Repository hosted by TU Delft Library

This paper summarises the current situation and knowledge on magnesium forging as well as its prospects for further development. It is structured according to the sequential areas of the processing chain: feedstock, forging process, finishing, and components. Attention is given to some specific metallurgical aspects of magnesium forging and the resulting mechanical properties. Further, the weight-saving potential of magnesium forgings over conventional solutions (notably in aluminium), prospective application areas (especially for transport means), demonstrator components and costs are addressed.

Wrought magnesium alloy products have favourable attributes compared to castings with regard to mechanical properties. In addition they provide a complementary class of shapes and geometries. These are the current drivers for developments within the field of magnesium forging technology. Within this context, the European research project MagForge is being conducted with the overall goal to provide tailored and cost-effective technologies for the industrial manufacturing of forged magnesium alloy components. One of the present impediments to magnesium forging is the availability of suitable alloys: Forgeability in terms of the robustness of processing windows and the resulting product quality in terms of mechanical performance need to be improved. Hence this is one of the main topics of the project. This paper reports on the progress made in this particular area. The strategy adopted for advancement consists in inducing grain refinement and stabilisation of the microstructures by rare-earth and complementary modifications of the commercial AZ80 and ZK60 forging alloys. In line with common practice, forging slugs were prepared from cast billets by preextrusion. The quality of the modified alloy slugs is assessed on the basis of laboratory-scale cone-forging trials. The results show a significant improvement over the existing alloys.

The increasing use of aluminium alloys in light weight structural applications is restricted mainly due to their lower room temperature formability compared to steels. Forming at higher temperature is seen as a promising solution to this problem. In the present investigation two Al-Mg-Si alloys (EN AW-6016 and EN AW-6061) were deep-drawn at room temperature and 250 C and their behaviour during drawing were compared. The effect of ram speed, drawing ratio, holding time, and temper was also investigated. Among the parameters investigated temperature was found to have the most significant effect on the force-displacement response. Because anisotropy has been an important concern during the deep-drawing process, this parameter was also investigated by looking at the earing profile. With increasing temperature the amplitude of earing decreased while the number of ears remained the same, indicating that there is no change in anisotropy with temperature. The cup thickness increases from the bottom of the cup to the flange with a local minimum around the mid-height of the wall. © 2013 Elsevier B.V.

Longitudinal weld seams are an intrinsic feature in hollow extrusions produced with porthole dies. As these joins occur along the entire extruded length, it is desirable that these weld seams have a minimal impact on the structural integrity of the extrudate. In particular, defects associated with weld seam formation should be avoided. In this research, the occurrence of defects related to material flow inside the extrusion tooling is studied. In lab-scale experiments, EN AW-6060 and EN AW-6082 aluminium alloy billets are formed into strips by means of the direct hot extrusion process. By utilising model dies with an internal obstruction similar to the supports present in porthole dies, a strip with a central longitudinal weld seam is formed. The effects of different geometries of the weld-chamber and the processing conditions on the quality of the weld seam are investigated. Characterisation is performed through mechanical testing, focusing on the ability of the weld seam area to accommodate plastic deformation, and microstructural analysis provides insight into the defects related to unsound metal flow. Through computer simulations, conditions related to weld seam formation are modelled and correlated with the experimental results. The experimental results demonstrate that metal flow controlled by the die geometry causes defects leading to inferior mechanical performance of the extrudate. It is further argued that current weld seam formation criteria utilised in finite element modelling need enhancement to incorporate these flow related defects. cop. 2014 Elsevier B.V.

Thermal and thermo-mechanical modeling and characterization of solid state lightening (SSL) retrofit LED Lamp are presented in this paper. Paramount Importance is to design SSL lamps for reliability, in which thermal and thermo-mechanical aspects are key points. The main goal is to get a precise 3D thermal lamp model for further thermal optimization. Simulations are performed with ANSYS and Covent or Ware software tools to compare different simulation approaches. Modeled thermal distribution has been validated with thermal measurement on a commercial 8W LED lamp. Materials parametric study has been carried out to discover problematic parts for heat transfer from power LEDs to ambient and future solutions are proposed. The objectives are to predict the thermal management by modeling of LED Lamp, get more understanding in the effect of lamp shape and used materials in order to design more effective LED lamps and predict light quality, life time and reliability.

