Subsequent Machining Research Articles

Near Net Forming Research for Fin-Typed LED Radiator.

Pure aluminum radiator is the best choice for heat dissipation of various LED products at present. Its forming methods include common extrusion, die casting, forging, etc. Compared with other forming technologies, the LED radiator formed by cold forging has good heat dissipation performance, but there are some disadvantages in the forming process, such as uneven deformation, large material consumption and low die life. The cold forging process of pure aluminum fin-typed LED radiator is analyzed by the finite element method. The calculation results show that equal fillet structure of concave die is improper, leading to serious uneven flow velocity distribution during aluminum forging, inconsistent fin length, and warpage tendency. The gradient fillet structure of concave die is adopted. Numerical simulation and production test show that the gradient fillet structure design can significantly reduce the uneven metal flow. The extruded fins have a uniform length, which is conducive to reducing subsequent machining and production cost.

Investigation of the material combination 20MnCr5 and X45CrSi9-3 in the Tailored Forming of shafts with bearing seats

The Tailored Forming process chain is used to manufacture hybrid components and consists of a joining process or Additive Manufacturing for various materials (e.g. deposition welding), subsequent hot forming, machining and heat treatment. In this way, components can be produced with materials adapted to the load case. For this paper, hybrid shafts are produced by deposition welding of a cladding made of X45CrSi9-3 onto a workpiece made from 20MnCr5. The hybrid shafts are then formed by means of cross-wedge rolling. It is investigated, how the thickness of the cladding and the type of cooling after hot forming (in air or in water) affect the properties of the cladding. The hybrid shafts are formed without layer separation. However, slight core loosening occurres in the area of the bearing seat due to the Mannesmann effect. The microhardness of the cladding is only slightly effected by the cooling strategy, while the microhardness of the base material is significantly higher in water cooled shafts. The microstructure of the cladding after both cooling strategies consists mainly of martensite. In the base material, air cooling results in a mainly ferritic microstructure with grains of ferrite-pearlite. Quenching in water results in a microstructure containing mainly martensite.

Characterization of Residual Stresses and Grain Structure in Hot Forging of GH4169

Residual stresses (RS) in hot forging severely degrade the machining accuracy and stability of super alloy parts. This is the main reason for deformation during subsequent mechanical machining. RS need recognition, as well as the microstructure and properties achieved by forging. In this study, a simulation and experimental research on the single-pass compression of GH4169 are presented. RS variations with forging temperature, loading speed, and cooling speed are established by finite element (FE) simulation. Based on the FE results, an experiment is conducted at a temperature of 1020 ℃, loading speed of 25 mm/s, and press amount of 16 mm, immediately followed by water cooling. A new layer-stripping method is put forward for the high-efficiency measurement and correction of interior RS. Compared with the traditional strain gauge layer-stripping method, the measurement efficiency of the new layer-stripping method is increased by 10 times. Meanwhile, grain photographs are collected and grain size evolution is summarized; thus, the RS is characterized and evaluated from the angle of grains. It is demonstrated that the RS level rises with the increase in forging temperature, loading speed, and cooling speed, while the cooling method influences both the stress value and distribution. Compressive RS changes to tensile, while the average grain size reduces from the surfaces to the center. In the compressive regions, stress values share the same rules as grain size, while, in the tensile regions, they are contrary. The RS levels are divided according to the grain degree standard. According to the residual stress and grain distribution law of the blank, the optimal position of the part in the blank can be determined. Compared with the center position of the part in the blank, the residual stress of the part is reduced by 70%. The results provide useful strategies for the better design of forging technology, qualification examinations, and subsequent mechanical machining.

Finite element simulation on residual stress during immersion quenching and pre-stretching of Al7055 thick plates

During the immersion quenching process, severe temperature changes and significant temperature differences between the core and superficial area can give rise to high residual stresses within the aluminum alloy thick plate to cause subsequent machining distortion of the thin-walled part. Reducing the residual stress within the thick plate can effectively minimize the distortion in part. In this research, ABAQUS software was adopted to simulate the internal temperature and stress fields of thick plates of Al7055 alloy during the quenching and pre-stretching processes, sequentially and respectively. In addition, the x-ray residual stress measurement method was used to measure and characterize the surface residual stress of the plates. The results indicate that the medium temperature affects residual stress inside the thick plate significantly during the immersion quenching process. The level of which inside the plate gradually increases as the medium temperature decreases. Further, the pre-stretching treatment can effectively reduce residual stresses within the thick plate, and the residual stress level gradually decreases with the pre-stretching ratio increasing. The experimental results verify the feasibility of numerical simulation to predict the status of quenching stress in thick plates. Subsequently, a simulation study for material removal processes was carried out based on the above studies. The results reveal that the thin-walled part’s machining distortion degree improves as the raw thick plate’s initial quench residual stress level decreases.

