Mark S. Pridham

Material property changes associated with laser forming of mild steel components

Abstract Laser forming is a technique developed over the last 5 years or so which allows the forming and bending of metallic components without the need for hard tooling. This makes it a potentially useful prototyping and limited production technique. The laser forming method makes use of the laser’s ability to heat very localised regions of material very quickly. The large temperature gradients set up within the material create internal stresses, which result in permanent deformation. A variety of sharp folds and curved surfaces can be generated by this technique. Work carried out on this technique both by ourselves and others has tended to concentrate on developing control systems and mathematical models in order to gain an insight into the mechanics of the technique. An area neglected so far, however, is how the rapid and repeated heating and cooling cycles associated with laser bending alter material properties. If components produced by this technique are to be used in practice it is vital that any weaknesses (or benefits) imparted by the technique are identified. This paper gives a brief introduction on the general principles and applications of the laser forming technique. An analysis of the effects of the technique on the material properties of mild steel is then presented and this is followed by a discussion on how, in practice, components formed using the thermally based laser forming technique are likely to function in comparison to more conventionally produced parts.

Issues concerning the measurement of transformation temperatures of NiTi alloys

The transformation between martensite and austenite in shape memory alloy is characterized by four transformation temperatures: martensite start temperature (Ms), martensite finish temperature (Mf), austenite start temperature (As) and austenite finish temperature (Af). In actuator designs that rely on the shape memory effect, it is important to obtain an accurate measure of these transformation temperatures, especially As and Af, because they determine the functional temperature range of the actuator. Several methods of determining these temperatures have been reported, but their accuracy and coherence are not clear. The transformation temperatures of NiTi wire under different heat treatment conditions were measured by the three commonly used methods: differential scanning calorimetry (DSC); an electrical resistance method, which uses a sudden change in resistance as an indication of transformation; and an applied loading method, where a macroscopic displacement indicates the transformation. The results show that the transformation temperatures measured by DSC do not correspond to those measured by the other two methods, which are similar. The applied loading method is most effective for providing practical information about the stress-dependent transformation temperatures. The electrical resistance method gives clearly determined points for Ms and Mf in the cooling resistance–temperature curve, but As and Af are not clearly identifiable in the heating process.

Improvements to laser forming through process control refinements

Laser forming is a process that uses the energy of relatively high powered lasers to cause permanent deformation to components by inducing localised thermal stresses. It is envisaged that this material processing technique will find a number of commercial applications. This paper briefly discusses laser forming and the development of a basic process monitoring and control system used to overcome variability problems due to the complex nature of the lasers themselves and the manner in which they interact with material. It then goes on to show how the basic control system was modified, using increased feedback data sampling, time delays and a modified control algorithm which takes account of the forming rate in addition to the error. The effect of these developments is then illustrated by a series of tests which show the modifications significantly improve process tolerances.

Controlled laser forming for rapid prototyping

It is now well‐known that laser forming offers considerable potential for rapid prototyping and manufacture of sheet metal components. However, a problem with the technique to date has been the generally poor repeatability of the process as a whole on a run‐to‐run or‐day‐to‐day basis. Efforts to rectify the problem by careful selection and pre‐setting of the process parameters, based on empirical data, have proved unsatisfactory due to the inherent variability of the laser system. Elimination of the variability would require conditions which would be impractical for production situations. In order to overcome this difficulty a closed loop control system based on monitoring component deformations, as they approach target values, has been developed. In response to the deformation data, available at each forming pass, the forming parameters are adjusted so that forming continues at an optimum rate to a predetermined tolerance. Discusses the production of laser‐formed components and shows how the implementation of a feedback control system can negate the inevitable process variability, thereby significantly aiding the efficient and accurate production of components.

A feedback control system for laser forming

Laser forming is a relatively new technique which is beginning to find applications in a number of areas. The technique involves passing a high powered laser beam over the surface of a material to induce thermal deformation. If the technique is to find widespread commercial application then control of the process will be crucial. The case is made for using feedback control as opposed to a knowledge based system to counter the difficulties arising from the range and unpredictability of the process variables. The development of a basic feedback system is described and the results for forming operations with the system implemented are presented. These indicate the success of the system in producing deformations of predetermined magnitude. Recommendations for future developments and refinements are proposed.

