AM Technique Research Articles

Warpage control in thermoplastic ABS parts produced through material extrusion (MEX)-based fused deposition modeling (FDM)

PurposeAdditive manufacturing (AM) is a layer-by-layer technique that helps to create physical objects from a three-dimensional data set. Fused deposition modeling is a widely used material extrusion (MEX)-based AM technique that melts thermoplastic filaments and selectively deposits them over a build platform. Despite its simplicity and affordability, it suffers from various printing defects, with partial warping being a prevalent issue. Warpage is a physical deformation caused by thermal strain incompatibility that results in the bending of the printed part away from the build platform. This study aims to investigate the warpage characteristics of printed parts based on geometrical parameters and build orientations to reduce the warpage extent.Design/methodology/approachCuboidal samples of thermoplastic acrylonitrile butadiene styrene ranging from 5 to 80 mm were printed using a commercial MEX system. A Taguchi method-based design of experiment trial was performed to optimize the placement and orientation of the part for minimal warpage.FindingsIt was found that a lower value of the “in-plane” aspect ratio and a more prominent part thickness are favorable for minimal warpage. The part should always be placed near the region with the highest temperature (least thermal gradient) to minimize the warpage.Originality/valueA novel dimensionless parameter (Y) is proposed that should be set to a minimum value to achieve minimal warpage. The results of this study can help improve the design and part placement for the MEX technique, thus elevating the print quality.

Influence of printing parameters on part density in L-BPF of Ti-6Al-4V and correlation with static mechanical properties measured using indentation testing

Abstract A complete workflow of parameter development and process qualification of a part manufactured using laser powder bed fusion (L-PBF) AM technique requires data of porosity, pore morphology, distortion and static mechanical properties. Using a combination of two non-destructive techniques: Computed tomography and indentation testing, the influence of printing parameters on defect types and part density in L-PBF of Ti-6Al-4V is described. In addition, the results using the combined method are compared to conventional testing methods.

In‐depth investigation and industry plan for enhancing surface finishing of 3D printed polymer composite components: A critical review

AbstractAdditive manufacturing (AM) is pivotal in modern manufacturing, allowing diverse materials to create functional parts. Polymer composites, with superior properties like enhanced mechanics and conductivity, are highly regarded. Fused deposition modeling (FDM) is a favored, cost‐effective AM technique. Despite popularity, FDM struggles with poor surface quality, impacting final part properties. This challenge affects composite part performance, prompting a need for surface quality enhancements in AM processes, like FDM. The review explores essential post‐treatments required to optimize composite parts for various applications. It examines surface finishing techniques for FDM 3D printed polymeric composites, categorizing them into chemical, thermal, and physical methods. Recent research is extensively analyzed, offering a comprehensive guide for academia and industry. The review covers diverse aspects, including properties, surface morphology, electrical conductivity, and mechanical properties pre‐ and post‐finishing methods. Additionally, it summarizes leading companies worldwide providing surface finishing services for 3D printed polymer parts. Discussions on future trends, challenges, and research gaps in this field are included, emphasizing its significance in refining surface finishing methods for FDM 3D‐printed polymeric composites and encouraging broader industrial adoption of AM. This overview serves as a crucial roadmap for engineers, scientists, customers, and companies interested in surface finishing services.

A state-of-the-art review of 4D printing techniques, stimuli activated materials and future perspectives

In recent years, 4DP has gained significant interest in the field of AM, functional prototyping, soft robotics, biomaterials, pharmaceutical, and textiles. 4DP is an AM technique that integrates 3DP with stimuli-responsive materials. When exposed to various external stimuli, such as temperature, light, pH, magnetic field, electricity, and moisture; the structures formed by 4DP technique are space time dependent and can change their shape and/or properties over the period of time. Herein, this overview presents stimuli activated smart materials used in 4DP, 4DP techniques and practical applications of 4D printed smart structures. This overview also elucidates challenges and future perspectives in terms of, synthetization of different polymers, multi material printing, sustainability, pre-programming knowledge, and latent applications of 4D printed structures.