Features for designing of forging technology of unmachined counter-weighted crankshaft forgings

Recommendations for improving of the forging technology of crankshafts on crank presses are considered. The results of physical experiments carried out on the technological equipment to determine the actual values of the press tensile strength by transitions are presented. Simulation of the forming process in the QForm environment is made. The existence of significant displacing force at the die split is established and the possibility of its elimination at the expense of applying lock-fixing element with certain geometrical parameters is shown. Specific technical solutions implemented in production, which allowed to obtain forgings without subsequent machining of counterweights, are given.

Research Status of Subsequent Machining of Laser Cladding Layers

Background: As an important method of remanufacturing, laser cladding can be used to obtain the parts with specific shapes by stacking materials layer by layer. The formation mechanism of laser cladding determines the “Staircase effect”, which makes the surface quality can hardly meet the dimensional accuracy of the parts. Therefore, the subsequent machining must be performed to improve the dimensional accuracy and surface quality of cladding parts. Methods: In this paper, chip formation, cutting force, cutting temperature, tool wear, surface quality, and optimization of cutting parameters in the subsequent cutting of laser cladding layer are analyzed. Scholars have expounded and studied these five aspects but the cutting mechanism of laser cladding need further research. Results: The characteristics of cladding layer are similar to that of difficult to machine materials, and the change of parameters has a significant impact on the cutting performance. Conclusion: The research status of subsequent machining of cladding layers is summarized, mainly from the aspects of chip formation, cutting force, cutting temperature, tool wear, surface quality, and cutting parameters optimization. Besides, the existing problems and further developments of subsequent machining of cladding layers are pointed out. The efforts are helpful to promote the development and application of laser cladding remanufacturing technology.

Stiffening near-net-shape functional parts of Inconel 718 LPBF considering material anisotropy and subsequent machining issues

Additive manufacturing of metallic parts has witnessed significant development. The laser powder bed fusion (LPBF) process has emerged as a key process for manufacturing thin-walled components. However, despite the significant advances, post-machining operations are required for the improvement of dimensional and surface quality, specifically in low stiffness components. This study proposed an iterative design methodology for improving the stiffness of such LPBF components. In the first stage, an initial assessment of the machining conditions was performed to obtain a first approach to the relationship between tooth passing, natural, and chatter frequencies. Thereafter, a method to improve Young's modulus was proposed by evaluating the elastic anisotropy of different manufacturing configurations. Furthermore, LPBF lateral stiffeners were included in the workpiece design, thereby converting the final workpiece into a temporary assembly. The proposed methodology was applied in a case study for finishing thin-walled bent ducts and a comparison for surface quality between stiffened and non-stiffened ducts using the proposed methodology was conducted. A significant roughness and cutting force reduction (≈50%) were obtained for the stiffened duct.

Influence of the manufacturing parameters of an AlMg5 wire\u2013based hybrid production process on quality and mechanical properties

Hybrid manufacturing processes are known for combining the advantages of additive manufacturing and more traditional manufacturing processes such as machining to create components of complex geometry while minimising material waste. The trend towards lightweight design, especially in view of e-mobility, gives aluminium materials an important role to play. This study examines the use of aluminium alloys in laser metal wire deposition (LMWD) processes with subsequent subtractive machining, which is considerably more difficult due to the different process-related influences. The investigations are focussed on the influence of the differently controlled laser power on the shape accuracy, the microstructure, and the hardness of the AlMg5 test components after the LMWD process with subsequent subtractive machining by turning. The long-term goal of the investigations is to increase the stability of the hybrid production process of AlMg5 components with defined dimensional accuracy and mechanical properties.

Selective Laser Melting of Pre-Alloyed NiTi Powder: Single-Track Study and FE Modeling with Heat Source Calibration.