The increased electrical currents used to drive light emitting diode (LED) cause significant heat generation in the solid state lighting (SSL) system. As the temperature will directly affect the maximum light output, quality, reliability and the life time of the SSL system, thermal management is a key design aspect in terms of cost and performance. Particularly for consumer SSL system, natural convection cooling is cheaper and more reliable than the forced air cooling heat sinks. Although with less efficiency, natural convection heat sink is a good compromise between economy and thermal performance of SSL systems. In this work, the thermal performance of two geometrically different passive heat sink designs for consumer SSL applications is numerically simulated. The heat sink performance is simulated for two orientations: LED up and LED down orientation. Simulation runs for the two designs at the two orientations, in order to investigate the thermal performance of the heat sinks with natural convection cooling. Meanwhile, the radiation effect is considered. With passive cooling, the natural convection plays an important role, and results show that if free ambient air flow is blocked by the heat sink design and the performance reduces considerably. Furthermore, the volume of free air in the luminaire is expected to have significant impact to the heat sink thermal performance. Therefore, the thermal performance for different volumes of luminaire enclosures is also investigated in this work. To analyze the modeling results, a straightforward calculation of the thermal resistance between the LED junction and the environment is applied. In the results, the thermal resistances of LED junction to environment decreases but the air velocity gradually increases with the increasing luminaire volume. In conclusion, although more study is needed for validation of the optimal volume and shape for natural convection, the results in this work can already be used to guide the design of luminaires. In future work, the simulation on real model of bulb or luminaires will be applied to investigate what extend designs based on natural convection and radiation principles can be exploited to manage the LED junction temperature. © 2011 IEEE.

The surface quality of aluminium extrusion products can be hampered by undesired surface features like die lines and pickups. In particular the presence of pickups is considered as undesirable. Surface pickups appear as intermittent torn marks on the aluminium extrusion products, often terminated with a protruding lump rising above the surface up to hundreds of microns in height. Using a model calculating initiation, growth as well as detachment of the lumps on the die bearing surface, the surface quality of aluminium extrusion products can be predicted. The results of the model can be presented in terms of surface quality diagrams, where contour lines of the (normalized) calculated number of lumps are presented in terms of exit speed and extrudate surface temperature. These diagrams are unique for a certain combination of geometry of the extrudate and the aluminium alloy and can be used to optimise process conditions with respect to surface quality within a certain process window. In order to validate the model, the size and number of pickups on the surface of a labscale extruded strip of AA6063 have been measured. The results of the model are being compared to the experiments and show good agreement. © (2012) Trans Tech Publications.

Biomedical applications are an emerging field of interest for magnesium technology, envisioning biodegradable implants that resorb in the human body after having cured a particular medical condition (such as artery clogging or bone fractures). This challenges research in a sense that the materials to be used need to dissolve in vivo in a controlled fashion without leaving harmful remainders and while maintaining sufficient strength and other (mechanical) attributes as long as necessary. To comply to the requirements, magnesium alloys as well as their processing routes into implants need to be tailored. While new alloy compositions are receiving ample attention, the paper at hand addresses the processing issue. The application of choice is the (cardio)-vascular stent. Different steps in manufacturing magnesium AZ-alloy stent tube are considered, including equal channel angular pressing, extrusion and subsequent drawing operations. Results show that the processing route has an important influence on the microstructure of the finished stent tube and hence on its functional performance.

In hollow aluminium extrusions, longitudinal weld-seams are formed through a solid-state bonding process at elevated temperatures and under conditions of interfacial pressure and plastic deformation. For structurally loaded components, sound weld seams are imperative. In our research, a weld seam integrity indicator as a means of quantifying bonding efficiency is introduced and the feasibility of this concept is investigated by means of lab-scale experiments. Characterisation of these weld seams through mechanical testing, provides a basis for an estimation of the weld seam indicator introduced in this paper, thus demonstrating the feasibility of this concept.