Effect of heat treatments on microstructural and tribological properties of 3d printed 18Ni-300 maraging tool steel made by selective laser sintering process

The effect of heat treatments on the tribological and microstructural properties of 18Ni-300 maraging steel produced using the AM technique, which allows complex geometries, was investigated. The tribological properties of the samples were examined under dry atmospheric conditions through sliding wear tests at varying loads and speeds while maintaining a fixed distance. Studies utilizing spectrographic have unveiled that the material's structure is predominantly layered, consisting primarily of dense acicular martensite, along with recycled austenite and intermetallic components. The highest hardness was recorded in sample S1 with 49.7 HRC, while the lowest hardness was measured in sample S3 with 19.5 HRC. In terms of wear resistance, the lowest wear loss, at 0.098 g, was seen in the S1 sample, whereas the S3 sample displayed the highest wear loss at 0.209 g. With these findings, it can be concluded that the appropriate heat treatment significantly enhanced the tribological properties of 18Ni-300 steel.

HYbrid Titanium Alloys produced by laser powder bed Fusion using Ti-5553 and Ti–42Nb powder recycling

Over the years, persistent investigations have been conducted in search of unique properties in the field of materials. In titanium alloys, this relentless pursuit is no exception, especially when aiming to combine often antagonistic characteristics such as ductility and high mechanical strength. Intending to achieve properties rarely obtainable through other processing methods, a new class of titanium alloys called HYbrid Titanium Alloys (HYTA) emerges. The primary objective of this study is to create a HYTA that harmonizes the ductility observed in Ti–42Nb with the characteristic mechanical strength of Ti-5553. To achieve this feat, the technique of Additive Manufacturing by Laser Powder Bed Fusion (AM by LPBF) was chosen to produce an alloy with 20 in wt. of Ti-5553. Additionally, considering the costs associated with powders for LBPF, the effect of incorporating recycled powders in the process was investigated. In this context, the effects of processing these powders were evaluated, analyzing the influence of morphology and oxygen content on the resulting samples. In this way, in addition to contributing to an in-depth understanding of Hybrid Titanium Alloys, it was possible to seek the optimization of costs associated with the AM technique. As a result, an alloy with unique properties was obtained with an ultimate tensile strength of 725 MPa and 24 % elongation in the as-built condition.

Recycling as a Key Enabler for Sustainable Additive Manufacturing of Polymer Composites: A Critical Perspective on Fused Filament Fabrication.

Additive manufacturing (AM, aka 3D printing) is generally acknowledged as a "green" technology. However, its wider uptake in industry largely relies on the development of composite feedstock for imparting superior mechanical properties and bespoke functionality. Composite materials are especially needed in polymer AM, given the otherwise poor performance of most polymer parts in load-bearing applications. As a drawback, the shift from mono-material to composite feedstock may worsen the environmental footprint of polymer AM. This perspective aims to discuss this chasm between the advantage of embedding advanced functionality, and the disadvantage of causing harm to the environment. Fused filament fabrication (FFF, aka fused deposition modelling, FDM) is analysed here as a case study on account of its unparalleled popularity. FFF, which belongs to the material extrusion (MEX) family, is presently the most widespread polymer AM technique for industrial, educational, and recreational applications. On the one hand, the FFF of composite materials has already transitioned "from lab to fab" and finally to community, with far-reaching implications for its sustainability. On the other hand, feedstock materials for FFF are thermoplastic-based, and hence highly amenable to recycling. The literature shows that recycled thermoplastic materials such as poly(lactic acid) (PLA), acrylonitrile-butadiene-styrene (ABS), and polyethylene terephthalate (PET, or its glycol-modified form PETG) can be used for printing by FFF, and FFF printed objects can be recycled when they are at the end of life. Reinforcements/fillers can also be obtained from recycled materials, which may help valorise waste materials and by-products from a wide range of industries (for example, paper, food, furniture) and from agriculture. Increasing attention is being paid to the recovery of carbon fibres (for example, from aviation), and to the reuse of glass fibre-reinforced polymers (for example, from end-of-life wind turbines). Although technical challenges and economical constraints remain, the adoption of recycling strategies appears to be essential for limiting the environmental impact of composite feedstock in FFF by reducing the depletion of natural resources, cutting down the volume of waste materials, and mitigating the dependency on petrochemicals.

