Comparison of Different Additive Manufacturing Methods Using Optimized Computed Tomography

Additive manufacturing (AM) allows for fast fabrication of three dimensional objects with the possibility of use of considerably less resources than would be the case in traditional manufacturing. AM is a fast and cost effective method which boasts the ability to produce components with a previously unachievable level of geometric complexity in end user industrial applications in areas such as the aerospace and automotive industries. However these processes currently lack reproducibility and repeatability with some ‘prints’ having a high rate requiring rework or even scrapping. It is therefore imperative that robust quality systems can be implemented such that the waste level of these processes can be eliminated or decreased. This study presents an artefact that has been optimised for characterisation using computed tomography (CT) with representative AM internal channels and structures. Furthermore the optimisation of the CT acquisition conditions for this artefact is presented in light of analysis of form, internal feature dimensions and position and material porosity

Assessing the material loss of the modular taper interface in retrieved metal on metal hip replacements

Measuring the amount of material loss in the case of revised hip replacements is considered to be a prerequisite of understanding and assessing the true in vivo performance of the implant. This paper outlines a method developed by the authors for quantifying taper material loss as well as more general taper interface parameters. Previous studies have mostly relied on visual inspection to assess the material loss at the taper interface, whereas this method aims to characterize any surface and form changes through the use of an out-of-roundness measurement machine. Along with assessing the volumetric wear, maximum linear penetration and taper contact length can also be determined. The method was applied to retrieved large head metal-on-metal femoral heads in order to quantify the material loss at this junction. Material loss from the female femoral head taper can be characterized as a localized area that is in contact with the stem taper surface. The study showed that this method has good repeatability and a low level of interoperability variation between operators

Comparison of Different Additive Manufacturing Methods Using Computed Tomography

Additive manufacturing (AM) allows for fast fabrication of three dimensional objects with the use of considerably less resources, less energy consumption and shorter supply chain than would be the case in traditional manufacturing. AM has gained significance due to its cost effective method which boasts the ability to produce components with a previously unachievable level of geometric complexity in prototyping and end user industrial applications, such as aerospace, automotive and medical industries. However these processes currently lack reproducibility and repeatability with some ‘prints’ having a high probability of requiring rework or even scrapping due to out of specification or high porosity levels, leading to failure due to structural stresses. It is therefore imperative that robust quality systems be implemented such that the waste level of these processes can be significantly decreased. This study presents an artefact that is optimised for characterisation of form using computed tomography (CT) with representative geometric dimensioning and tolerancing features and internal channels and structures comparable to cooling channels in heat exchangers. Furthermore the optimisation of the CT acquisition conditions for this artefact are presented in light of feature dimensions and form analysis. This paper investigates the accuracy and capability of CT measurements compared with reference measurements from coordinate measuring machine (CMM), as well as focus on the evaluation of different AM method

Comparison of Different Additive Manufacturing Methods Using Optimized Computed Tomography

Additive manufacturing (AM) allows for fast fabrication of three dimensional objects with the use of considerably less resources, less energy consumption and shorter supply chain than would be the case in traditional manufacturing. AM has gained significance due to its cost effective method which boasts the ability to produce components with a previously unachievable level of geometric complexity in prototyping and end user industrial applications, such as aerospace, automotive and medical industries. However these processes currently lack reproducibility and repeatability with some ‘prints’ having a high probability of requiring rework or even scrapping due to out of specification or high porosity levels, leading to failure due to structural stresses. This study presents an artefact that is optimised for characterisation of form using computed tomography (CT) with representative geometric dimensioning and tolerancing features and internal channels and structures comparable to cooling channels in heat exchangers. Furthermore the optimisation of the CT acquisition conditions for this artefact are presented in light of feature dimensions and form analysis. This poster investigates the accuracy and capability of CT measurements compared with reference measurements from coordinate measuring machine (CMM), as well as focus on the evaluation of different AM methods

The evolution of Geometrical Product Specification and Verification in the field of surface metrology and the challenges of its vocational training

