Department of Mechanical and Mechatronic Engineering

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Chemical etching investigation on surface finish and fatigue behaviour of laser powder bed fusion produced Ti-6Al- 4V.
(Stellenbosch : Stellenbosch University, 2021-12) Malcolm, J. S.; Becker, Thorsten Hermann ; Stellenbosch University. Faculty of Engineering. Dept. of Mechanical and Mechatronic Engineering.
ENGLISH ABSTRACT: An experimental methodology which implemented the practice of chemical etching was developed with the aim of reducing the surface roughness of laser powder bed fusion (LPBF) produced Ti-6AI-4V and thereby, improving the high cycle fatigue (HCF) performance. Hydrofluoric-nitric acid solutions, HF-HNO3, were used in the chemical etching experiments. The concentration of the HNO3 acid was fixed at 20 % while the concentration of the HF acid was varied to alter the solution concentration. The surface roughness of the LPBF produced Ti-6AI-4V samples were measured using a skidded contact profiler instrument. The subsequent chemical etching investigations revealed that an equivalent reduction in surface roughness can be achieved using either a low 2 % HF acid, or a high 12 % HF acid etching solution concentration. The solution temperature investigation demonstrated that the mass removal rate of a chemical etching process can be increased with raising solution temperature, albeit at the expense of process control. From the insights obtained from the chemical etching investigations, the recommended solution to reduce the surface roughness is a 2 % - 4 % HF acid concentration in conjunction with a solution temperature that is below room temperature. Etching durations of 60 minutes and longer allow for the minimum surface roughness that chemical etching as-built (AB) LPBF samples can be achieved. The internal pores in LPBF produced samples become exposed at the surface during the chemical etching process which limits the average surface roughness, Ra, from reducing to values significantly lower than Ra ≈ 2 μm. The HCF testing conducted was tension-tension cyclic uniaxial testing with a stress ratio, R, of R = 0.1. The LPBF produced Ti-6Al-4V fatigue samples were separated into four groups: the AB samples and then the three remaining sample groups which were chemically etched in a 4%HF20%HNO3 solution for 5, 15 and 90 minutes, respectively. The fatigue samples which received the 90 minute etch had both the lowest average surface roughness, Ra = 1.84 ± 0.03 μm, and the largest improvement in fatigue performance with reference to the AB sample’s fatigue behaviour. All the fatigue failed samples had crack initiation sites at the surface of the sample. The fabrication process-induced surface roughness acted as crack initiation sites for the AB and 5 minute etch sample groups. Chemical etching caused the controlling defect responsible for fatigue crack initiation to change from surface roughness to internal defects which were brought to the surface by the chemical etching mechanism. The opened subsurface defects acted as stress raisers which caused the fatigue crack initiations for the 15 and 90 minute etch sample groups.
Critical powder characteristics for high quality laser powder bed fusion
(Stellenbosch : Stellenbosch University, 2024-02) De Waal, A; Blaine, DC; Stellenbosch University. Faculty of Engineering. Dept. of Mechanical and Mechatronic Engineering.