Structural and aeroelastic characteristics of wing model for transonic flutter wind tunnel test fabricated by additive manufacturing with AlSi10Mg alloys

In this paper, the potentials of wing models fabricated by metal additive manufacturing for transonic flutter wind tunnel testing are investigated. The additive manufacturing technique would be a tool for the fast production of wind tunnel models at low cost and enable multiple experiments with various wing designs while precisely realizing designed geometries and structural properties for aeroelastic evaluations. The novelties in this paper are: 1) a concept of the transonic flutter wing model, which can be used to investigate phenomena involving transonic aeroelastic instabilities in transonic flutter wind tunnel testing, fabricated with the AM technique for cost-effective manufacturing and extension of design freedom, and 2) its design and manufacturing methodology combining the AM technique and post machining treatments. As a proof-of-concept study, this paper mainly investigates the feasibility of a transonic flutter wing model fabricated by the proposed methodology based on the AM technique and post-processing, which realizes not only aerodynamic shapes but also designed elastic and modal characteristics, to evaluate typical transonic aeroelastic responses such as the LCO and the transonic dip. The geometrical accuracy of additively manufactured wing models is firstly accessed. Structural/aeroelastic characteristics of an additively manufactured wing model for wind tunnel testing are then evaluated numerically and experimentally. Finally, the aeroelastic characteristics of the fabricated flutter wing model are also investigated by performing a transonic flutter wind tunnel experiment. Through a series of evaluations, the feasibility and capability of such AM-based transonic flutter wing models for transonic wind tunnel testing are demonstrated.

A hybrid additive manufacturing process for production of functional fiber-reinforced polymer composite structures

Fiber reinforced polymer composites (FRPCs) are valued for their high strength and lightweight and have found applications from aerospace to renewable energy. Additive manufacturing (AM, or 3D printing) of FRPCs is of great interest in recent years due to the manufacturing flexibility offered by AM. Direct ink write (DIW) 3D printing is a popular choice for FRPC 3D printing due to its low cost and open framework for broad material choices. However, previously explored techniques for AM of FRPCs are hindered by heavy reliance on fiber orientation dictated by extrusion toolpath and often require specialized hardware to print FRPCs with low viscosity and long cure time thermoset epoxy matrices. In this paper, we introduce a single-stream hybrid DIW AM technique for the creation of mechanically robust FRPC functional structures where the matrix is DIW 3D printed, and pre-epoxy impregnated (prepreg) woven carbon fiber (CF) fabrics are robotically placed. Additionally, functional components, such as conductive elements, can be readily integrated. We investigated the impact of prepreg woven CF reinforcement on a two-stage photo-thermal thermoset resin matrix on the mechanical characteristics of manufactured FRPCs. We also combined these efforts to fabricate functional structures, including strain sensors for in-situ deformation monitoring, heating elements, and embedded light sensors. This study found that the proposed single-stream hybrid DIW AM process could be a facile approach to fabricate high-strength FRPC functional structures.