Surface metrology is the science of the measurement on micro/nano surface which plays an important role in the control of the manufacture process of a workpiece and the prediction of its performance. In the last three decades, there is an evolution in the field of surface metrology, such as the emerge of many new data collection methods, the development of novel data processing techniques and the shift of characterisation technology from profile paradigm toward areal paradigm. Therefore, in the last 10 years, ISO have developed and released lot of new standard documents to standardise the specification and verification procedure of surface texture assessment in the framework of its technique langrage, i.e. Geometrical Product Specification and Verification (GPS). It empowers engineers with a richer and unambiguous language to detail their requirement with the less specification uncertainty. However, an engineer has difficulty to develop his/her skill in use this technique language due to the increase of its complexity and flexibility. To this end, an EC founded project is undertaking to develop e-learning platform and workshop to deliver the latest development of GPS to engineers. It will use the student-centred method which focused user friendly learning environment, the learning outcomes (i.e. skill and knowledge), self-directed learning etc. This three-year project will be completed at end of 2018. In the beginning of the project, it is of importance to develop a better understanding the challenge of training and user requirement. Therefore, this paper reviews the evolution of GPS in the field of surface metrology with highlighting the issues from training aspect. It explores the current ISO standard documents and training materials, together with a short review of education theory and practise in the information era. A survey has undertaken from industrial participants which outline the requirements of its main users. The outcome of this research maps the field of this training project

The Impediments to Nigeria Understanding Oil Production Volumes, Losses and Potential Solutions

The issue of the quantity of oil produced or missing has traditionally been played down in Nigeria. This is evident as no one in or outside Nigeria is able to quote a totally reliable production volume or loss figure. The aim of this study is thus to search for the root causes as to why there are difficulties in ascertaining the quantity of crude oil produced or missing per day and for potential solutions. The research assesses the present situation and problems requiring solution concerning Nigerian oil and gas measurement control. This is achieved through an intensive review of each of the notified bodies responsible for Nigeria’s oil and gas measurement control, using secondary resources. The bodies reviewed in this study are the Department of Petroleum Resources and the Weights and Measures Department. The study has identified knowledge impediments among the designated bodies. Also discovered were inadequate measurement equipment and absence of measurement guidelines, thus, no mechanisms were in place to address any mismeasurements or losses that are discovered. Provision of comprehensive training to the regulatory body to provide it with the necessary “teeth” to ensure effective delivery of it regulatory function has therefore been recommended as the key solution

Characterization of defects/porosity in additive manufactured components using computer tomography

The key barrier for many industries in adopting additive manufacturing technologies is the lack of quality assurance and repeatability. Defect/porosity analysis is the most important inspection step for any additively manufactured components. This paper presents a method for the detection of defects/porosity in additive manufactured components using computer tomography. A Nikon XTH225 industrial CT was used to analyse the relative size and location of the defects and assess the capability of the inspection process based on different levels of X-ray detector magnification. To reduce the number of process variables, all the measurement process parameters, such as filament current, acceleration voltage and X-ray filtering material and thickness, are kept constant. The acquired data processing, surface determination process and defect analysis was carried out using the VgStudio Max (Volume Graphics, Germany) software package. One Ti6AL4V component built using an Arcam Q10 electron beam melting machine (EBM) was used. The results obtained from the XCT scan are compared to the physical defect analysis, by sectioning the component and confirming pore size and location using focus variation interferometry. The effect of surface determination, repeatability and results’ accuracy are discussed. The main focus of the study is on providing best practice regarding the selection of inspection parameters such as magnification to accurately perform the defect detection