ENGLISH ABSTRACT: The acceptance of Additive Manufacturing (AM) as a reliable manufacturing technology in South Africa and globally, is still limited. This is due to concerns about replicability and variations in quality. Efforts to enhance AM standards aim to address these challenges by focusing on understanding raw metal materials. Characterizing and comprehending powder materials pose significant challenges for the AM community, especially in technologies utilizing a powder bed. Powder behaviour during spreading is a relatively new research area that is not fully understood. Academia and industry practitioners are actively exploring these concepts to enhance AM's quality, reliability, and replicability. This research aims to identify and validate critical metal powder characteristics that include the powder morphology, particle size distribution, moisture content, flowability and various densities of the powder, and their relation to efficient powder spreadability characteristics that effectively describe the powder layer quality. Furthermore, the relationship of these characteristics to consistently building high-quality final parts using the AM technique known as laser-powder bed fusion (L-PBF) must be established. The results obtained in this project aid the Collaborative Programme in Additive Manufacturing (CPAM) strategy that is focused on identifying L-PBF as a reliable and accepted manufacturing technique in South Africa as well as globally. Standardised and customised powder characterisation testing was conducted on different powders that included two spherical powders that are designed for LPBF, namely, commercially pure titanium (CP Ti) and Ti-6Al-4V, as well as a CP Ti powder that is produced by the hydride-dehydride process (HDH), that displays an angular particle shape and is not typically used for L-PBF. The results from the powder characterisation experiments successfully identified critical powder characteristics that influence powder spreadability and, consequently, the as-built part quality. Thus, the impact of powder characteristics on the spreadability of the powder and the resultant direct effect on the powder layer quality is demonstrated, emphasizing the importance of aligning spreading parameters with the critical powder characteristics that include powder flowability, packing ability, and settling rate for consistent, high-quality powder layers. A criteria system that validates whether a powder is suitable for L-PBF, based on the powder characteristics, was identified, and used to determine whether a mixture of the spherical and HDH CP Ti powders is useable for L-PBF as a more cost-effective solution. It was determined that a 90:10 mass ratio mixture of the spherical to HDH CP Ti powders delivered suitable spreadability results for L-PBF.
Development of titanium alloys for laser powder bed fusion with the use of d-optimal design
(Stellenbosch : Stellenbosch University, 2023-03) Barnard, Christoffel Jacobus; Becker, Thorsten Hermann ; Ter Haar, Gerrit; Stellenbosch University. Faculty of Engineering. Dept. of Mechanical and Mechatronic Engineering.
ENGLISH SUMMARY: Titanium is a costly and difficult material to manufacture, therefore it has primarily been used in high-value applications. The excellent material properties of titanium are what makes the material difficult to manufacture using traditional manufacturing methods. In the growing industry of additive manufacturing titanium has been adopted as one of the primary materials. The ability to manufacture intricate parts with little waste being very suited to eliminate the problems of titanium manufacturing. Titanium and its alloys have been extensively used in the expensive laser powder bed fusion process, primarily the Ti6Al4V alloy. The manufacturing process parameters for this alloy have thus been thoroughly optimised. The inherent cost of this process and of titanium make it expensive to develop new alloys for this manufacturing process. This research study investigates the use of D-optimal design experiments to determine the optimal process parameters for the TC11 and Ti6246 α+β titanium alloys and to construct a mathematical model that predicts the porosity of the alloys depending on the process parameters. A D-optimal design experiment was designed with the use of Design Expert software to find the optimal process parameters and to determine the terms of the mathematical model. Promising process parameter combinations from this experiment was then used to manufacture tensile samples to investigate the mechanical properties of the as-manufactured alloys and to compare them to the wrought alloys. The Ti6Al4V alloy was also manufactured as another way to determine the efficacy of the D-optimal approach. The Ti6Al4V alloy was manufactured with densities ranging from 97 to 100 %. The mathematical model was able to predict the porosity of samples with an overall error of 0.13 %. The TC11 alloy was manufactured with densities ranging from 97 to 100 %. The mathematical model was found to be able to predict the porosity of samples with an overall error of 0.513%. The mechanical properties of the TC11 tensile samples were superior to the wrought alloy in tensile strength and had similar elongation. Tensile strengths and elongation of 1511 MPa and 16.8 % were found. The Ti6246 alloy was manufactured with densities ranging between 96.5 to 100 %. The mathematical model was able to predict the porosity of samples with a mean error of 0.129 % up to 1.95 %. Tensile samples were manufactured with tensile strengths superior to that of the wrought alloy, but with very low ductility for moderate and high VED levels (167 and 250 J/mm3). Tensile samples manufactured at low VED level showed superior elongation and tensile strength compared to the wrought alloy, but with a much lower yield strength.