Self-healing diamond/geopolymer composites fabricated by extrusion-based additive manufacturing

Geopolymers are easily molded and mixed with various additives to form composite materials with desired functions. In this study, the gradient diamond/geopolymer composites (DGP) are fabricated based on the extrusion-based AM technique. Diamond disturbs the chain structure of the geopolymers, making the slurry easier to extrude with delayed solidification time and reaction rate. The interfacial characterization shows that a larger volume secondary mullite are formed from mullite phase under the condition of sufficient thermal transform from the diamond, thus achieving the self-healing process of pores and cracks at room temperature. The results of heat treatment of the DGP prove that the diamond effectively optimizes the thermal stability with fewer pores and cracks. Self-healing of damage in engineering materials can be achieved by reducing the crack widths during service process, which highly extends the life cycle and saves the operating cost. 3D printed geopolymer/diamond composites are successfully fabricated with self-healing ability and thermal stability. • Diamond/geopolymer (DGP) composites fabricated by extrusion-based additive manufacturing. • The DGP has a role in effectively delaying solidification time and increasing printing efficiency. • The DGP enables self-healing process of pores and cracks at room temperature. • The DGP could effectively enhance the stability of the sintered parts at 1200 °C.

Comparative study of functional outcomes of arthroscopic anterior cruciate ligament reconstruction using anteromedial portal and translateral all-inside technique

BackgroundAnterior Cruciate Ligament (ACL) injuries are common in the active population of the Armed Forces. Symptomatic instability prompts individuals to seek a cure or a sheltered appointment. Despite the increasing numbers of ACL reconstructions performed, the outcomes have not been so spectacular with only a meager percentage of our patients returning to preinjury levels of activity. With the premise that an all-inside ACL reconstruction is likely to result in better functional outcomes, the aim of this study was to compare the short-term functional outcomes of a large consecutive series of patients undergoing ACL reconstruction using the translateral all-inside ACL reconstruction technique (AI) and standard anteromedial portal technique (AM) with a minimum follow-up of one year. MethodsA total of 240 patients with isolated ACL tear underwent ACL reconstruction via the AI or AM technique. Their preoperative and postoperative scores were compared to look for any significant differences in functional outcomes. ResultsThe two groups were matched for age, BMI, mechanism of injury, and interval from injury to surgery. There was no difference in their preoperative scores. Postoperatively, although there were significant improvements across both groups, there was no significant difference between the groups at any point of time. ConclusionThe AI technique has garnered interest in recent literature in addressing ACL injuries. This study found no discernible benefit of the AI technique when compared to the AM technique in terms of functionality following an ACL reconstruction at any point of time up to 1 year following surgery.

Investigations on the effect of heat treatment on laser powder bed fusion built SS316L alloy

The present investigation aims at studying the changes happening in the mechanical and microstructural characteristics of additively manufactured Stainless Steel SS316L components post heat treatment. The samples printed through the Laser powder bed fusion based AM technique is considered for investigation. The samples are heat-treated at 650 °C and 1100 °C for 2 h and furnace cooled. The heat treatment is found to impart a substantial impact on the mechanical and microstructural characteristics of the component. The as-built SS316L sample possesses a micro-hardness value of 221.5HV, a tensile strength of 529 MPa, and total elongation of 28.5%. The heat treatment of samples at 1100 °C for 2 h lowers the micro hardness value (175.6HV), tensile strength (510 MPa) and increases the ductility (83.5%). The grain size, porosities and dislocation density are reduced during the heat treatment and the mechanical characteristics get modified. Hence, heat treatment can be considered as an effective method for altering the mechanical characteristics of the additively manufactured SS316L functional components.

Hybrid Transtibial Femoral Preparation for Transphyseal Anterior Cruciate Ligament Reconstruction: A Radiographic Comparison With the Transtibial and Anteromedial Portal Techniques.

