D. Busquets

Analysis of bending strength of porous titanium processed by space holder method

Abstract Porous titanium specimens have been produced by means of the space holder method. Titanium grade 3 (TiCP3) has been used as material and ammonium bicarbonate as spacer. Process parameters (compaction pressure, metal grain size range and spacer volume) influence on porosity morphology and distribution and bending strength has been analysed. The results denote an important loss of strength when comparing samples sintered without and with spacer. On the other hand, a higher bending strength was observed in the porous samples with a smaller size of ammonium bicarbonate particles. Finally, the evolution of bending strength with the compaction pressure depends on the spacer volume, having a direct dependency for reduced amounts and inverse for higher contents.

Stiffness variation of porous titanium developed using space holder method

Abstract The excellent properties of Ti have resulted in its generalised use for bone implants. However, Ti is very stiff in comparison with human cortical bone, and this creates problems of bone weakening and loosening of the implant. This article discusses the mechanical properties (flexural and compressive strength, and stiffness) of porous Ti–6Al–4V specimens developed using the space holder method. These properties are examined relative to the production process parameters: compacting pressure and sintering time, as well as temperature, and the addition of spacer and its particle size. It is seen that when spacer is added, compressive strength decreases with the application of compacting pressure and that these are the most influential parameters. The developed pieces show a closed and unconnected porosity. Small additions of spacer (25 vol.-%) reduce stiffness to around half of that shown by the solid material, and the resulting pieces are strong enough to be used as bone substitute.

Problems in laser repair cladding a surface AISI D2 heat-treated tool steel

The aim of the present work is to establish the relationship between laser cladding (LC) process parameters (power, process speed, and powder feed rate) and AISI D2 tool steel metallurgical transformations, with the objective of optimizing the processing conditions during real reparation. It has been deposited H13 tool steel powder on some steel substrates with different initial metallurgical status (annealed or tempered) using a coaxial LC system. The microstructure of the laser clad layer and substrate heat-affected zone (HAZ) was characterized by optical microscopy, scanning electron microscopy, and electron backscattered diffraction. Results show that the process parameters (power, process speed, feed rate, etc.) determine the dimensions of the clad layer and are related to the microstructure formation. Although it is simple to obtain geometrically acceptable clads (with the right shape and dimensions) in many cases some harmful effects occur, such as carbide dilution and non-equilibrium phase formation, which modify the mechanical properties of the coating. Specifically, the presence of retained austenite in the substrate–coating interface is directly related to the cooling rate and implies a hardness diminution that must be avoided. It has been verified that initial metallurgical state of the substrate has a big influence in the final result of the deposition. Tempered substrates imply higher laser absorption and heat accumulation than the ones in annealed condition. This produces a bigger HAZ. For this reason, it is necessary to optimize the process conditions for each repair in order to improve the working behaviour of the component.

Microstructure and Mechanical Behavior of Porous Ti–6Al–4V Processed by Spherical Powder Sintering

Reducing the stiffness of titanium is an important issue to improve the behavior of this material when working together with bone, which can be achieved by generating a porous structure. The aim of this research was to analyze the porosity and mechanical behavior of Ti–6Al–4V porous samples developed by spherical powder sintering. Four different microsphere sizes were sintered at temperatures ranging from 1300 to 1400 °C for 2, 4 and 8 h. An open, interconnected porosity was obtained, with mean pore sizes ranging from 54.6 to 140 µm. The stiffness of the samples diminished by as much as 40% when compared to that of solid material and the mechanical properties were affected mainly by powder particles size. Bending strengths ranging from 48 to 320 MPa and compressive strengths from 51 to 255 MPa were obtained.