Unique functional properties such as the low stiffness, superelasticity, and biocompatibility of nickel–titanium shape-memory alloys provide many applications for such materials. Selective laser melting of NiTi enables low-cost customization of devices and the manufacturing of highly complex geometries without subsequent machining. However, the technology requires optimization of process parameters in order to guarantee high mass density and to avoid deterioration of functional properties. In this work, the melt pool geometry, surface morphology, formation mode, and thermal behavior were studied. Multiple combinations of laser power and scanning speed were used for single-track preparation from pre-alloyed NiTi powder on a nitinol substrate. The experimental results show the influence of laser power and scanning speed on the depth, width, and depth-to-width aspect ratio. Additionally, a transient 3D FE model was employed to predict thermal behavior in the melt pool for different regimes. In this paper, the coefficients for a volumetric double-ellipsoid heat source were calibrated with bound optimization by a quadratic approximation algorithm, the design of experiments technique, and experimentally obtained data. The results of the simulation reveal the necessary conditions of transition from conduction to keyhole mode welding. Finally, by combining experimental and FE modeling results, the optimal SLM process parameters were evaluated as P = 77 W, V = 400 mm/s, h = 70 μm, and t = 50 μm, without printing of 3D samples.

Application of the vacuum casting technology in the production of small-size gas turbine engine blade machines

The application of casting technologies in the production of parts and assemblies of small-size gas turbine engines is justified in the paper. The technology of vacuum casting in gypsum molds was tested during the production of an experimental centrifugal compressor of a small-size gas turbine engine. On the basis of a 3D model of the designed centrifugal compressor, computational studies of vacuum casting were carried out and rational parameters of the technological process were determined. Prototypes of the developed centrifugal compressor of a small-size gas turbine engine were made. The results of calculations and the performed technological experiment confirmed the fill rate of the gating form and the absence of short pour. The distribution of shrinkage porosity and cavities corresponds to the design values and is concentrated in the central part of the casting that is subjected to subsequent machining. The area of the blades, disc and sleeve is formed without defects. The use of casting technologies in the production of parts and assemblies of small-size gas turbine engines assures the required quality with a comparatively low price of the finished product, making it possible to achieve the balance between the cost of the technology and the quality of the product made according to this technology.

Determining the temperature values on the surface of complex curvilinearly bent agricultural machine parts during the formation of powder coatings by thermal methods

The development of modern technologies for the restoration and hardening of agricultural machine parts require the development of a mathematical apparatus that simulates the involved processes. For this purpose, the models should take into account the technological features of the process of hardening and restoration of machine parts, including: cleaning; determination of geometric indicators; surface preparation; choice of technology; subsequent machining of the resulting surface; surface quality control. One of the priority directions in the development of technologies for the restoration and hardening of parts is the formation of coatings on machine parts using powder materials applied by the gas-thermal method. One of the most important conditions for obtaining high-quality coatings is temperature control on the surface of the part. In this regard, it is of scientific interest to study the effect of the distance from the surface of the part to the flame nozzle when applying self-fluxing powder on the time-average temperature at the point of coating, the maximum temperature on the back side of the part, and the total power transferred to the part by the gas burner. This paper presents a mathematical model for determining the above-mentioned influence, and also presents the results obtained using a computer program. The novelty of the model is the ability to use it to study temperature values for complex curved-bent machine parts (crankshafts and camshafts of agricultural machines). To check the reliability of the dependences obtained, experiments were carried out and the convergence with theoretical developments was confirmed.

Modification mechanism of collaborative ions implanted into 4H-SiC by atomic simulation and experiment

Ion implantation is increasingly applied to assist machining micro/nano devices in academic research and industry. A method of heavy ions assisted light ions implantation was proposed to modify 4H-SiC for reducing difficulty in ultra-precision machining. First, a three-dimensional molecular dynamics (MD) simulation model for ion implantation into SiC surface was established. The lattice damage mechanism of SiC under implantation of different ions including H+, Cu2+, and collaboration of Cu2+ and H+was studied. Projection range of the model with collaboration of Cu2+ and H+ was the largest. Then, the workpiece after ion implantation was annealed to study residual stress. The results show that high-temperature annealing contributes to reducing residual tensile stress. Finally, a MD model with length of 1530 Å along the Z-axis was built to compare with actual experiments using the same ion implantation parameters. The resulted subsurface damage values were quite close which demonstrates feasibility of the MD model. Surface morphology, cross-section of the defects, and three-dimensional ion distribution on the implanted samples surface were also observed and displayed. As demonstrated by simulation and experiments, the method of collaborative ion implantation can effectively modify surface of hard and brittle materials and bring benefits for subsequent ultra-precision machining.