Background:Transphyseal anterior cruciate ligament (ACL) reconstruction remains the most commonly used technique for pubescent patients. The transtibial (TT) drilling technique creates vertical and central femoral tunnels to minimize the physeal area of injury at the expense of a nonanatomic femoral tunnel. The hybrid TT (HTT) technique offers the potential of an anatomic femoral position with tunnel geometry similar to that using the TT technique.Purpose/Hypothesis:The purpose was to perform a radiographic comparison of the HTT technique with TT and anteromedial portal (AM) techniques in adolescent patients undergoing transphyseal ACL reconstruction. It was hypothesized that femoral tunnels created during HTT would be similar to TT tunnels but significantly more vertical and central than AM tunnels.Study Design:Cohort study; Level of evidence, 3.Methods:We retrospectively screened primary transphyseal ACL reconstructions performed in adolescents at our institution between 2013 and 2019. The youngest 20 eligible patients were selected from each technique cohort: TT, AM, and HTT. Postoperative radiographs were assessed for the coronal femoral tunnel angle, as well as the location of the tunnel-physis penetration on the anteroposterior and lateral views. Physeal lesion surface area was calculated. Data were compared among the 3 groups using 1-way analysis of variance followed by pairwise comparisons.Results:Included were 47 patients with a mean ± SD age of 14.3 ± 1.2 years (n = 9 with TT, 18 with AM, and 20 with HTT techniques). The coronal tunnel angle was significantly more vertical in the TT (60.7° ± 7.2°) and HTT (54.4° ± 5.7) groups as compared with the AM group (48.8° ± 5.9; P = .0037 and P = .02, respectively). There was no significant difference between the TT and HTT groups (P = .066). The only significant finding regarding femoral tunnel location was that the HTT tunnels (28.9% ± 4.8%) penetrated the physis more centrally than did the AM tunnels (20.0% ± 5.1%; P = .00002) on lateral radiographs.Conclusion:The HTT technique presents an option for transphyseal ACL reconstruction, with femoral tunnel obliquity and estimated physeal disruption similar to that of the TT technique and significantly less than that of the AM technique. The HTT technique also results in the most central physeal perforation of all techniques, predominantly in the sagittal plane.

Additive manufacturing of in-situ strengthened dual-phase AlCoCuFeNi high-entropy alloy by selective electron beam melting

The application scope and market demand for additive-manufactured high-entropy alloys (AM HEAs) have broadened of late. However, a long-standing problem associated with AM HEAs is their limited ductility. In this study, a dual-phase AlCoCuFeNi HEA, consisting of body-centered cubic (BCC) solid solution matrix with uniformly dispersed face-centered cubic (FCC) structured precipitates, was fabricated by selective electron beam melting (SEBM). SEBM involved a preheating process can enable the formation of Cu-rich FCC phases with needle-like and spherical morphologies, as well as nanotwins that in-situ precipitated from the metastable BCC(B2) matrix. The compressive strength and ductility of the SEBM HEA were superior to those of the HEAs processed by selective laser melting (SLM) AM technique. Furthermore, using selective electron beam remelting (SEB-RM) during SEBM could result in a higher relative density, finer microstructure, and enhanced compressive properties. Particularly, the SEB-RM sample exhibited a better compressive strength of 2572 MPa, a yield strength of 870 MPa, and a strain of 18.3%. The improved mechanical properties of SEB-RM samples could be ascribed to the refined grains and the formation of FCC precipitates, mostly along the grain boundaries. This provides new insights into the dual-phase HEAs—fabricated via a combination of the SEBM additive manufacturing process and selective electron beam remelting—that exhibit in-situ strengthening.

Direct-ink-writing (DIW) 3D printing functional composite materials based on supra-molecular interaction

Direct ink writing (DIW) is the most versatile AM technique in terms of materials development, it enables the creation of complex 3D shapes using any material by formulating a paste with controlled rheology. One of the main challenges of DIW is to design and formulate the viscoplastic and self-healing soft inks that easily flow under shear and quickly recover upon deposition. Researchers often look for flexible approaches that can be adapted to formulate printing inks with a wide range of materials. Here we report a supra-molecular interaction system composed of triethanolamine and ammonium oleate for application in DIW technology. Almost any materials (eg. rubber, plastic, ceramic, metal and composites) can be integrated into the ink system, and then can be 3D printing via shear-thinning DIW method. Furthermore, the solid contents of the ink system are higher than 80%, avoiding the formation of porous structure and dimensional changes after shaping. Present DIW method was used to construct sensors based on multi-material for application in real-time monitoring human health. This work may provide an new method to develop 3D printing materials for various practical applications.

