johnson–Cook (JC) constitutive model was modified. The modified constitutive model considers the effects of the strain rate and temperature softening coefficients in the strain hardening stage to predict the mechanical behavior of LENS-316L at high temperature and strain rates. Model predictions were in agreement with the experimental data. This study provides important insights for applying the LENS-316L complex construction in a dynamic load impact environment.

Interface bonding behavior and failure mechanism of joining Tribaloy T-800/AISI 4140 via laser engineered net shaping

As a potential material for surface coating and repair applications, Tribaloy T-800 was deposited on AISI 4140 alloy steel substrate through laser engineered net shaping (LENS) to investigate its compatibility with the substrate material with a focus on interface bonding behavior and failure mechanism. It was hypothesized that the interface bonding failure between these two dissimilar materials occurs under a high degree of lattice mismatch across the interface. The mismatch relates with the misfit dislocation caused by different crystal lattice parameters and crystalline structures. The hypothesis is proved experimentally and theoretically in this study and the following findings are obtained: Fe element diffuses from the substrate due to an increased diffusivity, resulting in a brittle intermetallic Wairauite α’-(CoFe) at the planar transition layer. A large lattice misfit (more than 20%) exists between the FCC α’-Co solid solution matrix and the BCC α’-(CoFe) Wairauite phase, indicating an incoherent interface with a large amount of dislocations at the interface. It implies that there is severe local stress occurs in this area where cracks could easily arise and then propagate along the interface.

Simulation and experimental analysis of melt pool evolution in laser engineered net shaping

In this work, the evolution of melt pool under single-point and single-line printing in the laser engineered net shaping (LENS) process is analyzed. Firstly, the basic structure of the melt pool model of the LENS process is established and the necessary assumptions are made. Then, the establishment process of the multi-physical field model of the melt pool is introduced in detail. It is concluded that the simulation model results are highly consistent with the online measurement experiment results in terms of melt pool profile, space temperature gradient, and time temperature gradient. Meanwhile, some parameters, such as the 3D morphology and surface fluid field of the melt pool, which are not obtained in the online measurement experiment, are analyzed. Finally, the influence of changing the scanning speed on the profile, peak temperature, and temperature gradient of the single-line melt pool is also analyzed, and the following conclusions are obtained: With the increase in scanning speed, the profile of the melt pool gradually becomes slender; The relationship between peak temperature and scanning speed is approximately linear in a certain speed range; The space temperature gradient at the tail of the melt pool under different scanning speeds hardly changes with the scanning speed, and the time temperature gradient at the tail of the melt pool is in direct proportion to the scanning speed.

Identification and development of a new local corrosion mechanism in a Laser Engineered Net Shaped (LENS) biomedical Co-Cr-Mo alloy in Hank’s solution

In this work, the microstructure and the corrosion behavior of a biomedical grade Co-Cr-Mo (CCM) alloy manufactured by laser engineered net shaping (LENS) was characterized. Electron microprobe maps identified the micro-segregation of Cr and Mo to the cell boundaries in the microstructure of the LENS alloys. Cyclic polarization in Hank’s solution and study of the corroded surface under scanning electron microscopy (SEM) revealed the development of micro-galvanic cells and new crack-like features which served as active sites of localized dissolution of the alloy. Based on this evidence, a new corrosion mechanism that constituted of selective dissolution of Cr/Mo-depleted regions and development of crack-like features at junctions between the various cell types was proposed for LENS CCM alloys.

Microstructures and mechanical properties of crack-free 316L stainless steel and Inconel 625 joint by using Laser Engineered Net Shaping

The additively manufactured (AM) joint of 316L stainless steel (SS316L) and Inconel 625 (IN625) with flexible structural adjustability and good joint strength are pivotal components for aerospace and shipbuilding fields. As a promising metal additive manufacturing technology, Laser Engineered Net Shaping (LENS) technology brings possibilities for the combination of dissimilar metals. In this study, a gradient SS316L/IN625 joint with a composition ranging from 100% SS316L to 100% IN625 alloy was successfully prepared using LENS. The compositions, microstructures of each layer, and the interfaces between every two layers of the SS316L/IN625 joint were investigated. The results showed that the microstructure and elements were uniformly distributed along the deposition direction. No second phase or crack was detected in each layer or interface was observed in the SS316L/IN625 joint. The microhardness increased from 198 to 265 HV with increasing the content of IN625. The SS316L/IN625 joint showed a tensile strength of 672 MPa and a yield strength of 486 MPa.