Combination of structured light scanning and external fiducials for coordinate system transfer in hybrid manufacturing

This paper describes a method for establishing and transferring coordinate systems through multiple hybrid manufacturing operations. To demonstrate the approach, an additively manufactured preform is finish machined to produce the desired part geometry. A set of external fiducials is temporarily attached to the preform using a polymer frame. The assembly is inspected using a structured light scanner and the resulting scan is used to define an alignment and coordinate system which respects the physical requirements of the manufacturing processes. The coordinate system is then used to program subsequent machining operations. Once the part is set up on the milling machine, the fiducials are used to establish the shared coordinate system for the machining operation using standard on-machine probing. After the part is machined, the same fiducial/scanning process is repeated for a second machining operation to complete the part (i.e., some features could not be accessed in the first setup). Finally, the method performance is assessed.

Influence mechanism of defects on the subsurface damage and structural evolution of diamond in CMP process

Surface defects of materials will deteriorate their mechanical properties and limit the applications. Understanding the effect of surface defects on machining performance remains a challenge. Therefore, it is of paramount significance to insight into the structural evolution and subsurface damage induced by defects. Herein, we construct a polishing model with initial defects to make the simulation more realistic, and elucidate the effects of initial defects on the subsurface damage and structure evolution via ReaxFF molecular dynamics simulations. Simulation results show that the structural evolution starts from defect edge, and the substrate with initial defects will produce more amorphous damage layers compared with the ideal substrate. The thickness of the amorphous damage layer is roughly the same as the depth of the initial defects. Moreover, smaller pit defects have a little effect on the surface morphology. Nevertheless, larger pit defects will cause poor surface quality and more severe subsurface damage. Our simulation results are of significance for further understanding the influence of initial defects on subsequent machining quality and subsurface damage, and provide theoretical support for the processing of diamond from an atomic perspective.

Possibilities of the Utilization of Ferritic Nitrocarburizing on Case-Hardening Steels.

This paper is devoted to the possibilities of the utilization of chosen chemical heat treatment technologies on steels used for manufacturing highly stressed components of military vehicles and weapons systems. The technologies chosen for this research are plasma ferritic nitrocarburizing and ferritic nitrocarburizing in a gaseous atmosphere. These technologies were applied on a steel equivalent 1.5752 (i.e., CSN 41 6426), which is suitable for carburizing. Chemical composition of the steel was verified by optical emission spectrometry. An observation of a microstructure and an assessment of the parameters of obtained white layers were performed by optical microscopy. Morphology and porosity of the surface were observed by electron microscopy. The depth of diffusion layers was evaluated in accordance with ISO 18203:2016(E) from the results of microhardness measurements. A friction coefficient was obtained as a result of measurements in accordance with a linearly reciprocating ball-on-flat sliding wear method. Wear resistance was assessed by employing the scratch test method and a profilometry. The profilometry was also utilized for surface roughness assessment. It was proved that both tested chemical heat treatment technologies are suitable for surface treatment of the selected steel. Both technologies, ferritic nitrocarburizing in plasma and a gaseous atmosphere, are beneficial for the improvement of surface properties and could lead to a suppression of geometrical deformation in comparison with frequently utilized carburizing. Moreover, the paper presents a procedure that creates a white layer-less ferritic nitrocarburized surface by utilizing an appropriate modification of chemical heat treatment parameters, thus subsequent machining is no longer required.

Sequential Processing of Cold Gas Sprayed Alloys by Milling and Deep Rolling.

Cold gas spraying (CS) is a solid-state material deposition process which, in addition to the flexible repair of individual component areas, also enables the build-up of larger samples. The layers are created on a substrate by the impact-induced bonding of highly accelerated micrometer particles. Since melting does not occur, the material composition can be varied flexibly and independently of material-specific melting points. In this work, the influence of the described forming process on subsequent machining by milling and deep rolling is investigated. The process forces measured during milling and the surface topography after milling and deep rolling were influenced by the material composition and the CS-related properties, e.g., high material hardness or particle bonding. In contrast to prior assumptions, deep rolling was shown to have no influence on the determined hardness depth profile for the investigated materials. Future work will focus on additional analyses, such as the determination of half-widths, to obtain further insight on the material behavior.

