Minglei Qu, Qilin Guo, Luis I. Escano, Samuel J. Clark, Kamel Fezzaa, Lianyi Chen
Keyhole pore formation is one of the most detrimental subsurface defects in the laser metal additive manufacturing process. However, effective ways to mitigate keyhole pore formation beyond tuning laser processing conditions during keyhole mode laser melting are still lacking. Here we report a novel approach to mitigate keyhole pore formation during laser powder bed fusion (LPBF) process by using stable nanoparticles. The critical keyhole depth for keyhole pore generation (i.e., the largest keyhole depth without keyhole pore formation) during LPBF of Al6061 increases from 246 µm to 454 µm (85% increase) after adding TiC nanoparticles. In-depth x-ray imaging studies and thermo-fluid dynamics simulation enable us to identify that two mechanisms work together to mitigate keyhole pore generation: (1) adding nanoparticles prevents the keyhole from collapsing by increasing the liquid viscosity to impede the protrusion development; (2) adding nanoparticles slows down the keyhole pore movement by increasing the liquid viscosity, resulting in the recapturing of the pore by the keyhole. We further demonstrate that adding TiC nanoparticles can also eliminate the keyhole fluctuation induced keyhole pore during LPBF of Al6061. Our research provides a potential way to mitigate keyhole pore formation for defect lean metal additive manufacturing.
The reaction of aluminum powder fuel in a steam-combustion reaction is highly exothermic and can be used for thermal power plants to produce energy. This concept is ideal for use in underwater vehicles, where the presence of molecular oxygen negates the use of standard combustion reactions for power and energy generation. A significant technical challenge associated with the use of aluminum powder as a fuel is that powders of fine particle size are cohesive and have poor flow properties due to attractive interparticulate interactions. Reducing the inter-particulate cohesion through particle surface modification enables successful fuel delivery. This dissertation uses hydrophobic, siloxane based surface treatments to reduce the inter-particulate cohesion of aluminum particles and thus improve the flowability. Chapter 3 focuses on the bulk property enhancement of solution phase methyltriethoxysilane surface modified particles, which was revealed to improve bulk density, reduce cohesion, and improve aeration properties, even at fine particle size. Chapter 4 describes the design of a gas phase deposition process using volatized polydimethylsiloxane in a fluidized bed, which also subsequently improved the flowability of the treated powder. Chapter 5 discusses the development of an in situ technique to study fluidization behavior in an aerated environment through powder column expansion and estimation of minimum fluidization velocities. Overall, this work contributes to the development of a powder fuel to support the design of an aluminum powder fuel-steam power plant for use in underwater power and energy generation.
Aluminum F357 is a widely used material for casting in the aerospace and additive manufacturing industry. Heat treatments are commonly applied to some aluminum alloys to modify their properties. With a further study on the aging and performance of the F357 with 3D printing technology, several industries benefit from this; military, automotive, and aerospace are some examples because of the numerous components cast in service. This work presents the mechanical properties of F357 specimens fabricated with EOS technology and subjected to heat treatments. Heat treatment conditions were applied to tensile specimens and tested. Furthermore, the specimens were subjected to artificial thermal aging for 100 h and 1000 h at two different temperatures (285 ºF and 350 ºF), and their mechanical properties were also determined. Finally, remarks on the comparison between the heat treatments and the effect of thermal aging on the microstructures and mechanical properties of the specimens will be presented.
