Key Engineering Materials Vol. 957

Abstract: The use of plasticizers usually has a beneficial effect on the ductile properties of polymers due to a reduction in the interaction of the polymer chains. As a result of this reduced interaction, there is also a reduction in the tensile strength of the material up to 20.3 MPa. To improve the interaction of polymer and plasticizer without affecting the ductility of the material, and potential migration, a reactive extrusion (REX) process is proposed in this research, by using several organic peroxides in poly(lactide)-dibutyl itaconate (DBI) plasticized formulations. Test specimens are obtained by extrusion and injection moulding and the overall mechanical properties, thermal degradation and biodegradation were measured. The tensile strength increased up to 25.1 MPa and with an elongation above 270%.

Abstract: The development of composite materials has meant, over the last few decades, a challenge for research in relation to the search for new ways of manufacturing. Currently, a wide variety of materials and processes have a high degree of development and industrial applications, but the potential for improvement in this area promotes the search for new systems that allow obtaining products with better mechanical characteristics and better manufacturability. Within this framework of the development of processes and applications, the following combination of manufacturing technologies is proposed: thermoforming and overmolding of carbon fiber reinforced thermoplastics with continuous and short fibers respectively. The aim of this work is to provide a hybrid technology to manufacture sandwich panels whose stiffening core geometry can be highly optimized based on the load requirements considered in the design of the product of interest. The proposed process consists of the prior shaping by thermoforming process of a continuous fiber-reinforced thermoplastic laminate on one of the surfaces of the manufacturing mold that will later be used for injection of the core structure with a counter-mold adapted for this purpose. The injection of the short fiber reinforced thermoplastic will be carried out in such a way that one of the surfaces of the resulting cavity is directly in contact with the previously formed laminate, giving rise to an overmolding. After this operation, a closing laminate will be formed that will give the panel the desired resistance. A detailed analysis of the different stages of this process is necessary, especially with regard to the processing parameters (e.g. injection temperature, pressure, polymer velocity, and interface temperature), orientation of the fiber resulting from the injection process, the adhesion of the polymer between stages, and the geometric limitations of considered sub-processes. The analysis is performed using a finite element method (FEM) software and analytical approaches.

Abstract: Polyoxymethylene (POM) is a semi-crystalline engineering thermoplastic polymer employed in the manufacturing of precision parts for engineering applications requiring high dimensional stability and good frictional resistance properties. Due to its lower cost and ease of integration into automated manufacturing processes, laser marking is the state of art method vastly employed for marking products aiming to enhance traceability and accountability. Laser marking of polymers can be challenging depending on the outcome of interactions between the material and the laser radiation for a specific wavelength. Low absorption for a wide wavelength range is usual on transparent and white thermoplastics, which is also the case for natural POM. In this work, in a bid to determine the range of feasible process parameters that ensure good-quality markings, which are necessary to developing a lean manufacturing-focused laser marking process, both natural and pigmented POM were laser-marked, and the quality of the substrates and markings were analyzed using a variety of analytical methods. Results indicated a marked difference in the laser mark-ability of natural and pigmented POM, which is attributed to marked differences in laser absorption abilities of natural and pigmented POM.

Abstract: Composite structures, such as glass fiber reinforced polyether-ether-ketone (PEEK) and polyamide (PA66), usually undergo drilling operations for subsequent assembly. A typical problem with these composites is damage around the drilled surface due to a possible non-homogeneous cutting of the fibers. In this context, the delamination is evaluated after a cryogenic drilling. Thus, the objective of this paper is to determine the feasibility of cryogenic drilling considering surface damage after cryogenic machining, at hole the entry and exit. Experimental test were carried out in a machining center at a temperature close to -130 °C using liquid nitrogen, LN₂, as cooling environment. The diameter of the drill is 6 mm and the drill tip is polycrystalline diamond (PCD). The plate material is PEEK-GF30 and PA66-GF30. The delamination factor was obtained using a three-dimensional measurement device with an optical sensor and a focus-variation device. The results obtained are favorable regarding the potential use of cryogenic machining.

Abstract: In recent years, developments in medical devices have led to research in drug release mechanisms. Although important advances have been made, some critical points still exist to investigate. Regarding materials to be used for drug purposes some natural materials seem to be a biocompatible future solution. Silk fibroin (SF) is one of the proposed candidates to satisfy the needs of drug release technologies due to its biodegradability in a tunable range of time with non-toxic end products. This work aims to study the dip coating process over stainless steel and polyurethane tubes to obtain micro-coating layers for drug release purposes. The effect on the number of cycles (2, 4, and 8) and evaporation time between cycles (10, 20, and 30 seconds) was studied. The layer thickness of the coating and the degradation rate in water were analyzed. Results showed that silk fibroin coatings at the microscale can be achieved. Furthermore, a strong influence of the evaporation time over the layer thickness with a maximum decrease of 66,1% as the evaporation time increases and an increase of 63,8% as the number of cycles increases. Results showed a high degradation rate in PBS with a 70,5% of weight loss relative to the initial weight of SF degraded within 3 hours.