This work deals with thermal simulation and characterization of solid state lightening (SSL) LED Lamp in order to get precise 3D thermal models for further lamp thermal optimization. Simulations are performed with ANSYS-CFX and CoventorWare software tools. The simulated thermal distribution has been validated with thermal measurement on a commercial 8W LED lamp. Materials parametric study has been carried out to discover problematic parts for heat transfer from power LEDs to ambient. The objectives are to predict the thermal management by simulation of LED lamp and environment and to get more insight in the effect of lamp shape and materials used in order to design more effective LED lamps. © 2011 IEEE.

Longitudinal weld seams are an intrinsic feature in hollow extrusions produced with porthole dies. The formation of longitudinal weld seams is a solid bonding process, controlled by the local conditions in the extrusion die. Being the weakest areas within the extrusion cross section, it is desirable to achieve adequate properties of these weld seams. In our research, the concept of a weld seam integrity indicator as a means of quantifying bonding efficiency is introduced. The value of this indicator depends on a number of factors: the material flow within the die weld chambers, an adequate pressure level acting on the weld planes and finally the evolution of the metal microstructure. Optimisation of the welding conditions leads to a higher value of the weld seam integrity indicator and thus to improved weld seam properties. The objective of the research presented in this paper is to assess the feasibility of this concept. In lab-scale experiments, AA6060 and AA6082 aluminium alloy billets were formed into strips by means of the direct hot extrusion process. By utilising porthole dies a central longitudinal weld seam is formed. The effect of different geometries of the weld chamber and the processing conditions on the quality of the weld seam are investigated. Characterisation of these weld seams through mechanical testing, focusing on the ability of the weld seam area to accommodate plastic deformation following the onset of plastic instability, and microstructural analysis provides insight into bonding performance. The outcome of this characterisation provides a basis for an estimation of the weld seam indicator. Through computer modelling, the particular process conditions related to weld seam formation are calculated and correlated with the experimental results. The experimental results clearly demonstrate that weld seam formation is controlled by a combination of factors as described above. Inadequate fulfilment of these conditions, verified by the FEsimulations, is the cause of inferior weld seams, associated with low values of the weld seam integrity indicator. Through further elaboration of the concepts presented in this work, the weld seam integrity indicator is to be developed, with the future aim of predicting the weld seam performance through finite element simulations. © (2010) Trans Tech Publications.

In hollow aluminium extrusions produced with porthole dies, longitudinal weld-seams are present throughout the entire extruded length, due to rejoining of metal streams inside the extrusion tooling. These weld-seams, formed by a solid-state bonding process at elevated temperatures and under conditions of interfacial pressure and plastic deformation, are commonly the weakest areas in the extrusion cross-section. Therefore, predictive control of the weld-seam formation process is essential. For this purpose a conceptual model is developed, containing all relevant features pertaining to the solid-state bonding process as this occurs in aluminium extrusion. This paper describes an experimental programme aimed at exploring the effects of tooling geometry, process conditions and alloy composition on weld-seam performance. In lab-scale experiments, strips were extruded from AA6060 and AA6082 billets, using tooling geometries in which a bridge was incorporated to form a weld-seam. Keeping the profile geometry constant, the internal geometry of the tooling was varied, resulting in different bonding conditions. The obtained weld-seams were characterised by means of transverse tensile tests. Associated microstructural characterisation of the extrusions and inspection of fracture surfaces was performed in order to relate the mechanical performance to microstructural features. The results clearly show that the evolutionary microstructural response of the alloy, determined by the local thermomechanical conditions inside the die, has a significant effect on the weld-seam performance. It is therefore concluded that, besides a criterion for suitable flow conditions and interfacial pressure, a predictive tool for weld-seam quality must incorporate the evolutionary microstructural response of the alloy in question.

