Recently, additive manufacturing (AM) has gained much traction due to its processing advantages over traditional manufacturing methods. However, there are limited studies which focus on process optimization for surface quality of AM materials, which can dictate mechanical, thermal, and tribological performance. For example, in heat-transfer applications, increased surface quality is advantageous for reducing wear rates of vibrating tubes as well as increasing the heat-transfer rates of contacting systems. Although many post-processing and in situ manufacturing techniques are used in conjunction with AM techniques to improve surface quality, these processes are costly and time-consuming compared to optimized processing techniques. With improved as-built surface quality, particles tend to be better fused, which allows for greater wear resistance from contacting tube surfaces. Additionally, improved surface quality can reduce the entropy and exergy generated from flowing fluids, in turn increasing the thermodynamic efficiency of heat-transferring devices. This review aims to summarize the process-optimizing methods used in AM for metal-based heat exchangers and the importance of as-built surface quality to its performance and long-term energy conservation. The future directions and current challenges of this field will also be covered, with suggestions on how research in this topic can be improved.
Abstract Surface texture influences friction and transfer layer formation during sliding. In the present paper three well-defined surface textures, namely unidirectional, 8-ground, and random, were employed on steel plates. Experiments were conducted on a tribological couple – zinc pin against steel plate – using an inclined pin-on-plate sliding tester. It was observed that the occurrence of stick-slip motion, the transfer layer formation and the value of coefficient of friction as well as its two components, namely adhesion and plowing, depend primarily on surface texture. The effect of surface texture on coefficient of friction was attributed to the variation of the plowing component of friction for different surfaces. The plowing component of friction was highest for the surface texture that promotes plane strain conditions while it was lowest for the texture that favors plane stress conditions at the interface. Dimensionless quantifiable roughness parameters were formulated to describe the degree of plowing and hence the plane strain/stress type deformations taking place at the asperity level.
In the present investigation, efforts were made to study the different frictional responses of materials with varying crystal structure and hardness during sliding against a relatively harder material of different surface textures and roughness. In the experiments, pins were made of pure metals and alloys with significantly different hardness values. Pure metals were selected based on different class of crystal structures, such as face centered cubic (FCC), body centered cubic (BCC), body centered tetragonal (BCT) and hexagonal close packed (HCP) structures. The surface textures with varying roughness were generated on the counterpart plate which was made of H-11 die steel. The experiments were conducted under dry and lubricated conditions using an inclined pin-on-plate sliding tester for various normal loads at ambient environment. In the experiments, it was found that the coefficient of friction is controlled by the surface texture of the harder mating surfaces. Further, two kinds of frictional response, namely steady-state and stick-slip, were observed during sliding. More specifically, stead-state frictional response was observed for the FCC metals, alloys and materials with higher hardness. Stick-slip frictional response was observed for the metals which have limited number of slip systems such as BCT and HCP. In addition, the stick-slip frictional response was dependent on the normal load, lubrication, hardness and surface texture of the counterpart material. However, for a given kind of surface texture, the roughness of the surface affects neither the average coefficient of friction nor the amplitude of stick-slip oscillation significantly.
Mechanical rock cutting is a process encountered in different engineering applications including rock excavation, mining and deep oil well drilling. Rock mechanical properties vary with depth in the subsurface and also at different geographical locations due to different environmental conditions. Understanding of fragmentation mechanisms in specific rock materials allows the determination of optimum cutting parameters that improve cutting efficiency and increase tool life during cutting operations. In the present investigation, numerical models that accurately predict the rock fragmentation and stress profiles in the rock slab during cutting were developed using the explicit finite-element method (FEM). In the numerical models, a damage material model was utilized to capture the rock fragmentation process and a rigid steel cutter (at different rake angles) was displaced at different velocities against a stationary rock slab. Rock slabs with significantly different mechanical properties were incorporated with a constant friction factor and a cutting depth of 1 mm. The variation of cutting forces and stresses, and fragmentation of the rock slab were analyzed. The simulation results indicated that the explicit FEM is a powerful tool for simulating rock cutting as the formation of fragments were distinctly observed at different cutting conditions. The rock mechanical properties and tool rake angle were found to have the most significant effect on the rock fragmentation during cutting operations. The cutting forces were also influenced by mechanical properties of the rock and tool rake angle.
Friction-based welding processes are considered as very efficient solid-state metal joining processes due to soundness of the welded joint with remarkably less energy consumption and environmentally friendly. The terminology behind these processes is to make use of elevated temperatures caused by the mechanical friction at contacting surfaces to fuse materials together. Several different friction-based welding techniques are classified briefly explaining their mechanisms to show a comparison between friction welding (FRW) and friction stir welding (FSW). In FRW contact between the specimens itself induce friction, while FSW uses a non-consumable rotating tool to fuse material at the junction. Numerous advantages and drawbacks exhibited by both the processes are compared by overviewing the current researches. Also extensive focus on the various factors influencing the properties of FSW joints such as rotational speed, welding speed, axial force, tool geometry, and defects are studied. FSW displayed superiority in welded joints as regards adaption to modern technologies, optimized process parameters, and the ability to join a wide variety of dissimilar metals and alloys.
Understanding the fundamental tribology associated with the cutting of rock under extremely high pressure and high temperature (HPHT) conditions are extremely important in mining and drilling operations. In this paper, a rock fragmentation process is simulated during mechanical cutting of rock using an explicit finite element code, LS-DYNA. In the simulation, a cutting tool is orthogonally moved against a stationary rock material. Rock material properties have been incorporated using an advanced damage constitute material model. Simulations were performed for various rake angles at different cutting velocities and cutting depths. The variation of cutting forces, stresses, rock fragment morphology and the character of fragment formation have been investigated. Overall, the results indicate that the explicit FEM is a powerful tool for simulating rock cutting and the fragmentation process. More specifically, the separation of rock fragments from the base rock slab was accurately predicted using the numerical model. The cutting forces and rock fragment characteristics were strongly influenced by rake angel when compared to cutting tool velocities for a given depth of cut. This information is shown to be highly pertinent to better understanding cutting rates and tool wear.
In this study, nanocrystalline (NC) aluminum (Al) and magnesium (Mg)-doped Al bulk components were fabricated using a hybrid manufacturing process that combines cryomilling and high-pressure cold spray (HPCS) additive deposition techniques. Yttria-stabilized zirconia (YSZ) was also added during the HPCS process to improve deposition efficiency and build-up thickness via peening. The evolution of morphology, crystallite size, and elemental composition of both cryomilled powders and cold-sprayed (CS’ed) components was systematically characterized using X-ray Diffraction (XRD), Scanning Electron Microscopy (SEM), and Transmission Electron Microscopy (TEM). Mechanical characterization was performed using Vickers microhardness and uniaxial tensile testing, while the tribological behavior was assessed using sliding wear tests under dry/lubricated conditions. XRD analysis revealed that increased cryomilling duration led to significant crystallite refinement, which directly correlated with enhanced hardness and strength. This mechanical strengthening was accompanied by an increase in coefficient of friction (COF) and lower wear rates. The results also showed that the Mg-doped Al exhibited superior hardness, tensile strength, and tribological performance compared to pure Al. The study further explores the underlying mechanisms responsible for these enhancements, highlighting the potential of solute-assisted grain boundary stabilization in tailoring high-performance NC Al alloys.
