Solid-State Metal Additive Manufacturing for Structural Repair

Recent developments in additive friction stir deposition (AFSD)

A comprehensive review of friction stir techniques in structural materials and alloys: challenges and trends

Additive friction stir deposition (AFSD) is a solid-state metal additive manufacturing technique, which utilizes frictional heating and plastic deformation to create large deposits and parts. Much like its cousin processes, friction stir welding and friction stir processing, AFSD has seen the most compatibility and use with lower-temperature metals, such as aluminum; however, there is growing interest in higher-temperature materials, such as titanium and steel alloys. In this work, we explore the deposition of an ultrahigh-temperature refractory material, specifically, a tantalum–tungsten (TaW) alloy. The solid-state nature of AFSD means refractory process temperatures are significantly lower than those for melt-based additive manufacturing techniques; however, they still pose difficult challenges, especially in regards to AFSD tooling. In this study, we perform initial deposition trials of TaW using twin-rod-style AFSD with a high-temperature tungsten–rhenium-based tool. Many challenges arise because of the high temperatures of the process and high mechanical demand on AFSD machine hardware to process the strong refractory alloy. Despite these challenges, successful deposits of the material were produced and characterized. Mechanical testing of the deposited material shows improved yield strength over that of the annealed reference material, and this strengthening is mostly attributed to the refined recrystallized microstructure typical of AFSD. These findings highlight the opportunities and challenges associated with ultrahigh-temperature AFSD, as well as provide some of the first published insights into twin-rod-style AFSD process behaviors.

Additive friction stir deposition (AFSD), a derivative technology of friction stir welding (FSW), is now rapidly growing in industries that require bulk additive manufacturing. For mass adoption to occur, deposition consistency and quality along with thermal inputs and their impacts need to be better understood and improved. In both AFSD and FSW, it has been demonstrated that properties of depositions or welds vary along their length in response to in-process temperatures. Research using temperature control to maintain weld temperatures in FSW has been performed for some years now with significant results in both quality and reliability, but this technology is just starting to be applied toward AFSD. This work describes the implementation of a temperature control method that was proven successful in FSW to AFSD. It demonstrates the accuracy achievable using that method for AA 7050 T7451 compares the controlled results against a fixed RPM deposition, reviews results of temperature control over multi-pass AFSD, and outlines future work for AFSD involving temperature control.KeywordsAFSDAdditive friction stir depositionTemperature controlPID control

Additive friction stir deposition (AFSD) is a novel additive manufacturing (AM) technology in which solid-state, friction-stirred metal is deposited layer-by-layer to build three-dimensional parts. Unlike the mainstream fusion-based AM methods, AFSD does not cause metal melting and solidification. Therefore, AFSD can eliminate defects such as lack-of-fusion, key-holing, and large residual stress. Currently, the understanding of AFSD is based on the friction stir welding (FSW) process that has been widely studied for the past two decades. However, the material feeding and spreading in AFSD is essentially different from FSW and can complicate the thermal field and material flow. In this work, a computational fluid dynamics (CFD) model is created to simulate the temperature and fluid flow for AFSD of aluminum 7050 alloys. The predicted temperature is validated against both literature and thermocouple measurements. The current work lays the foundation for a quantitative understanding of AFSD process physics and the simulation-guided process design to tailor the thermal-mechanical field.

Chapter 1 - Introduction

Additive friction stir deposition of metallic materials: Process, structure and properties

In the dynamic field of metal 3D printing, solid-state additive manufacturing (AM) techniques like Additive Friction Stir Deposition (AFSD) are revolutionizing the production and repair of critical parts in various industries due to their efficiency, environmental benefits, and enhanced material properties. AFSD stands out by enabling the creation of high-strength, fully dense non-ferrous and ferrous alloys that exhibit superior mechanical characteristics. This study offers a detailed examination of the fatigue crack growth and fracture behaviors of non-ferrous (AA6061-T6) and ferrous (4340 steel) alloy specimens processed through AFSD. Employing numerical methods, the mixed-mode fracture properties and propagation of cracks in the structures are evaluated. The model involves modified compact tension specimens (MCTS) subjected to tensile loads to analyze fatigue crack propagation and determine Paris Law constants. The study utilizes ANSYS software’s Separating Morphing Adaptive Remeshing Technology (SMART) tool to predict crack trajectories and fatigue growth under constant amplitude loading. Results from the numerical models, showing stress intensity factors, crack extension, and stress deformation, are compared with analytical outcomes to validate the precision of the models in simulating the mechanical integrity of AFSD-produced parts. Additionally, a comparative analysis of the mechanical properties between traditionally produced alloys and those manufactured via AFSD highlights significant improvements, thereby confirming the potential of AFSD for high-strength structural and repair applications with both ferrous and non-ferrous alloys.