Effects of defects on mechanical properties in metal additive manufacturing : a review focusing on X-ray tomography insights
(Elsevier, 2019) Du Plessis, Anton; Yadroitsava, Ina; Yadroitsev, Igor
ENGLISH ABSTRACT: X-ray tomography has emerged as a uniquely powerful and non-destructive tool to analyze defects in additive manufacturing. Defects include unintended porosity, rough surfaces and deviations from design, which can have different root causes and can vary significantly among samples. Powder material properties, non-uniform delivery of the powder layer, deformation during manufacturing, deviations from optimal process-parameters caused by changes in the laser beam, the optical components and the scanning system operation, may result in lack of fusion pores, metallurgical pores, keyhole pores, etc. These different types of pores have different typical sizes, shapes and 3D distributions. All types of defects have effects on the mechanical properties of a final part. The use of X-ray tomography to visualize pores in parts (non-destructively) prior to mechanical testing has allowed us to improve our understanding of the effect of this porosity on the mechanical properties of the part (also referred to as “effect of defect”). This can provide the possibility to discriminate critical defects from harmless ones, and thereby build confidence in additivemanufacturing processes. This paper reviews the current state of knowledge with regard to the “effect of defect” in metal additivemanufacturing, and highlights some relevant examples from our recent work.
Fatigue crack growth rate threshold of laser powder bed Fusion Ti-6AI-4V
(Stellenbosch University, 2021-12) Dhansay, N. M.; Becker, Thorsten Hermann ; Stellenbosch University. Faculty of Engineering. Dept. of Mechanical and Mechatronic Engineering.
ENGLISH ABSTRACT: Typically, producing Ti-6Al-4V through the laser powder bed fusion (LPBF) technique, results in the material having large residual stresses and martensitic microstructure. These stresses and microstructure have been shown to result in Ti-6Al-4V having poor fatigue properties. However, the insight into the fatigue failure mechanisms caused by the residual stress and microstructure has been limited. LPBF is one of many additive manufacturing (AM) techniques in which parts are built in a layer- wise manner with the use of powdered material and consolidated through high power laser melting. This allows for complex geometries and previously impossible geometries to be manufactured with minimal material wastage. Many industries are aware of the potential in this manufacturing technique and have shown interest in it becoming a viable option for the manufacturing of some of their components. In particular, the use of LPBF produced Ti-6Al-4V is of interest to the aerospace and biomedical industries. This is because Ti-6Al-4V is already well established in many existing industries. For LPBF produced Ti-6Al-4V to be a viable option in industry, researchers need to have an adequate understanding of the material and the implications for its mechanical properties. Fatigue property investigations have largely focused on the fatigue life approach (crack initiation) and Paris regime (region II, crack propagation). However, in recent years, the near-threshold regime (region I, crack propagation) has become of interest, albeit limited in approach. The consensus within literature shows that the large tensile residual stresses, martensitic microstructure and porosity results in poor fatigue properties. Unfortunately, the insight into the fatigue fracture mechanisms brought about by the residual stress and microstructure is not yet well established. Furthermore, the near-threshold fatigue crack growth rate regime (FCGR) experiences crack closure mechanisms which result in premature near-threshold values. The majority of the near-threshold investigations on LPBF produced Ti-6Al-4V do not account for the crack closure mechanisms and therefore produce premature results. As a result of the low crack growth rates achieved in the near-threshold regime, a window into observing the fatigue crack initiation mechanisms is obtained. More specifically, how the fatigue crack initiation mechanisms are influenced by residual stress, martensitic microstructure and changing microstructural morphology. Literature has shown that porosity acts as crack initiation sites and reduce the fatigue life of a component. Furthermore, surface and near-surface porosity have been shown to have a more severe impact on fatigue life than internal porosity. It is through using the near-threshold FCGR approach in which one can calculate the allowable pore size under operational loads. Using the load-shedding technique to obtain near-threshold FCGRs, the results showed anisotropic behaviour dependent on residual stress levels and R-ratios. The fatigue fracture mechanisms were predominantly governed by transgranular quasi-cleavage mechanisms. Furthermore, the fracture is directed by PBG morphology which results in anisotropic crack- closure effects. In addition, primary α lath orientation is governed by the crystallographic texture of the PBG which influences the mechanisms of crack propagation. With the increase in grain size, the presence of β improves the near-threshold FCGRs in the duplex anneal (DA) condition due to the superior plastic flow abilities more than in the as-fabricated and stress relief conditions. This study investigates the near-threshold FCGRs of LPBG produced Ti-6Al-4V in the as- fabricated, stress relief and bi-modal conditions in three build orientations. In addition, crack closure mechanisms are accounted for by implementing variable R-ratio testing. This research presents the influencing mechanisms of residual stress and microstructure on fatigue behaviour.