Additive manufacturing of strong silica sand structures enabled by polyethyleneimine binder

Binder Jet Additive Manufacturing (BJAM) is a versatile AM technique that can form parts from a variety of powdered materials including metals, ceramics, and polymers. BJAM utilizes inkjet printing to selectively bind these powder particles together to form complex geometries. Adoption of BJAM has been limited due to its inability to form strong green parts using conventional binders. We report the discovery of a versatile polyethyleneimine (PEI) binder for silica sand that doubled the flexural strength of parts to 6.28 MPa compared with that of the conventional binder, making it stronger than unreinforced concrete (~4.5 MPa) in flexural loading. Furthermore, we demonstrate that PEI in the printed parts can be reacted with ethyl cyanoacrylate through a secondary infiltration, resulting in an increase in flexural strength to 52.7 MPa. The strong printed parts coupled with the ability for sacrificial washout presents potential to revolutionize AM in various applications including construction and tooling.

Hybrid Transtibial Femoral Preparation for Transphyseal Anterior Cruciate Ligament Reconstruction: A Radiographic Comparison to Transtibial and Anteromedial Portal Techniques

Background:Transphyseal anterior cruciate ligament (ACL) reconstruction remains the most commonly used technique for pubescent patients. The principles of creating vertical and central femoral tunnels are well accepted to minimize physeal area of injury and are typically accomplished with a transtibial (TT) technique. This, however, may come at the expense of a non-anatomic tunnel. The hybrid transtibial (HTT) technique offers the potential of combining an anatomic femoral position with tunnel geometry similar to the TT technique but has never been assessed in a clinical cohort.Hypothesis/Purpose:We hypothesized that tunnels created by a HTT technique would be similar in orientation and physeal location to TT tunnels, but significantly more vertical and central than tunnels created with an anteromedial portal (AM).Methods:We retrospectively screened all ACL reconstructions performed in children aged 10 to 16 years, at our institution between 2013 to 2019, with the requirements of having a transphyseal reconstruction and an available post-operative radiographs. Radiographs were then assessed for the coronal femoral tunnel angle (FTA), as well as the location of the tunnel-physis penetration on the AP (LTAP) and lateral (LTL) views. Physeal lesion surface area was calculated. Data were compared between the three groups using ANOVA.Results:Forty-seven patients met eligibility criteria with 9 TT, 18 AM, and 20 HTT patients. Mean patient age was 14.3 +/- 1.2 years. The FTA was significantly more vertical in the TT (60.7o +/-7.2) and HTT (54.4o +/- 5.7) groups as compared to the AM group (48.8o +/- 5.9); p = 0.0037 and p = 0.02 respectively. There was no significant difference between the TT and HTT groups ( p = 0.066). The LTAP was not significantly different between groups (p = 0.097). The LTL demonstrated that the HTT tunnels penetrated the physis at a more central location in the sagittal plane (28.9% +/- 4.8%) than the AM tunnels (20.0% +/- 5.1%, p = 0.00002), but was statistically indistinguishable from the TT (24.4%+/- 4.0%, p= 0.066) tunnels.Conclusion:The hybrid transtibial technique presents an option for transphyseal ACL reconstruction, with femoral tunnel obliquity and estimated physeal disruption similar to the TT technique, significantly less than the AM technique. The HTT also results in the most central physeal perforation of all techniques, predominantly in the sagittal plane. With the known ability of the HTT technique to recreate an anatomic femoral footprint, this may represent the “best of both worlds” for transphyseal ACL reconstruction.Tables/Figures:Figure 1:Figure 2:Figure 3:Figure 4:

