جستجوی مقالات مرتبط با کلیدواژه "selective laser melting" در نشریات گروه "مواد و متالورژی"

The selective laser melting process is one of the additive manufacturing processes of metals in which a laser beam melts selected areas on a powder bed and creates the parts in layers. One of the important challenges of this method is to make a part without any defects and reaching 100% density. The material selected for this study is stainless steel 316L powder and the NOURA M100P machine was used to make the samples. In this research, six samples with different energy densities were made from 160 j/mm3 to 260 j/mm3. After making the samples, their density was measured by the Archimedes method. A sample made with an energy density of 220 j/mm3 has the highest relative density of 97.93. By examining the microstructure of the printed samples, it was determined that the samples have grain boundary cracks and incomplete fusion cavities and keyhole phenomenon.

Considering the nature of the additive manufacturing process and the produced layered structure, the possible gradient in microstructure can be predictable. For this purpose, the morphology and microstructure of the cross-section from top to bottom was evaluated. The morphology of the last and first printed layers, were also investigated.

Ti-10Mo was printed using mixed powder in 120 layers, each thickness of 25µm, by selective laser melting (SLM) with a laser power of 95 W, a scanning speed of 600 mm.s-1, and a hatching distance of 88 µm under argon atmosphere. Density was measured, and the constituent phases were identified by XRD. The microstructural feature was studied by optical and scanning electron microscopies.

The printed samples were dense, and the relative density was about 98.53%. Details in microstructural evaluation show spectacular Mo-enriched rims, which reveal the circumstance of Mo dissolution in molten Ti and homogenization, consequently. Also, a gradient in Mo dissolution is seen along the cross-section. So that, at the top, the sides of molten pools that are mostly Mo enriched are seen as thick white and bright rims in electron microscopy and as white to light purple in optical microscopy. However, at the bottom, the rims seem to be really thinner and smoother, which can be in consequence of enhanced diffusion of the Mo to Ti matrix. Here, the promoted diffusion could be in the result of heat transfer from the newly printed layer to the previous printed ones.

In this study, the addition of organic methionine inhibitor (as an eco-friendly inhibitor) to 0.1 M sulfuric acid media on corrosion resistance of 316L austenitic stainless steel (fabricated by rolling method and three-dimensional (3D) printing method) was investigated. Open-circuit potential electrochemical test and impedance, and structural tests such as optical and electron microscopy and x-ray photoelectron spectroscopy were conducted. The results showed that the corrosion resistance in the presence of inhibitor was higher than the sample without inhibitor and the inhibitory efficiency of methionine was increased up to 64% and the resistance to surface transfer between metal oxide and electrolyte was improved up to 2.77 times. The addition of methionine reduced the surface roughness and accumulation of the surface cavities. The chemical and physical adsorption mechanism of the inhibitor (negatively charged side adsorption of the methionine molecule with positively charged anodic regions of the metal surface) occurred at all points on the surface of the sample with the inhibitor. Also, the amount of oxygen in the cavities was reduced and the distribution of sulfur was uniform. The thickness of the passivator oxide layers was calculated more than the sample without inhibition due to the addition of inhibitor.

Additive manufacturing as the process of manufacturing engineering parts in a layer by layer manner, has been used for a few decades. Between different additive methods, selective laser melting is one of the most promising techniques. This is due to the high manufacturing quality specially when fabricating metallic compounds. Among different metallic compounds Inconel 625 has one of the most compatible alloys with additive processes due to the high strength properties, excellent weldability. Achieving properties in as built parts comparable to that of the conventionally manufactured counterparts, has been a challenge. These samples were built with linear heat input of 0.125, 0.150 and 0.175 joules per millimeter. Data suggested and increase in average grain size with increase in applied heat input. Also due to the different heating cycles specimen built with different heat input experience, grain structure and elements distribution in the specimen has changed. Also variation in heat input didn’t cause significant difference in mechanical properties of the samples; however, they have exhibited higher mechanical properties compared to casted counterparts and shown properties comparable to that of the wrought parts. Manufactured parts had significantly higher hardness compared to conventionally manufactured counterparts even that of the wrought ones. Based on optical microscopy studies, higher mechanical properties of the specimen can be considered a result of the fine grain size of the fabricated parts.

IN718 alloy is a high strength Nickel base superalloy that mainly used in moderate temperatures and corrosive service conditions. In the present study, samples produced by selective laser melting (SLM) at 1040 °C for 120 minutes were prepared under solution treatment. After solution, the samples were cooled in three cooling media of water, air and extinguished furnace with open door. Finally, the samples were aged in three standard conditions for 48 and 72 hours. To investigate the effect of cooling rate after solution and aging time on precipitates characteristics and microstructure of samples, Field Emission Scanning Electron Microscope (FESEM), X-ray diffraction (XRD) and hardness test were used. The results showed that increasing the aging time under constant solution conditions increases the volume fraction and the size of the secondary phases. Also, reducing the cooling rate in the solution step leads to increasing the hardness of the samples in the aging stage. Finally, with increasing aging time, the maximum hardness of the sample was 794H. v.

The prediction and control of solidification induced microstructure is an important issue during the product/process design stage in the selective laser melting additive manufacturing. It helps to avoid undesirable microstructures and possibly additional required post heat treatment, consequently improving the quality of manufactured parts. However, the direct numerical simulation of microstructure formation in this process is computationally very expensive, even by employing the state-of-the-art available computational resources. In the present study, an indirect approach based on empirical law is employed to predict the solidification microstructure. For this purpose, the macro-scale nonlinear heat equation include phase change effect is solved using the conventional finite element method and the local cooling rate and thermal gradient within the freezing interval is computed accordingly for Ti6-Al4-V alloy. Then, this information is projected on the empirical solidification microstructure map of this alloy to predict local microstructure, and the effects of process parameters like the laser power, laser effective radius (laser focus), scanning speed and scanning strategy on the solidification microstructure are investigated. According to the results, by decreasing the laser power, increasing the effective laser radius and laser scanning speed, the resulted grains morphologies can be varied gradually from the columnar to mixed columnar and equiaxed and completely equiaxed microstructures.