Characterizing edge-wear in ceramic-on-ceramic acetabular cups

The use of fourth generation ceramic as an orthopaedic biomaterial has proved to be a very efficient and has gained popularity for primary hip surgery in the last 8-10 years. Cumulative percentage probability of revision after 7 years for uncemented CoC is 3.09% and for hybrid CoC is 2.00%, this compares favourably with traditional metal-on-UHMWPE uncemented at 3.05% and hybrid at 2.35% (12th Annual Report - NJR, 2015). Such ceramic-on-ceramic hip prostheses are being implanted in ever younger, more active patients, and yet very few long-term large cohort retrieval studies are yet to be carried out due to the survivorship of the implants. It has been seen in previous studies that levels of wear in ceramic-on-ceramic bearing surface can be of the order of 0.2 mm3/million cycles (Al-Hajjar, Fisher, Tipper, Williams, & Jennings, 2013). This is incredibly low when compared to studies that characterize wear in other bearing surface combinations. It has also been reported that an unusual stripe pattern of wear can occur in some in-vivo retrieved cups (Macdonald & Bankes, 2014) and it has further been postulated that this is caused by cup edge loading (Walter, Insley, Walter, & Tuke, 2004). The combined measurement challenge of stripe wear occurring at the edge of a low-wear ceramic-on-ceramic device is considerable, a solution to which is presented here. Current literature on wear measurement of such cases has been confined to in-vitro simulator studies and use of gravimetric measurement which by definition has limitations due to the lack of spacial characterisation. This paper details a novel method for measuring edge-wear in CoC acetabular liners. The method has been employed in an in-vitro study where it has been benchmarked against gravimetric measurements. These liners were measured on a CMM to determine the volume of material loss. The measurements were conducted as a blinded post-wear study akin to measurement of retrieved components. The most challenging part of this novel method was to create a reference geometry that replicates the free form edge surface of the ‘unworn’ cup using the residual post-wear surface. This was especially challenging due to the uncontrolled geometry at the cup edge and intersection of geometric features at this point. To achieve this, the geometry surrounding the wear patch was used to create a localized reference feature that minimised the effect of global form errors caused by hand polishing in the edge area. Furthermore, the reference geometry is compared with the measured surface to determine the linear penetration and volumetric wear loss. Result of this novel method can be seen in Fig 1 and Fig 2. The findings have been compared to gravimetric results and a bar graph comparing two results can be seen in Fig 3. Overall the accuracy of the method for this cohort was 0.03-0.2 mm3 when compared to gravimetric reference measurements. This compares very favourably with previously published wear measurement methods and gives confidence in the ability to measure such small measurement volumes over complex geometry

Identifying The Pattern of Material Loss at the Head-Neck Junction Wear Helps Determine the Mechanism of Failure of Metal on Metal Total Hip Replacements

Material loss at the Head-Neck junction accounts for a third of the total volume material loss in contemporary metal-on-metal total hip replacements. It is speculated that the material loss is the result of corrosion and mechanical wear (fretting). High volumes of material loss have been reported, especially from the head taper. There is only one report on characterizing the pattern of material loss and this was in a very small number of cases (n=5). Our aim was to identify the different material loss patterns at the head taper and their corresponding mechanisms We retrospectively analysed a series of retrieved Large Head Metal on Metal Total Hip Replacements (155 cups, 155 femoral heads and 4 stems). We measured material loss on the bearing surfaces and the head-neck junction using well-published metrology methods. Furthermore we collected patient (age, gender and time of primary/revision operations), pre-revision (cobalt and chromium blood metal ion, oxford hip score, cup orientation and implant position) implant (cup and head size, manufacturer and corrosion severity) data. Finally we used surface analysis techniques (microscopy and spectroscopy) to identify fretting, imprinting and the material composition of debris. We devised a novel four-group classification and two blinded engineers classified the material loss patterns using wear maps derived from the metrology analysis We observed four distinct patterns of taper surface material loss at our retrieval centre and we set out to characterize these types and relate them to patient, implant and clinical variables. The four groups of material loss patterns were defined as: (1) Low wear (n= 63), (2) Open-end band (n=32), (3) Stripped material loss (n=54) and (4) Coup-Countercoup (n=6) (Figure). The Interobserver Reliability Kappa score was 0.78 (p<0.001) indicating substantial agreement between the two examiners. Analysis of variables between the groups identified significantly different head sizes (highest: Group 2, p=0.000), corrosion severity (highest: Group 2, p=0.004) and time to revision (highest: Group 3, p=0.040). We identified four different material loss patterns each with its own mechanism. Corrosion was identified as the principal mechanism in Groups 1 and 3. Group 1 head-neck junctions are thought to have a better seal with less fluid ingress in the junction. Group 3 head-neck junctions are attacked by corrosion either circumferentially, or unilaterally, along the whole engagement length. Mechanically assisted corrosion was the principal mechanism in Group 2. The higher friction torque opens up the open-end part of the junction and the ingressing fluid accelerates the corrosion. Extensive fretting was also observed under the scanning electron microscope. Intra-operative surgical damage was identified as the principal mechanism in Group 4, with only 6 components. The patterns and the mechanisms of material loss at the head-neck junction contribute to the understanding of large head metal-on-metal hip replacements. As a result, better implants can be designed in the future. Clinically, these findings suggest that head size and head taper-trunnion fit are the main factors that determine the longevity of the head-neck junction. On the other hand, patients selection does not influence the integrity of the junction