V.A. Medrano, E. Arrieta, J. Merino, B. Ruvalcaba, K. Caballero, B. Ramirez, J. Diemann, L.E. Murr, R.B. Wicker, D. Godfrey, M. Benedict, F. Medina
AlSi7Mg (F357) alloy specimens were fabricated in two different laser powder bed fusion (LPBF) systems: EOS M290 and SLM 280HL. Vertical (Z) and horizontal (XY) orientations were fabricated, and five different thermal post processes were applied to samples, individually. According to ASTM F3318-18, the considered thermal conditions were as-built, stress relieved (SR1), HIP, T6 and HIP+T6. Subsequently, the individual samples were aged at 140 °C and 177 °C for 100 h and for up to 1000 h. Tensile specimens were machined down from the aged samples and tested as per ASTM E8/E8M-21. While the yield stress (YS), elongation (%), and Vickers microindentation hardness (HV) were somewhat different for the as-built components, the general trends for the different heat treatments were essentially the same. As-built and SR1 treated microstructures were dominated by microdendritic cells, while the HIP, T6 and T6 + HIP component microstructures consisted of recrystallized grains containing eutectic Si particles of various sizes and shapes within the grain interiors and the grain boundaries; which gave rise to wide-ranging mechanical properties. As an example of these widely-ranging mechanical properties, it was observed that components fabricated in the Z or build direction in the EOS system exhibited a YS, elongation, and HV of 225 MPa, 13%, and HV120, while when HIPed and unaged exhibited values of 87 MPa, 25%, and HV51, respectively. These same HIPed components when aged at 177 °C for 1000 h exhibited values of 81 MPa, 41% and HV44. The mechanical properties of the unaged, HIPed and aged fabricated in Z direction in SLM system were 85 MPa, 31%, HV51, and 80 MPa, 42%, and HV47, respectively, providing support for LPBF system fabrication compatibility. These measured mechanical property values represent a small fraction of the more than 1600 mechanical property measurements (YS, UTS, elongation, and hardness (HV)) in this study.
This work presents the foundation and results of a study of the pulse laser powder bed fusion process (P-LPBF). It focuses on the unique opportunity offered for the additive manufacturing (AM) industry to control the microstructure and mechanical properties of high strength aluminum alloys.
A literature review has been presented covering the topics; additive manufacturing, high strength aluminum alloys, solidification physics, and evaporation phenomena. The work goes on to present results of a P-LPBF processing study of Al-Zn-Mg-Cu and Al-Mg-Si alloys. It was found that as built specimens of 98.0±0.5 Vol% density were achievable for the Al-Zn-Mg-Cu alloy, whereas samples with 99.3±0.4 Vol% density were produced from the Al-Mg-Si alloy.
Solidification microstructures were examined and found to present highly refined second phases with coarse melt pool regions for both alloys. Quantitative analysis of the solidification behaviour of the AlZn-Mg-Cu alloy was conducted with the CGM and KGT microstructure development models. It was concluded that neither alloy exhibited significant departure from equilibrium solidification behaviour. The bulk chemical analyses of P-LPBF specimens of each alloy indicated that significant solute loss had occurred. Standard T6 heat treatment procedures were applied to the samples, yielding the expected microstructure and hardness results. The observed hardness of the Al-Zn-Mg-Cu alloy samples were found to fall below the wrought material while Al-Mg-Si alloy samples were able to slightly exceed the wrought properties. Solute loss was identified as the main variable driving this phenomenon. The AA7175 composition was more sensitive to solute loss as it depended more heavily on precipitation strengthening, while AA6061 made significant use of the Hall-Petch hardening mechanism, allowing it to tolerate substantial solute loss while maintaining the desired mechanical properties.
In summary, this work has investigated the microstructural, mechanical, and chemical properties of two solidification crack susceptible aluminum alloys. The findings clearly demonstrate promising results for the feasibility of P-LPBF AM processing of high strength aluminium alloys.
Erfan Maleki, Sara Bagherifard, Asghar Heydari Astaraee, Simone Sgarbazzini, Michele Bandini, Mario Guagliano
Post-processing methods can be crucial in addressing the associated anomalies of the as-built state of additively manufactured materials. In this study, for the first time, the effects of gradient severe shot peening as a novel mechanical surface treatment, along with other types of shot peening treatments, including conventional, severe, and over shot peening processes were investigated individually and combined with heat treatment on fatigue behavior of hourglass AlSi10Mg samples fabricated by laser powder bed fusion. Detailed experimental characterizations in terms of microstructure, porosity, surface texture, hardness and residual stresses as well as rotating bending fatigue behavior were conducted. The experimental results revealed a significant fatigue behavior improvement after applying gradient severe shot peening treatments due to their remarkable capacity to modulate surface texture, known as a side effect of peening, besides surface layer nanocrystallization, enhanced hardness, and high compressive residual stresses.