Feasibility of additive manufacturing for patient-specific knee replacements.
(2021-12) Nortje, B. D.; Van der Merwe, J. H.; Becker, Thorsten Hermann
ENGLISH ABSTRACT: Osteoarthritis causes the degradation of the articular cartilage of the knee. This results in a loss of function of the joint. With a total knee replacement surgery, these articular surfaces are removed and replaced with artificial components. The ability to integrate patient-specific implant components can improve the results of a total knee replacement. The focus of this research is to investigate the feasibility of manufacturing methods required for a femoral knee replacement component for patient-specific implants. Through understanding what has been accomplished in literature, a multi-directional pin-on-flat wear tester is developed. This machine is based on the motion that is observed by the knee articulation in specific. A cross-shear ratio is designed to achieve similar wear characteristics to that seen in a knee replacement. This development is validated through the use of Co-Cr-Mo samples. The developed and implemented machine is utilised to perform an assessment of laser powder bed fusion, a powder bed based additive manufacturing technique, for use in patient-specific femoral knee replacements. Focus is placed on the use of Ti-6Al-4V and Co-Cr-Mo; where the latter is used for a benchmark to indicate the relative wear properties of the former. The average wear rates that resulted after 3 x 106 cycles were 2.58 mg/MC and 2.63 mg/MC for Co-Cr-Mo and Ti-6Al-4V, respectively. Additive manufacturing provides the ability to manufacture a near net shape component that is suited for the patient’s geometry. Orthopaedic surgeons consider the full and natural functioning of the replaced joint to be an indication of the operation’s success. The unsatisfactory performance of an implant can be attributed to the incorrect tension of the ligaments surround- ing the joint; this is often found to be a result of geometric intolerances of the standard implant sizes in relation to the patient’s natural joint. This can be rectified through the use of additive manufacturing which provides the ability to manufacture components of high geometric tolerances. A feasibility analysis is done with regards to cost and suitability of man- ufacturing processes for patient-specific femoral knee replacements. Additive manufacturing is compared to CNC machining to assess the feasibility of pur- suing a patient-specific femoral knee replacement that is manufactured from Ti-6Al-4V. The device that was developed illustrated satisfactory performance in ac- cordance to what is expected from literature. Through these tests, Ti-6Al-4V displayed suitable wear properties for use in articulating joints. The study places focus on the knee joint, but can be adapted for many other articulating joints with lower loading. It can then be shown that additive manufacturing may not be a feasible option for manufacturing based on cost, but may prove beneficial for reasons such as geometry complexities, surface finish, accuracy and extent to which the surfaces can be altered.
Fracture mechanics-based fatigue life assessment of additively manufactured Ti-6Al-4V
(Stellenbosch : Stellenbosch University, 2024-02) Macallister, N; Becker, Thorsten Hermann ; Blaine, DC; Stellenbosch University. Faculty of Engineering. Dept. of Mechanical and Mechatronic Engineering.
