Search

Magnesium alloys have gained interest in biomedical applications due to the mechanical properties (very similar to the human bones) and the capability to be absorbed (additional surgery steps after the implantation not necessary). But suitable manufacturing techniques are required for these alloys, since the standard ones (machining, casting) or even the newest (additive) cannot be successfully adopted in every situation or cannot guarantee the requirements in terms of precision or cost. In the present paper two sheet forming processes (the Super Plastic Forming and the Incremental Sheet Forming) have been investigated for manufacturing a resorbable cheekbone implant using the Mg alloy AZ31B. An experimental/numerical approach was adopted: specific tests were conducted for determining the material behaviour to be implemented in the process simulations used for designing both the manufacturing processes; finally some prototypes of the investigated case study were produced and the post forming characteristics (thickness, roughness, accuracy) evaluated. Both the processes allowed to successfully and optimally produce the resorbable prosthesis adopted as case study, which revealed to be characterised by good enough surface quality (especially in the case of the Super Plastic Forming) and uniform thickness distribution (especially in the case of the Incremental Sheet Forming).

In the last years, the precision and personalized medicine is pushing the biomedical research efforts towards the direction of implant surgery requiring only 1-step approach: this goal has been achieved after the introduction of resorbable implants. The resorbable prosthetic support is indicated for temporary prosthetic applications, such as bone fractures fixation, or all those conditions usually treated with metal implants then removed with a second surgery, just after the healing of the bone defect. Biodegradable, bioactive and customizable implants for the treatment of bone fractures, both efficient in bearing the functional loads, and showing good biocompatibility and degradation properties matching the bone tissue healing, are still lacking. These premises have led to consider Magnesium (Mg) and its alloys as very promising candidates for the development of temporary, resorbable implants. However, the very high corrosion rate of Mg is the main problem, not yet solved. The material needs to be properly treated/coated, as well as manufactured, in order to design the most suitable duration of the temporary prosthesis permanence in situ. An innovative and interdisciplinary approach has been developed within the M.Era-Net ISIDE project and it is here briefly detailed with a special focus on the highlighted application fields.

Magnetorheological Fluids (MRFs) are included in the so called “smart materials”: they are suspensions of magnetically responsive particles in a liquid carrier, whose rheological behaviour (e.g., its viscosity) can be changed quickly and reversibly if subjected to a magnetic field. Their application as forming medium in sheet metal forming processes is gaining interests in the recent years since the thickness and the strain distribution on the formed part can be affected by properly changing the properties of the MRF. In order to widely adopt MRFs in such processes, the evaluation of their rheological behaviour according to the applied magnetic field plays a key role. But there are still few works in the literature about the most effective way to characterise the MRFs to be used in sheet metal forming applications.In this work, the rheological behaviour of a MRF is carried out by means of an inverse analysis approach using data from bulge tests performed using an MRF as forming medium. Bulge tests were conducted on sheets having known properties using an equipment with a solenoid to generate the magnetic field, which was specifically designed and manufactured. Pressure rate and magnetic flux density were varied according to a Design of Experiments (DoE) while the strain experienced by the sheet material was acquired by means of a Digital Image Correlation (DIC) system in order to compare it with the numerical one. In particular, the fitting between numerical and experimental data was obtained by changing the MRF’s rheological properties using an inverse analysis technique. The proposed methodology allows to evaluate the MRF behaviour at different levels of both magnetic field and pressure rate, which are determinant for the FE simulation of sheet metal forming processes.

The hydraulic bulge test represents an effective experimental method to characterise sheet metals since the equivalent strains before failure are much larger than those measured during tensile testing and there is nearly no frictional effect on the results. Recently this test has been proposed not only for extracting data concerning the equi-biaxial strain condition, but to determine the forming limit diagram (FLD) in the range of positive minor strains. In the proposed methodology, different strain paths can be obtained by merely using a test blank having two holes with a suitable geometry and position to be tested, without the need of dies with elliptical apertures. However, a carrier sheet is necessary, thus implying results may be affected by friction effects. This paper proposes a new methodology for the determination of the right side of the Forming Limit Curve (FLC), based on the adoption of local heat treatments aimed at determining different strain paths on the blank to be tested while using the classical circular die for bulge tests. In particular, the formability of the alloy AA5754-H32 was investigated; 3D Finite Element simulations were conducted setting different laser strategies and monitoring the resulting strain path. Results revealed that the proposed methodology supports obtaining many additional points in the right side of the FLC, thus being effective and friction free.

Main problems related to the adoption of magnesium alloys for temporary orthopedic prostheses manufacturing are (i) the need of an efficient production process and (ii) the high corrosion rate compared with the bone healing time. In this work, the single-point incremental forming (SPIF) process, an effective and flexible solution for manufacturing very small batches even composed by one piece, was investigated. Tests were conducted on AZ31B-H24 sheets and were aimed at understanding the effect of temperature on the mechanical characteristics (microstructure, hardness, and roughness) of the sheet after the above-mentioned forming process and their correlation with both the corrosion rate and the cytocompatibility. In addition, after the forming process, samples processed by SPIF were coated by electrospun polycaprolactone (PCL) to reduce the corrosion rate and to further improve the cytocompatibility. Grain refinement was achieved thanks to the combined effect of temperature and strain rate during forming and finer grain size resulted to improve the magnesium corrosion resistance. In simulated body fluids, the electrospun PCL-coated samples exhibited a slower pH increase compared with uncoated samples. No indirect cytotoxic effects were detected in vitro for MC3T3-E1 cells for both coated and uncoated samples. However, cells colonization was observed only on electrospun PCL-coated samples, suggesting the importance of the polymeric coating in promoting the adhesion and survival of seeded MC3T3-E1 cells on the implant surface.

Single Point Incremental Forming (SPIF) is a technology that allows producing highly customized products using conventional computer numerical control machines (CNC). Magnesium is a biocompatible and biodegradable metal with mechanical properties close to those of cortical bones: it can be a viable solution for producing temporary biomedical prostheses, avoiding a second surgery to remove it. Its major drawback is its poor formability at room temperature and low corrosion resistance. The first problem can be solved working at temperatures higher than the room one; the second, by coating the Mg substrate with a higher corrosion resistant material. In this work, Mg alloy sheets (AZ31B) were formed by SPIF at elevated spindle speeds (in order to locally heat the part by friction); in addition, a thin layer of PCL was deposited by electrospinning on the SPIFed parts. A porous coating was thus obtained: this morphology resembles the extracellular matrix of the body and promotes cell growth for tissue healing or even the electrospun fibers of PCL could be drug releaser for infection reduction. Cytotoxicity tests performed in this work revealed that oxides produced by the SPIF process delay the cytotoxic effect of the as received Mg alloy, while the presence of the PCL coating had a positive effect on the corrosion of the investigated Mg alloy