ALBA 14 a long term low cost glider with water sampling capabilities

The concept of ALBA vehicles has been developed under the consideration of long term autonomous instrumentation for oceans research applications. Among their prospective missions, there are three of particular interests: water column survey, early event detection (oil spill and HAB) and water sampling, The concept of hybrid vehicles has been already proposed in diverse works [12]. The advantages provided by combining the capabilities of gliders, as a robust long term platforms, and self-propelled vehicles (AUV), with their great maneuverability promises interesting possibilities. A reduced payload capacity is then the drawback in hybrid vehicles since they should reserve additional space for housing all the different sub systems. In addition, external appendages and navigation systems such wings, propeller, propeller duct, etc. introduce additional drag and interference in the optimal seagoing efficiency in both modes of operation. Low cost concept applied to ALBA project allows the possibility of focusing on the research of interest, overshadowing the problems associated to the conservative strategy of avoiding planning mission where the vehicle safety could be compromised. A high number of low cost vehicles can simultaneously be deployed at the same cost than few units of standard commercial underwater autonomous vehicles AUV and Gliders. Keeping in mind that cutting edge capabilities usually provided by these commercial vehicles are not always required, the proposal vehicle Alba 14 SGL can fill the gap in the research community, providing interesting approaches to new prospective of surveys that, due to the high cost of commercial vehicles, had not been considered before. Continuing with this concept prototype of a glider Alba-14 has been designed and built as a test bed for testing its capabilities and feasibility of the low cost concept applied to unmanned vehicles for ocean research.

Fabrication of ultrafine and nanocrystalline WC-Co mixtures by planetary milling and subsequent consolidations

Abstract In this work ultrafine and nanocrystalline WC–Co mixtures were obtained by low energy milling in planetary ball mill. The effect of the processing conditions on the reduction and distribution of the grain sizes and the internal strains level were studied. The characterisation of the powder mixtures was performed by means of scanning and transmission electron microscopy and X-ray diffraction analysis. Observations through SEM and TEM images showed a particle size below 100 nm, after milling. The X-ray diffraction profile analysis revealed a WC phase refined to a crystallite size of 19 nm. The mixtures obtained have been consolidated and mechanical and microstructurally characterised. The results show improvements in resistant behaviour of the material consolidated from nanocrystalline powders, in spite of the grain growth experienced during the sintering. The best results were found for the material obtained by wet milling during 100 h, which presents values of hardness higher than 1800 HV.

Microstructural change of the HAZ in an MIG welded bond on an AA7020 aluminium alloy: stress corrosion crack growth rate in dissimilar metal welds

AW 7XXX-series aluminium alloys with mid-range performance qualities are used ever more frequently in the manufacturing of welded structures, due to the industrial and technological development of different welding methods that render the use of such structures ever more competitive. At the same time, this requires greater knowledge both of the welded bond and of the welding process or of microstructural change throughout the HAZ, in welds of this type. Many researchers have undertaken studies into the mechanical behaviour of the welded bond, others have devoted their attentions to metallurgical phenomena, whether concerning phenomena inherent to the area immediately surrounding the weld interface or concerning models or simulations of the welded structure; in addition, there are those who have made comparisons between different welding methods or who have dedicated their time to post-welding treatments. However, very few researchers have devoted their attentions to studying microstructural change throughout the HAZ on welded test pieces.

Application of the Zero-Order Reaction Rate Model and Transition State Theory to predict porous Ti6Al4V bending strength

Porous Ti6Al4V samples were produced by microsphere sintering. The Zero-Order Reaction Rate Model and Transition State Theory were used to model the sintering process and to estimate the bending strength of the porous samples developed. The evolution of the surface area during the sintering process was used to obtain sintering parameters (sintering constant, activation energy, frequency factor, constant of activation and Gibbs energy of activation). These were then correlated with the bending strength in order to obtain a simple model with which to estimate the evolution of the bending strength of the samples when the sintering temperature and time are modified: σY=P+B·[lnT·t-ΔGa/R·T]. Although the sintering parameters were obtained only for the microsphere sizes analysed here, the strength of intermediate sizes could easily be estimated following this model.