ENGLISH ABSTRACT: This dissertation presents a study on fracture mechanics-based fatigue life assessment for Additively Manufacturing (AM). The mature laser powder bed fusion (LPBF) process with the Ti-6Al-4V alloy in particular is selected for study, as it is well suited to the South African context with regard to economic climate, strong AM relationships and abundant mineral titanium reserves available. Furthermore, the Ti-6Al-4V alloy is a staple of aerospace, automotive and biomedical industries which are amongst the largest promoters for using AM technology, and for whom fatigue characterisation remains a prevalent topic as many end-use applications are intended for cyclic loading. Though significant research in fatigue behaviour exists, the conundrum of reliably certifying fatigue life in AM parts persists. This problem stems from the complex relation between AM print parameters, build orientation, surface roughness, inherent defects, residual stresses, meso- and microstructure; and establishing reportable fatigue strength baseline values required by industry. Moreover, as the AM environment promises saving in cost and time, full fatigue testing schemas are undesirable. As such, alternate damage-tolerant methods are becoming increasingly popular, where adopting fracture mechanics-based frameworks accompanied by limited or non-destructive testing could aid in certification. For this purpose, the dissertation first presents a novel version of the fatigue predictive NASGRO model where parameters are established that are unique to LPBF produced Ti-6Al-4V meso- and microstructures. In establishing these parameters for LPBF produced Ti-6Al-4V, the influence of process inherent microstructure, residual stress, and orientational dependant meso- structure is considered through examining near-threshold in combination with steady-state fatigue crack growth rates. The analysis shows that the descriptors of material constraint are sensitive to build orientation and microstructure. Furthermore, the effect of residual stresses is observed to not be severe. In this a clear effect of build orientation and meso- and microstructure is established for selecting NASGRO model parameters. Secondly, the proposed NASGRO formulation is translated into a comprehensive novel damage-tolerant fracture mechanics-based model to estimate fatigue life. Non-uniform defect populations, typical of AM material, in terms of size, shape and location are captured through X-ray tomography and surface profilometry and used as inputs modelled as equivalent crack lengths. The fatigue strength estimations are shown to be sensitive to fatigue crack growth rate threshold parameters and short crack growth mechanic descriptions. Furthermore, by introducing multiple crack initiations, the fatigue estimates are shown as distributions and are sensitive to defect number. Finally, sub-size specimen testing is investigated as a potentially elegant solution to accompany fatigue life assessments for threshold validation. Where results show inconsistent near-threshold fatigue behaviour linked to the microstructure. In this, considering unique meso- and microstructural features of LPBF produced Ti-6Al-4V, the domain and suitability in using sub-size specimens for fatigue crack growth rate threshold testing is discussed. Overall, this dissertation walks the path required in establishing reliable damage tolerant fatigue life estimation approaches for LPBF produced Ti-6Al-4V. Providing fundamental insights into interactions of fracture mechanic mechanisms and descriptions necessary for reliably modelling fatigue behaviour, therefore contributing to the developing frameworks and philosophies in AM to help in certification of fatigue performance of LPBF produced Ti-6Al-4V components.
Functionally stiff lattice structure for bone reconstruction
(Stellenbosch : Stellenbosch University, 2023-03) Heynemann, Marli; Van der Merwe, Johan; Becker, Thorsten Hermann ; Stellenbosch University. Faculty of Engineering. Dept. of Mechanical and Mechatronic Engineering.