As an alternative to current additive manufacturing (AM) techniques, additive friction stir deposition (AFSD) is a solid-state process that has been recently explored and does not require melting or remelting of the feedstock, which is a challenge in fusion AM processes. Once its yield point is reached, the material is deposited under elevated temperatures—a similar mechanism to that of friction stir welding (FSW), producing fully dense parts with more equiaxed and finer grain structures, potentially not requiring postheat treatment. Compared with direct energy deposition (DED) technologies, it benefits from reduced heat input and high build rates. It also allows for open-air deposition of reactive metals such as aluminum alloys. These alloys are used extensively in the aerospace industry, and demand is expected to double over the next decade. The 7XXX series is the hardest and strongest commercial grade among the aluminum alloys, making them useful for aircrafts, high-speed trains, and parts under high stress. Although research on AFSD development has mostly focused on aluminum alloys, advancements using 7XXX series have been either limited or nonexistent. In this paper, we analyze 7XXX series aluminum parts produced through AFSD by the introduction of different combinations of torque, deposition rate, and tool speed. Tensile and hardness tests are performed in different directions, including the interface between the baseplate and deposited material, assessing the overall strength of all AFSD parts carried out. Despite AFSD's potential, it is demonstrated that there is an opportunity for improvements, and further work (e.g., Charpy test, microstructural characterization, heat treatment) is required to comprehend the technologies’ impact and benefits.

Wire arc additive manufacturing (WAAM) has recently gained considerable attention due to its capability to manufacture large-size metal with a length of one meter or above, with good microstructural and mechanical properties. However, the manufacture of critical components exposed to extreme environmental conditions, such as high stresses, remains the focus of most research studies. The applications of WAAM in high-tech industries, such as aerospace and marine modes, remain limited due to metallurgical challenges such as oxidation, porosity, cracking, and deformation, especially for high-strength aluminium alloys, including 6XXX and 7XXX series. The aforementioned metallurgical challenges in WAAM are minimized to some extent by another emerging technology, known as additive friction stir deposition (AFSD). AFSD is capable of manufacturing large-size and high strength (strength equal to or greater than that of the raw material) industrial components with fewer metallurgical defects and refined microstructures. However, this technology is in its developmental stage and possesses some challenges, such as oxidation, which is currently an emerging topic for researchers in metal additive manufacturing (AM). This paper reviews the potential of various additive manufacturing (AM) techniques for the manufacture of high-strength components, using either unweldable virgin or recycled high-strength aluminium alloys. The study also provides a comprehensive overview of the importance of recycling aluminium, as well as the challenges of utilizing aluminium (Al) alloys within metal AM. Considerations related to microstructure, the mechanical properties and metallurgical defects in both these technologies are extensively discussed and compared. The study concludes that both technologies are still being developed and experience various metallurgical issues, which need to be addressed to fully utilize WAAM and AFSD for critical applications. Further, the AFSD process is shown to be a better alternative to the WAAM process in the fabrication of Al components, where it possesses less metallurgical issues, higher strength and more refined microstructures. The literature suggests ultimate tensile stress (UTS) and average elongation percentage during AFSD in the range of 197.3 MPa–306 MPa and 8.6%–39% for Al alloys, respectively. However, slightly better UTS values in the range of 344 MPa–349 MPa and significant reduction in average elongation percentage to 5% is noted during WAAM process. Furthermore, AFSD exhibited significantly higher microhardness values (40.8 HV–151.4 HV) when compared to WAAM (73 HV–111 HV). Accordingly, the study notes that further numerical and experimental studies are needed to fully understand material flow in stirring zones during the AFSD process.