S. Nam, E. Simsek, N. Argibay, O. Rios, H.B. Henderson, D. Weiss, E.E. Moore, A.P. Perron, S.K. McCall, R.T. Ott.
Al-Ce-based alloys are promising candidates for additive manufacturing (AM) due to their hot-cracking resistance and because they do not require heat treatment to obtain precipitation strengthening. Rapid solidification rates enabled by AM methods can lead to enhanced mechanical properties; however, the strengthening mechanisms over large composition ranges were unclear. Here, combinatorial synthesis by directed-energy deposition (DED) and hardness measurements were used to rapidly map the composition-dependent strength of the ternary Al-Ce-Mg system. Tensile testing and microstructure characterization of selected compositions were performed to elucidate the compositional dependence of the strengthening mechanisms. Al11Ce3 precipitates were present in all cases, and the maximum hardness (1.25 GPa) was measured for the Al-8Ce-10Mg composition. A combination of (i) Hall-Petch strengthening, based on the FCC-matrix-phase cell size; (ii) particle strengthening, based on Al11Ce3 volume fraction and size; and (iii) solid-solution strengthening, based on Mg composition of the matrix phase, were used to account for the measured strengths. Vickers hardness is shown to correlate well with ultimate tensile strength in these alloys, highlighting the value of surface-based techniques for rapid screening.
Matteo Vanzetti , Enrico Virgillito, Alberta Aversa, Diego Manfredi, Federica Bondioli, Mariangela Lombardi and Paolo Fino
Conventionally processed precipitation hardening aluminum alloys are generally treated with T6 heat treatments which are time-consuming and generally optimized for conventionally processed microstructures. Alternatively, parts produced by laser powder bed fusion (L-PBF) are characterized by unique microstructures made of very fine and metastable phases. These peculiar features require specifically optimized heat treatments. This work evaluates the effects of a short T6 heat treatment on L-PBF AlSi7Mg samples. The samples underwent a solution step of 15 min at 540 ◦C followed by water quenching and subsequently by an artificial aging at 170 ◦C for 2–8 h. The heat treated samples were characterized from a microstructural and mechanical point of view and compared with both as-built and direct aging (DA) treated samples. The results show that a 15 min solution treatment at 540 ◦C allows the dissolution of the very fine phases obtained during the L-PBF process; the subsequent heat treatment at 170 ◦C for 6 h makes it possible to obtain slightly lower tensile properties compared to those of the standard T6. With respect to the DA samples, higher elongation was achieved. These results show that this heat treatment can be of great benefit for the industry.
Kyle Tsaknopoulos, Caitlin Walde, Derek Tsaknopoulos and Danielle L. Cote
Gas-atomized powders are frequently used in metal additive manufacturing (MAM) processes. During consolidation, certain properties and microstructural features of the feedstock can be retained. Such features include porosity, secondary phases, and oxides. Of particular importance to alloys such as Al 6061, secondary phases found in the feedstock powder can be directly related to those of the final consolidated form, especially for solid-state additive manufacturing. Al 6061 is a heattreatable alloy that is commonly available in powder form. While heat treatments of 6061 have been widely studied in wrought form, little work has been performed to study the process in powders. This work investigates the evolution of the Fe-containing precipitates in gas-atomized Al 6061 powder through the use of scanning and transmission electron microscopy (SEM and TEM) and energy dispersive X-ray spectroscopy (EDS). The use of coupled EDS and thermodynamic modeling suggests that the as-atomized powders contain Al13Fe4 at the microstructure boundaries in addition to Mg2Si. After one hour of thermal treatment at 530 ◦C, it appears that the dissolution of Mg2Si and Al13Fe4 occurs concurrently with the formation of Al15Si2M4 , as suggested by thermodynamic models.