ENGLISH SUMMARY: The treatment of segmental bone loss with reconstructive procedures based on bone grafting is associated with the limitation of donor bone availability and complications at the harvest site, introducing a second surgical site on a patient, increased operating times and increased opportunity of infection. Endoprosthesis, used as a bone substitution, is an alternative reconstructive method which reduces the pain experienced after the procedure, operating time and healing period before a patient may resume load-bearing activities. However, unoptimised titanium implants used for bone substitution have higher stiffness properties than the surrounding bone. The incompatibility in stiffness between the implant and the surrounding bone causes stress shielding, which leads to bone resorption at the interface between the implant and the bone. The reduction in bone mass surrounding the implant leads to complications such as implant loosening, bone fractures and possibly implant failure, which leads to further corrective surgeries affecting the patient’s quality of life. This resulted in the investigation of using a lattice structure to reduce the apparent stiffness of an implant to match, or mimic, the stiffness of bone to reduce the stress shielding phenomenon. A body-centred cuboid unit cell was identified and investigated to match the longitudinal and transverse stiffness bone when repeated in a Ti-6Al-4V (ELI) lattice structure. An analytical model was used to design a body-centred cuboid unit cell with an expected longitudinal stiffness of 17.9 GPa and a transverse stiffness of 10.1 GPa, similar to human cortical bone's average longitudinal and transverse stiffness. The stiffness of the lattice structure was verified by performing compression tests on lattice test specimens according to ISO 13314. The measured elastic modulus of the lattice structure was 14.7 GPa in the longitudinal direction and 8.4 GPa in the transverse direction. The average longitudinal stiffness of bone varies between 14 GPa – 21.8 GPa; consequently, the unit cell stiffness could match the longitudinal stiffness of cortical bone. The measured transverse stiffness of the unit cell also falls within the range of the average transverse stiffness of cortical bone between 7.7 GPa and 12.5 GPa. Based on the compression test findings, the proposed lattice structure could mimic the anisotropic stiffness of bone. Therefore, implementing the proposed unit cell in the design of titanium bone substitution implants could reduce the stress shielding phenomenon. This would increase the success rate of titanium endoprostheses and improve a patient’s quality of life by reducing the effects of the complications associated with stress shielding.
High-temperature mechanical property characterisation of additively manufactured Inconel 718
(Stellenbosch : Stellenbosch University, 2024-02) Blackwell, MA; Neaves, Melody; Becker, Thorsten Hermann ; Stellenbosch University. Faculty of Engineering. Dept. of Mechanical and Mechatronic Engineering.
ENGLISH ABSTRACT: As aerospace technology advances and the need for more advanced and intricate parts rises, traditional manufacturing methods such as casting fall short. Additive manufacturing techniques, such as laser-powder bed fusion (LPBF), have shown promising results in creating the intricate parts required. Near full density parts have been achieved, making these parts viable for use; however, the additive manufacturing process results in a unique microstructure not found in their wrought counterparts. The high temperature performance of the LPBF parts is not well documented. These characteristics are crucial, given the demanding operating conditions that these aerospace parts will encounter. Inconel 718 (IN718) is one of the most widely used alloys for high temperature aerospace applications. This research investigates the high temperature mechanical properties of LPBF produced IN718 with a specific focus on identifying whether LPBF IN718 exhibits a similar rapid strength drop-off above 650 ˚C as the wrought material. Specimens were produced using a Concept Laser Mlab 200R Laser Cusing machine and heat treated according to one of two heat treatment schemes, one of which was chosen due to its potential to improve high temperature strength, with the other more standard heat treatment being chosen for comparison. Specimen quality and heat treatment effects were confirmed through density testing and microstructural analysis. Mechanical testing was performed using a Gleeble 3800 thermomechanical simulator which resistively heats the specimen, creating a temperature gradient across the gauge region. This allows for properties to be extracted over a range of temperatures from a single specimen. Displacement and full-field thermal data were simultaneously captured using a stereo digital image correlation system and infrared camera. These datasets were both temporally and spatially synchronised using a common triggering system and calibration procedure, respectively. This data was subsequently processed to extract temperature dependant mechanical properties; namely, Young’s modulus, Poisson’s ratio, yield strength, ultimate tensile strength and percentage elongation at failure. These properties were compared to those of both wrought and LPBF produced IN718 found in literature. It was found that the LPBF produced IN718 specimens exhibited a similar strength drop-off above 650 ˚C. In addition, the data showed that the LPBF IN718 specimens subjected to the first heat treatment scheme achieved strengths comparable to that of the wrought material and were able to achieve the minimum strength requirements defined in Aerospace Material Standard (AMS) 5663, while the specimens subject to the second, more standard, heat treatment scheme were unable to meet the necessary strength requirements. None of the LPBF specimens were able to meet the elongation requirements dictated by AMS5663 at 650 ˚C. Significant anisotropy of the material properties were observed depending on build orientation.