One-Step Enameling and Sintering of Low-Carbon Steels

Powder technology allows manufacturing complex components with small tolerances, saving material without subsequent machining. There is a growing trend in using sintered steel components in the automotive industry. Within 2020, about 2544 million US dollars was invested for manufacturing sintered components. Not only does this industry uses steel components, but the gas cooker industry also uses steel in its burners since they are robust and usually demanded by Americans, with forecasts of 1097 million gas cookers in 2020. Steel gas burners have a ceramic coating on their surface, which means that the burner is manufactured in two stages (casting and enameling). This work aims to manufacture the gas burners by powder metallurgy, enameling and sintering processes in a single step. To achieve this aim, the ASC100.29 iron powder has been characterized (flow rate, relative density and morphology); subsequently, the most suitable parameters for its compaction and an adequate sintering temperature were studied. Single-step sintering and enameling was achieved by compacting iron powder at 500 MPa and sintering at 850 °C for 5 min. The necessary porosity for mechanical anchoring of the coating to the substrate is achieved at this sintering temperature. Bending resistance tests, scratching, degradation under high temperature and basic solution and scanning electron microscopy were used to characterize and validate the obtained samples.

Influence of Subsequently Applied Mechanical and Thermal Loads on Surfaces Ground with Mechanical Main Impact.

To generate advanced properties for the wear resistance and fatigue life of components and allow for an improved, application-oriented development of part specifications, a precisely tailored initial machining or manufacturing process is necessary. In addition, it is important to know how subsequent machining steps or operational loads affect the components’ condition. Residual stresses are a meaningful measurand for evaluating the modifications that a machining process induces into the material. The desired modifications should be specified regarding the final state for the required operational behavior. Thus, the stability of the modifications can be considered so that they can be beneficial in service. This investigation is part of fundamental research in the field of the Collaborative Research Center (CRC) “Process Signatures”. By applying defined selected loads, the effects on machined surface layers are investigated since machined components are exposed to further loads during use. For this reason, experimental process chains are applied in this work to grind-strengthened specimens as possible application cases and corresponding loads. These experimental process chains consist of defined mechanical and thermal loads, which are applied to the specimens using a thermal and mechanical testing system. Furthermore, it is investigated how these additional loads affect the modifications previously introduced by the grinding process. The influence of the additional loads is evaluated by using radiographic and electron microscopic examinations. It can be observed that the sequence, as well as the type of the applied loads, play a significant role in the development of the modifications.

Influence of microstructural defects and the surface topography on the fatigue behavior of “additively‐subtractively” manufactured specimens made of AISI 316L

AbstractAs additive manufacturing offers only low surface quality, a subsequent machining of functional and highly loaded areas is required. Thus, a sound knowledge of the interrelation between the additive and subtractive manufacturing process as well as the resulting mechanical properties is indispensable. In this work, specimens were manufactured by using laser‐based powder bed fusion (L‐PBF) with substantially different sets of process parameters as well as subsequent grinding (G) or milling (M). Despite the substantially different surface topographies, the fatigue tests revealed only a slight influence of the subtractive manufacturing on the fatigue behavior, whereas the different laser‐based powder bed fusion process parameters led to pronounced changes in fatigue strength. In contrast, a significant influence of subtractive finishing on the fatigue properties of the defect‐free continuously cast (CC) reference specimens was observed. This can be explained by a dominating influence of process‐induced defects in laser‐based powder bed fusion material, which overruled the influence of surface machining. However, although both laser‐based powder bed fusion parameter sets resulted in substantial defects, one set yielded similar fatigue strength compared to continuously cast specimens.

Investigation of High-Depostition-Rate Additive Manufacturing of Ti-6Al-4V via Laser Material Deposition

Ti-6Al-4V is the most prominent titanium alloy widely used e.g. for aerospace applications. Conventionally, many Ti-6Al-4V aerospace components are produced by a multi-stage hot forging process followed by subsequent machining which often generates a high amount of scrap. Additive manufacturing (AM), such as powder-based laser material deposition (p-LMD), enables parts to be made with geometric freedom and near-net-shape, but so far lacks high deposition rates. The present study proposes high-deposition-rate laser material deposition manufacturing using a large laser beam diameter and increased scanning speed to achieve deposition rates up to 5 kg/h. As Ti-6Al-4V is prone to oxygen pick-up, the process was performed in an inert atmosphere. We determined suitable process windows for tracks without fusion defects and low porosity and investigated microstructure and hardness.