Kyle Tsaknopoulos, Caitlin Walde, Derek Tsaknopoulos, Victor Champagne and Danielle Cote
Aluminum 5056 is a work-hardenable alloy known for its corrosion resistance with new applications in additive manufacturing. A good understanding of the secondary phases in Al 5056 powders is important for understanding the properties of the final parts. In this study, the effects of different thermal treatments on the microstructure of Al 5056 powder were studied. Thermodynamic models were used to guide the interpretation of the microstructure as a function of thermal treatment, providing insight into the stability of different possible phases present in the alloy. Through the use of transmission electron microscopy (TEM) and energy-dispersive X-ray spectroscopy (EDS), combined with thermodynamic modeling, a greater understanding of the internal microstructure of Al 5056 powder has been achieved in both the as-atomized and thermally treated conditions. Evidence of natural aging within these powders was observed, which speaks to the shelf-life of these powders and the importance of proper treatment and storage to maintain consistent results.
Kyle Tsaknopoulos, Caitlin Walde, Victor Champagne, and Danielle Cote
Metal additive manufacturing processes often use gas-atomized powder as feedstock, but these processes use different methods for consolidation. Depending on the consolidation temperature, secondary phases may be retained during processing, making it important to understand powder microstructure prior to consolidation. Commercial alloy compositions are typically used for these powders because they have been widely studied and qualified; however, the microstructure of the powder form of these compositions has not been studied. This paper aims to understand the commercial Al 6061 powder: how the internal microstructure of the powder differs from wrought both in the as-manufactured and thermally-treated conditions. A specific focus is put on the Mg-rich phases and their morphologies. This was accomplished through transmission electron microscopy, scanning transmission electron microscopy, and energy dispersive x-ray spectroscopy. Both the size and morphology of the phases in the powder differ greatly from those in the wrought form.
Caitlin Walde, Kyle Tsaknopoulos, Victor Champagne, Danielle Cote
Al 7075 is a heat-treatable Al–Mg–Zn alloy widely used in the aerospace industry. Recently, it has found application as feedstock for metal additive manufacturing (MAM). It has been shown that wrought alloy compositions in powder form differ in microstructure and properties from their conventional form. Given this, it is important to understand the microstructure of the powders prior to use in MAM processes. This work studies as-atomized gas-atomized Al 7075 powders and the effect of thermal treatments on microstructure. Extensive electron microscopy revealed the presence of T-phase, Al7Cu2Fe, and Mg2Si in the as-atomized condition. Thermal treatments were performed at 465 °C and 480 °C to homogenize the microstructure; however, S-phase was unexpectedly present in the samples treated at 465 °C. In both 465 °C and 480 °C treatments, T-phase was not fully dissolved after the 60-min treatment. Guided by thermodynamic modeling, these results indicate a shift in local equilibria in these powders.
International Journal of Material Science and Research 1 (2018) 50-55
A. Bhagavatam, A. Ramakrishnan, V.S.K. Adapa, G.P. Dinda, et al.
Additive manufacturing (AM) has become one of the most important research topics with its ability to manufacture a wide range of alloys like steel, nickel-based super alloys, titanium alloys, aluminum alloys, etc. Al 7075 is not a friendly alloy for laser metal deposition (LMD). This paper reports the successful development of LMD process for deposition of defect-free Al 7075 alloy. By preheating the substrate to 260°C the residual stress decreased and eliminated the hot/solidification cracks in the deposit. LMD is a rapid cooling process due to which the gas bubbles of Mg and Zn are trapped in the deposit. These are identified as gas porosity because of the partial evaporation of low boiling point elements like magnesium and zinc present in this alloy. The least porosity observed was 0.08% at 29 J/mm2 of energy input. The SEM and EDS investigation of as-deposited Al 7075 revealed the segregation of Cu, Mg, and Zn rich phases along the inter dendritic regions and grain boundaries. Cu, Mg, and Zn rich phases at the inter dendritic regions dissolved into the α-Almatrix after heat treatment. The XRD scan of laser deposited Al 7075 revealed the presence of Al2CuMg and MgZn2 precipitation hardening phases.