Influence of powder characteristics on the spreadability of pre-alloyed tungsten-carbide cobalt
(Southern African Institute for Industrial Engineering, 2021) Govender, Preyin; Blaine, Deborah Clare; Sacks, Natasha
With rising interest in additive manufacturing (AM) techniques, there is an increased focus on research that evaluates critical parameters that guide the selection of powders that are suitable for AM. One such parameter is a powder’s spreadability, described by metrics such as powder bed density and percentage coverage. This study focused on three spray-dried WC-Co powders (two 12 wt% and one 17 wt% Co) and evaluated the influence of typical powder characteristics, such as particle size and shape, apparent density, and flow rate, on their spreadability. It was found that particle size distribution influenced the powder spreadability. Larger particles hindered the even spreading of powder over the base plate, resulting in low powder bed density and percentage coverage. This also correlated with the powders’ apparent densities. The flow rate and angle of repose gave an indication of how cohesive the powders are. The more cohesive a powder, the poorer the spreadability, resulting in a lower powder bed density and percentage coverage.
Mechanical behaviour of additive manufactured lattice structures
(Stellenbosch : Stellenbosch University, 2020-03) Tshikwand, Georgino Kaleng; Blaine, Deborah; Du Plessis, Anton; Stellenbosch University. Faculty of Engineering. Dept. of Mechanical and Mechatronic Engineering.
ENGLISH ABSTRACT: Lattice structures are open-cell, strut-based structures made up of unit cells that are tessellated in 3 orthogonal directions. Depending on the physical design of the unit cell, the lattice structure can be designed for customized stiffness, strength, and specific strain energy absorption. This allows for the design of lightweight, load-bearing structures, suitable for functional engineering applications. 5 X 5 X 5 octet-truss and diamond lattice structures of a specified relative density were designed using equations that relate the relative density to the strut dimensions. Computer-aided design (CAD) models of the structures were created and used to produce these lattice structures of Ti6AL4V using the additive manufacturing (AM) technique: laser powder bed fusion (L-PBF). Finite element analysis (FEA) was used to simulate the uniaxial compression of the lattice structures, yielding predictions for the stress distribution in the lattice struts, and allowing for the prediction of the deformation and failure modes. 3D solid and 1D beam elements were used for this purpose. A prediction of the global mechanical properties and deformation mechanism of the lattice structures was established for both types and compared. A comprehensive flowchart describing the FEA approach taken in order to predict the mechanical behavior of L-PBF lattice structures is provided. Mechanical uniaxial compression of the as-built lattice structures was conducted. Global mechanical properties were determined from the load-deformation data. The progressive collapse of struts under the load was analyzed in order to categorize the failure as stretch- or bending-based. Micro-computed tomographic (μCT) analysis of the as-built structures showed that the struts were thicker as compared to the CAD structures. Thus new CAD models were created with the strut thickness correlating to the actual average dimension. This resulted in improved prediction of the mechanical response of the as-built structures. A comparison of the FEA predictions and the experimentally measured mechanical properties of the lattice structures was carried out. It was determined that both the 3D solid and 1D beam structures predicted the actual mechanical properties of the octet-truss lattice structure within an error margin of less than 25 %. However, for the diamond lattice structure, only the 3D solid structure predicted the actual mechanical properties with an error margin of less than 20 %. A study of the stress distribution across individual struts in the structure was used to explain the deformation mechanisms observed in the respective lattice structures. The octet-truss structure was found to deform by a combination of 45° and 135° shear bands caused by the stretching of the horizontal struts in those planes, whereas the diamond lattice structure was found to deform by 45° shear bands caused by strut bending.