Preparation and characterisation of novel polycaprolactone-chitosan blends

This research paper reports on the production of a biocompatible and biodegradable material to be used in a polymer stent used for counteracting the occurrence of anastomotic leakage following gastrointestinal surgery. Chitosan was blended with polycaprolactone in a solvent mixture of acetic acid and water. Membranes were formed with a range of 50/50%, 60/40%, 65/35%, 70/30% and 80/20% polycaprolactone/chitosan. The tensile properties of the blends were examined over a time period to access material degradation. In addition the biocompatibilities of the polycaprolactone/chitosan blends were tested for cytotoxic effect using primary tendon fibroblastic cells. This research concluded that the polycaprolactone/chitosan was non-toxic to the fibroblasts cells in-vitro. Analysis of the mechanical properties of the blends showed a range of mechanical strengths and polymer life spans. Overall, blends of 65/35%, 70/30% and 80/20% polycaprolactone/chitosan emerged as possible candidates for the production of a gastrointestinal stent.

Stent Design for Gastrointestinal Leakage

Stents are largely used to counteract and relieve duct or vessel obstructions. The purpose of this current study is to select a biodegradable, antibacterial and environmentally friendly material to design and prepare a stent to counteract anastomotic leakage following gastrointestinal surgery. A precast moulding process was used to prepare medium molecular weight chitosan scaffolds, for stent design. Chitosan samples were prepared by opening the bonds of the material in acetic acid, moulding the material and curing the mouldings in sodium hydroxide. The film like mouldings were wound around a metallic rod, fixing the layers together with un-cured chitosan and subsequently curing the chitosan layers together with sodium hydroxide producing the stent like shape. Tensile and tensile creep strength of the chitosan material was investigated. This research concluded that chitosan can be moulded into a stent like geometry. Mechanical testing showed the tensile and tensile creep strength were repeatable and could be predetermined. This indicates that chitosan is a suitable material for the further development of a biodegradable stent designed to counteract gastrointestinal leakage.

Issues Concerning the Measurement of Transformation Temperatures in NiTi Alloys

The transformation between martensite and austenite is characterized by four transformation temperatures: martensite start temperature (Ms), martensite finish temperature (Mf), austenite start temperature (As) and austenite finish temperature (Af). In actuator design that relies on the shape memory effect, it is important to obtain an accurate measure of these transformation temperatures, especially As and Af. Several methods of determining these temperatures have been reported, but their accuracy and coherence are not clear. Three methods were used to measure the transition temperatures of NiTi wire under different heat treatment conditions: differential scanning calorimetry (DSC); an electrical resistance method, which uses a sudden change in resistance as an indication of transformation; and an applied loading method, where a macroscopic change in displacement indicates the transformation. The results show that the transition temperatures measured by DSC do not correspond to those measured by the other two methods, which are similar. The applied loading method is the most effective for providing practical information about the stress-dependent transformation temperatures. The electrical resistance test gives clearly determined points for Ms and Mf in the cooling resistance-temperature curve, but As and Af are not clearly identifiable in heating process.

Disinfection of clinical materials using photo-activated tolonium chloride solution

Photo-activated disinfection is beginning to be used in dental surgery to treat deep seated bacterial infection. It works by combining a photosensitiser and light of a specific frequency to generate singlet oxygen which is toxic to many types of bacteria. It is suggested that this technique could be used as a means to help treat infection more generally. To do so, it needs to work with materials and geometries exhibiting different physical and optical characteristics to teeth. In these trials, samples of stainless steel and polymethylmethacrylate were exposed to bacterial solutions of Staphylococcus aureus and Staphylococcus epidermis. These were treated with tolonium chloride-based photo-activated disinfection regimes showing positive results with typically 4 log10 reductions in colony forming units. Tests were also carried out using slotted samples to represent geometric features which might be found on implants. These tests, showed disinfectant effect however to a much lesser degree.

Disinfection of Surgical Materials Using Photo-Activated Tolonium Chloride

Photo-activated disinfection is a technique which has recently become available for use in disinfection in dentistry. This technique makes use of the dye toluidine blue O coupled to a low powered laser to generate singlet oxygen which kills the bacteria without harming the patient. In this paper trials were performed to try to investigate the possibility of the technique for use in other medical applications, particularly related to implants, and orthopedic fixtures. In the trials, samples of stainless steel and polymethylmethacrylate (PMMA) were exposed to Staphylococcus Aureus and Staphylococcus Epidermis bacteria before being treated with a variety of irradiation exposures. Results found that if laser exposure energy exceeded 4.25 J/cm2 then a log reduction of 4 times CFU/cm2 might be expected. Results were similar for both the highly reflective stainless steel and transparent PMMA. Trials were also performed using light emitting diode technology and while these results were less effective, it is felt that the rapid rate of developments in this technology could lead to a cost effective route to rolling photo-activated disinfection out to many more medical applications.