Recently, Al-Si alloys samples have been manufactured at lab scale using various additive manufacturing processes, but so far there is no literature available to investigate the feasibility of fabricating Al-Si alloy component for automotive component applications using Direct Laser Metal Deposition (LMD) technique. This paper deals with the practical challenges of building single wall and block deposition (cuboid shapes) of eutectic Al-Si alloy using direct laser metal deposition process for developing automotive applications. Two scanning pattern, hatch pattern and single wall pattern were chosen to study the effect of scanning direction on mechanical properties as well as microstructural evolution. Microstructural investigation of single wall and block deposition using optical and scanning electron microscopy revealed a 99.9% dense component with very fine hypoeutectic microstructure. Tensile test sample extracted from block deposition showed an impressive elongation of 9% with an ultimate tensile strength of 225 MPa and tensile test sample of single wall showed an average elongation of 9.4% with an ultimate tensile strength of 225 MPa. This investigation revealed that direct laser metal deposition could successfully print the eutectic Al-Si alloy bracket on shock tower hood without any distortion or bending.
Microstructural, Metallurgical and Materials Transactions A 44 (2013) 2233-2242.
G.P. Dinda, A.K. Dasgupta, S. Bhattacharya, H. Natu, B. Dutta, J. Mazumder,
Direct metal deposition (DMD) technology is a laser-aided rapid prototyping method that can be used to fabricate near net shape components from their CAD files. In the present study, a series of Al-Si samples have been deposited by DMD in order to optimize the laser deposition parameters to produce high quality deposit with minimum porosity and maximum deposition rate. This paper presents the microstructural evolution of the as-deposited Al 4047 sample produced with optimized process parameters. Optical, scanning, and transmission electron microscopes have been employed to characterize the microstructure of the deposit. The electron backscattered diffraction method was used to investigate the grain size distribution, grain boundary misorientation, and texture of the deposits. Metallographic investigation revealed that the microstructural morphology strongly varies with the location of the deposit. The layer boundaries consist of equiaxed Si particles distributed in the Al matrix. However, a systematic transition from columnar Al dendrites to equiaxed dendrites has been observed in each layer. The observed variation of the microstructure was correlated with the thermal history and local cooling rate of the melt pool.
Surface and Coatings Technology 206 (2012), 2152-2160.
G.P. Dinda, A.K. Dasgupta, J. Mazumder
Laser melting of Al–Si alloys has been investigated extensively, however, little work on the microstructural evolution of laser deposited Al–Si alloys has been reported to date. This paper presents a detailed microstructural investigation of laser deposited Al–11.28Si alloy. Laser aided direct metal deposition (DMD) process has been used to build up solid thin wall samples using Al 4047 prealloyed powder. The evolution of macro- and microstructures of laser deposited Al–Si samples was investigated using X-ray diffraction, optical microscopy, scanning electron microscopy and electron backscattered diffraction techniques. Microstructural observation revealed that the morphology and the length scale of the microstructures are different at different locations of the sample. A periodic transition of microstructural morphology from columnar dendrite to microcellular structure was observed in each layer. The observed difference in the microstructure was correlated with the thermal history of the deposit.
Metallurgical and Materials Transactions B volume 51, pages2230–2239 (2020)
M. Skelton, C. V. Headley, E. J. Sullivan, J. M. Fitz-Gerald & J. A. Floro
Gas-atomized powders are commonly used in additive manufacturing, specifically laser powder bed fusion, due to their high flowability during recoating. Morphological changes can occur in particles that are irradiated by the laser during additive manufacturing, but are not incorporated into the melt pool. These irradiated particles will affect the rheology of the recycled powder in subsequent builds, potentially leading to failures due to uneven powder flow or spatial distribution. Thus, a better understanding of mechanisms that degrade the sphericity of powder after being laser irradiated is needed. This research examines morphological changes in Al and Al-Cu eutectic powders after laser melting. Two complementary approaches were taken. First, particles found along the edges of line scans following high-power (300 W) laser irradiation were characterized. The collected particles displayed morphological anomalies not observed in the as-received powder. Then, to gain a more quantitative and controlled perspective on morphological evolution, the same base powders were dispersed onto glass substrates and irradiated with a low-power (6.5 W) CW laser diode. This approach, which permits characterization of specific particles before and after laser irradiation, clearly shows laser-induced changes in the surface morphology of particles in the form of dents and rifts. These results suggest that isolated melting and resolidification of particles contained within their respective oxide shells can occur at the relatively low laser energy densities present at the edges of laser melt tracks. Thermal stresses developing in the oxide shell during cooling can account for the observed morphological changes in the context of shell-buckling theory.