Mechanical properties and in situ deformation imaging of microlattices manufactured by laser based powder bed fusion
(MDPI, 2019-09-09) Du Plessis, Anton; Kouprianoff, Dean-Paul; Yadroitsava, Ina; Yadroitsev, Igor
ENGLISH ABSTRACT: This paper reports on the production and mechanical properties of Ti6Al4V microlattice structures with strut thickness nearing the single-track width of the laser-based powder bed fusion (LPBF) system used. Besides providing new information on the mechanical properties and manufacturability of such thin-strut lattices, this paper also reports on the in situ deformation imaging of microlattice structures with six unit cells in every direction. LPBF lattices are of interest for medical implants due to the possibility of creating structures with an elastic modulus close to that of the bones and small pore sizes that allow effective osseointegration. In this work, four different cubes were produced using laser powder bed fusion and subsequently analyzed using microCT, compression testing, and one selected lattice was subjected to in situ microCT imaging during compression. The in situ imaging was performed at four steps during yielding. The results indicate that mechanical performance (elastic modulus and strength) correlate well with actual density and that this performance is remarkably good despite the high roughness and irregularity of the struts at this scale. In situ yielding is visually illustrated.
Using a lattice structure coupon sample for build quality monitoring in metal additive manufacturing.
(Stellenbosch : Stellenbosch University, 2023-11) Park, M; Du Plessis, A; Venter, MP; Stellenbosch University. Faculty of Engineering. Dept. of Mechanical and Mechatronic Engineering.
ENGLISH ABSTRACT: Additive manufacturing (AM), particularly Laser Powder Bed Fusion (L-PBF), has revolutionised the manufacturing industry by offering enhanced geometrical design freedom, part integration, and reduced production lead time. However, the presence of manufacturing defects during the L-PBF process poses challenges to the mechanical properties and build quality of the manufactured parts. Traditional quality assessment methods involve costly and time-consuming manufacturing, destructive or non-destructive analysis and mechanical testing. To address these challenges, this research proposes a novel approach for quality control using lattice coupon samples instead of solid mechanical test specimens. Lattice structures, comprising repetitive unit cells with struts and nodes, serve as effective indicators of build quality. The smaller size and volume of the lattice coupon samples result in significant cost and time savings compared to traditional test samples. The high sensitivity of lattices to parameter deviations enables their use in assessing build quality within an L-PBF system. These coupon samples provide reliable indicators of structural integrity and long-term performance of AM parts, simplifying the quality control process and optimising L-PBF manufacturing. Additionally, this research develops a Finite Element (FE) model for AM lattice structures under quasi-static compression, offering a powerful tool for the virtual testing of components, enabling the study of their mechanical behaviour without the need for costly physical prototypes. The FE model incorporates defect states of the AM lattice structures observed from computed tomography (CT) scanning. By taking input from the CT scans, the prediction of mechanical properties of lattices with a high accuracy rate of 83 % was achievable. This model represents a promising tool for developing manufacturing defect-incorporated lattice representative volume elements (RVEs) for use in the design of AM parts incorporating lattice regions. By replacing complex lattice structures with solid-infilled features in the form of RVEs in simulations, the computational expense can be significantly reduced. This approach allows for efficient exploration of the mechanical behaviour of latticed AM components while accounting for manufacturing defects, offering insights for design optimisation and material selection. Furthermore, this research aims to leverage the developed FE model and interpolation methods to predict the mechanical properties of lattice structures, reducing the reliance on physical printing and CT scanning. By utilising these computational tools, accurate estimations of the mechanical properties can be achieved, minimising the need for extensive experimental testing and CT scan. Overall, this research contributes to the advancement of quality control in AM by introducing lattice coupon samples as indicators of build quality and developing computational models for predicting their mechanical properties. These innovations lead to improved efficiency and optimisation of L-PBF manufacturing processes, benefiting industries that rely on AM technology.

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