Abstract: The use of advanced materials in new fields of applications is usually related with specific properties and great advantages. However, it may become an important technological challenge for the manufacturing and joining of these materials. Carbon Fiber Reinforced Plastic (CFRP) is widely used in aerospace or automotive industry, but currently is starting to be used as structural component in the building industry. In contrast to the thin parts developed for aircraft. the elements for construction are characterized by high thicknesses. This fact increases the abrasive effect of carbon fibre on cutting tools when it is machined by conventional processes. For this reason, non-conventional technologies. as Abrasive Water Jet Machining (AWJM), could be a suitable technology for this purpose. An operation of great interest for these materials is drilling. The industry demands the drilling of a wide range of increasingly large diameters. This, in combination with large thicknesses in CFRP, makes the machining process more difficult. Technologies such as orbital drilling are limited by the range of diameters that can be obtained and the abrasive wear between the material and the cutting-tool. Therefore, the use of abrasive waterjet cutting is proposed as an alternative technology capable of machining diameters in a wide range. One of the main limitations for the use of AWJM on large thickness parts of CFRP comes from the surface finish of the machined surfaces. where high values of roughness can be found. In this research, the effects of water jet cutting parameters have been evaluated for the machining of 17 mm thickness CFRP specimens. By variations on the traverse speed. abrasive mass flow rate and hydraulic pressure. differences in the performance and micro-geometrical characteristics of the machined surfaces were obtained, allowing to identify most significant affecting parameters of the process.

Abstract: Increasing demands on component functionality and weight, but also on the use of resources and cost-effectiveness, are leading to the increased use of hybrid components. The combination of diverse materials enables the use of positive properties of the individual material in one component. With regard to the production of hybrid components, the use of hybrid pre-joined semi-finished parts simplifies the joining process, as simple geometries can be used. A well-established process for joining dissimilar materials such as steel and aluminium is rotary friction welding. However, steel and aluminium form brittle intermetallic phases in the joining zone due to their low solubility. Therefore, in addition to the advantages, the use of pre-joined hybrid semi-finished parts also pose new challenges for the following process chain. As a result of thermomechanical stresses during forming, local failure of the joining zone may occur. Due to its small thickness and position within the component, the analysis of the joining zone is only possible by complex destructive testing methods. FE simulation therefore offers an efficient way to design and analyse forming processes for hybrid semi-finished parts, the development of damage in the process design and to reduce damage by process modifications. Therefore, within this study a numerical model of the forming process chain is developed considering inductive heating, transfer and forming. For a realistic description the flow behaviour of the monolithic materials as well as the bonding strength of the pre-joined semi-finished parts is determined in experimental tests. Based on the experiments a damage model is calibrated and used for the analysis of different process variants of hollow forward extrusion of pre-joined hybrid semi-finished parts of steel and aluminium.

Abstract: Due to increasing product requirements regarding lightweight, functional integration and resource efficiency, research into and use of hybrid parts are steadily increasing. Tailored Forming provides an innovative process chain for manufacturing hybrid parts by using pre-joined semi-finished products. In addition to the potentials, however, challenges also result in the production of hybrid components. In particular, the material combination of steel and aluminium is demanding due to strongly differing physical properties. An inhomogeneous temperature distribution within the pre-joined semi-finished part can be used to equalize flow properties during the forming process. However, processes are sensitive to temperature deviations resulting in critical stresses and failure of the final part. This study focuses on a process design of a hybrid bearing bushing consisting of the aluminium alloy EN-AW-6082 and the steel 100Cr6 using numerical simulation. First, a closed-die forging process is analysed regarding sensitivity to process fluctuations resulting in deviations in temperature distribution. To increase process stability, a new hollow forward-impact extrusion process is numerically designed and investigated regarding its potential to reduce critical stresses and thus the risk of part failure. Furthermore, a numerical model of inductive heating is used for the consideration of inhomogeneous temperature fields. Finally, hybrid bearing bushings are produced using closed-die forging and hollow-forward extrusion to evaluate numerical results.

Abstract: 3D printing technology has gained popularity among researchers since it can produce complex geometries, such as open-cell foam. The open-cell foam shows potential in a range of applications such as energy absorption, thermal management, filtering, and acoustic damping. However, the feasibility of the applications depends on the material used to construct the 3D printed open-cell foam and its physical properties e.g, pore size and porosity. Therefore, understanding the physical properties is crucial in classifying this new generation of open-cell foams. This study aims to determine the permeability of 3D printed foams using the Forchheimer equation and compared the results with a fractional estimation method to reduce the duration of future experiments. The fractional results were validated through computational fluid dynamics (CFD) simulations. The result shows that the proposed estimation method can be used to determine the permeability of 3D printed foam with a height of 60 mm or larger, and up to six times larger than 5 PPI (pores per inch). However, it is recommended to conduct simulations of large pore size foam using a 3D model to accurately describe the local velocities in the free stream region.

Abstract: A combined heat and power system has been proposed and evaluated thermodynamically to generate mechanical power and hot water for various applications. The combined system consisted of a gas turbine cycle (GT) to produce the main mechanical power. An Organic Rankine cycle (ORC) is used to convert some of the GT flue gases wasted heat to additional mechanical power. The hot water is produced from two heating levels, in first level the rejected latent heat from the ORC condenser is recovered. While in second heating level the high thermal energy available in the GT working fluid after the compression process is used through a heat exchanger. A thermodynamic simulation is carried out by ASPEN plus package and the thermophysical properties of the working fluids is obtained from REFPROP. The effect of increasing the gas burner flue gases temperature from (700-1250 °C) on the main system design parameters reveals that the overall net power output has declined by 3.5% but it is greater than the standalone GT cycle by 13.4 – 17.6%. In addition, the total thermal energy recovered has declined significantly by 53.7%. Furthermore, the GT thermal efficiency has increased from 16-25% and the combined system has converted the waste heat into hot water with a temperature between 93-48.5 °C to be used in different heating applications.