Progress of different methods for femoral tunnel positioning in anterior cruciate ligament reconstruction

To systematically review the progress of different methods for femoral tunnel positioning in anterior cruciate ligament (ACL) reconstruction and provide a clinical reference for treatment of ACL rupture. The literature about the femoral tunnel positioning in ACL reconstruction was widely reviewed. The advantages and disadvantages and the clinical results of each method were summarized. Currently in ACL reconstruction, methods for femoral tunnel positioning include transtibial technique (TT), anteromedial technique (AM), outside-in (OI), modified TT (mTT), and computer assisted surgery. There is no significant difference in the postoperative effectiveness between TT technique and AM technique. Compared with the TT technique, the OI technique has higher rotational stability of knee, but there is no significant difference in clinical results. The femoral tunnel located by mTT technique is closer to the anatomical placement than that of TT technique, but mTT technique is not effective for systematically anatomic femoral tunnel positioning, and further research is needed to prove its advantages. Different femoral tunnel positioning methods have their own advantages and disadvantages, and there is no definite evidence that one is superior than the rest.

Experimental Investigation on the Effect of Fused Deposition Modelling Parameters for HIPS Material by Experimental Design and MRO Techniques

Fused Deposition Modelling (FDM) is one of the popular AM technique which utilizes thin thermoplastic filaments to create prototypes and end use products. The parts processed through FDM method have poor mechanical properties and surface quality characteristics in comparison to conventional manufactured products. Extensive research is underway in various parts of the world to bring an enhancement in mechanical properties and other related characteristics of the parts produced through FDM for making the process highly suitable to produce parts for diverse applications. The present work considers slice height, infill density, shell thickness and raster angle varied din three levels (34) to study their effects over output responses such as specimen weight, flexural strain and flexural strength. Taguchi’s L9 orthogonal array is considered for preparing the experimental plan. Flexural testing is adopted for the evaluation of flexural strength. Grey Relational Analysis and Technique of Order Preference Similarity to the Ideal Solution methods have been adopted for multi response optimization of FDM parameters and the optimized parameter setting recommended have been validated through confirmation trials. The confirmation trail conducted has revealed that the setting recommended by TOPSIS A1B3C3D1 has shown higher flexural strength and slice height is found to be most significant factor. GRA method has recommended the combination A3B3C3D1 which shows lesser flexural strain and infill density is found to be significant from all the parameters considered.

Post-heat treatment design for high-strength low-alloy steels processed by laser powder bed fusion

A novel post-heat treatment design is implemented for additively manufactured copper-bearing high-strength low-alloy (HSLA) steels by understanding the processing-structure-property relationships. Hot isostatic pressing (HIP) is adopted to reduce the porosity from 3% to less than 1% for the HSLA-100 steel processed using laser powder bed fusion (LPBF). Quenching dilatometry is employed to design the parameters for the HIP cycle with an optimized cooling process. In order to achieve the maximum amount of martensite, drop cooling after HIP is found to be more suitable than controlled cooling. A subsequent cyclic re-austenitization is introduced to achieve effective grain refinement to compensate for the coarsened microstructure after HIP. The re-austenitization effectively leads to a 60% reduction in the prior austenite grain (PAG) size. The microstructure of the as-built and HIP HSLA steels before and after cyclic re-austenitization consists of martensite, bainite and martensite/retained austenite (M/A) islands. Tempering heat treatment is applied after HIP, to induce strengthening due to precipitation hardening, that is optimized through microhardness and microstructure characterization. The peak hardness is achieved at 5 h of aging and the microstructure consists of tempered martensite, bainite, and M/A islands. A significant increase in the tensile yield strength, as well as the ductility, is achieved in comparison with the as-built alloy with the tailored microstructure obtained after the application of the designed post-heat treatment to HSLA-100 steels processed using LPBF. As LPBF of HSLA steel is attempted for the first time, this work demostrates the significance of post-heat treatment design for the AM technique.