Being biocompatible and biodegradable, magnesium alloys are considered as the new generation biomedical implant materials, such as for stents, bone fixtures, plates and screws. A major drawback is the poor chemical stability of metallic magnesium; it corrodes at a pace that is too high for most prospective implant applications. Requirements for biodegradable implants are bio-compatibility, controlled biodegradability and sustainable mechanical properties. Various magnesium alloys containing Al, Zn, Y and rare-earth elements are analyzed in this respect. The alloys are compared on the basis of microstructure, tensile tests and potentio-dynamic polarization tests in simulated body fluid. The effects of semi-solid processing, hot extrusion, heat treatments and sterilization on corrosion resistance and tensile properties are investigated. AZ80 magnesium alloy with certain post-processing treatments fulfills the requirements best as a prospect implant material which has the potential for further improvement by trace alloying additions and surface modifications.

This paper studies the thermal influence of a light-emitting diode (LED) driver on a retrofit LED lamp, also reporting on a procedure for its thermal characterization and multiscale modeling. In this analysis, temperature is measured by infrared thermography and monitoring specific locations with thermocouples. Experimental results point out that temperature increases considerably in all lamp parts when the driver is installed in the lamp (up to 15% for LED board). The multiscale simulation approach is set with thermal parameters (thermal conductivity, emissivity, and LED board thermal resistance) measured from several parts of the lamp, reaching an agreement between experiment and simulation smaller than 10%. With this model, the driver temperature is investigated under operational conditions accounting for two alternative thermal designs. First, the driver is completely surrounded with a filling material (air completely removed, Case A), and, second, only the thermal contact between the board and the lamp is improved (air is kept, Case B). In both cases, the heat removal from the driver to the ambient by conduction is enhanced, observing that temperature decreases in its most heated components up to 10 °C in Case A, and up to 7 °C in Case B. cop. 1986-2012 IEEE.

This work deals with the extraction of key thermal parameters for accurate thermal modelling of LED lamps: air exchange coefficient around the lamp, emissivity and thermal conductivity of all lamp parts. As a case study, an 8W retrofit lamp is presented. To assess simulation results, temperature is monitored on the lamp by infrared thermography and by monitoring specific locations with thermocouples. As a result, experimental and simulation results show differences below 15%. cop. 2013 IEEE.

This paper summarizes a study into the thermal influence of an LED driver board on an SSL Lamp. Full thermal characterization of an LED lamp and driver board is carried out by infrared thermography and by monitoring specific locations with thermocouples. The experimental results have been used to set up and validate a predictive simulation model for both the SSL lamp and the driver board. With the obtained simulation model the driver board temperature is investigated under operational conditions for two alternative designs. © 2012 IEEE.

This work deals with a precise 3-D modelling of several LED board technologies mainly focused on thermal, thermo-mechanical evaluation and life time prediction to compare their performances. Main role of each LED board is to transport heat from LED die to heat sink and keep the thermal stresses in all layers as low as possible. Thermal stress has been inspected for the widest temperature range that can affect the LED boards (-40 to +125°C). Additionally, thermal stress cycles that lead to the LED board failure due to thermal fatigue have been calculated. Simulations have been completed with ANSYS structural analysis where temperature dependent stress-strain material properties have been taken into account. The objective of the analysis is to optimize not only the thermal management by thermal simulation of LED boards, but also to find potential problems from mechanical fatigue point of view. © 2012 IEEE.

Intelligent LED lighting systems can save up to 80% of energy compared to incandescent lighting systems. In order to provide these products at reasonable costs, integration and miniaturization are important steps [1]. Another attractive feature of LED systems is the claimed long life expectancy. The design lifetime of LED luminaries is typically 25 to 50 thousand hours [2, 3]. The long design lifetime, in combination with short-cycled technological innovations in LED packaging, pose challenges to traditional test-in reliability engineering approaches. Innovations in LED packaging are required to meet future cost target requirements at proven and designed-in lifetimes. Integral design of complex systems - such as LED systems merging optical, thermal, electrical, and mechanical disciplines - becomes more difficult [4, 9] as reliability engineering builds on the higher specialization per discipline. In this work we explore novel approaches for reliability engineering.