Numerical and experimental study on the thermal process during additive friction stir deposition

Additive friction stir deposition (AFSD) is a novel solid-state additive manufacturing method developed on the principle of stirring friction. Benefits from its solid-phase properties, compared with traditional additive manufacturing based on melting-solidification cycles, AFSD solves the problems of porosity, cracks, and residual stress caused by the melting-solidification process, and has a significant improvement in efficiency. In AFSD, the interaction between feedstocks and high-speed rotating print heads suffers severe plastic deformation at high temperatures below the melting point, ending up in fine, equiaxed recrystallized grains. The above characteristics make components by AFSD show similar mechanical behaviors to the forged ones. This article reviews the development of AFSD technology, elaborates on the basic principles, compares the macroscopic formability and material flow behavior of AFSD processes using different types of feedstocks, summarizes the microstructure and mechanical properties obtained from the AFSD of alloys with different compositions, and finally provides an outlook on the development trends, opportunities, and challenges to the researchers and industrial fields concerning AFSD.

Additive manufacturing (AM) has completely altered the traditional component manufacturing and qualification paradigm. It provides unitisation and topological optimisation opportunities simultaneously. Broadly, the additive manufacturing processes are classified as fusion-based or solid-state. The solid-state additive manufacturing processes are relatively nascent. Among these, friction stir-based processes involve intense shear deformation of material while building. In this review, we focus on friction stir additive manufacturing (FSAM) and additive friction stir deposition (AFSD). These friction stir welding derived techniques have ability to produce microstructures that lead to better mechanical properties than the conventionally processed parent alloys; in many cases overcoming the traditional strength-ductility tradeoff paradigm. The best way to capture this advantage is to conduct materials selection for build which benefit from the attributes of these processes. This review provides a systems approach framework and a conceptual process model to guide researchers. A case is built that the best mechanical properties can be obtained by alloy design for such disruptive and innovative manufacturing processes. The intrinsic and extrinsic limitations are highlighted to guide researchers in the field of FSAM and AFSD. While AFSD is readily applicable to lower melting temperature materials currently, applying it to high-temperature materials requires significant research and development on tool materials. Examples of materials processed by FSAM/AFSD include aluminium alloys, magnesium alloys, titanium alloys, steels and nickel-base superalloy. A physics-based process modelling framework applicable to FSAM/AFSD is provided. To fully validate such models, it is imperative to use machines with appropriate sensors that capture the machine parameters, tool health, and workpiece temperature.

In recent years, additive manufacturing (AM) has gained prominence in rapid prototyping and production of structural components with complex geometries. Magnesium alloys, which have a strength-to-weight ratio that is superior compared with steel and aluminum alloys, have shown potential in lightweighting applications. However, commercial beam-based AM technologies have limited success with magnesium alloys due to vaporization and hot cracking. Therefore, as an alternative approach, we propose the use of a near net-shape solid-state additive manufacturing process, additive friction stir deposition (AFSD), to fabricate magnesium alloys in bulk. In this study, a parametric investigation was performed to quantify the effect of process parameters on AFSD build quality including volumetric defects and surface quality in magnesium alloy AZ31B. In order to understand the effect of the AFSD process on structural integrity in the magnesium alloy AZ31B, in-depth microstructure and mechanical property characterization was conducted on a bulk AFSD build fabricated with a set of acceptable process parameters. Results of the microstructure analysis of the as-deposited AFSD build revealed bulk microstructure similar to wrought magnesium alloy AZ31 plate. Additionally, similar hardness measurements were found in AFSD build compared with control wrought specimens. While tensile test results of the as-deposited AFSD build exhibited a 20% drop in yield strength (YS), nearly identical ultimate strength was observed compared with the wrought control. The experimental results of this study illustrate the potential of using the AFSD process to additively manufacture Mg alloys for load bearing structural components with achieving wrought-like microstructure and mechanical properties.

Behavioral simulations and experimental evaluations of stress induced spatial nonuniformity of dynamic bulk modulus in additive friction stir deposited AA 6061