In vitro and in vivo corrosion measurements of magnesium alloys.

Machining of Composite Materials

Cutting edge geometries

Surface integrity of machined components has a critical impact on their performance. Magnesium alloys are lightweight materials used in the transportation industry and are also emerging as a potential material for biodegradable medical implants. Surface integrity factors, such as grain size, crystallographic orientation and residual stress, have been proved to remarkably influence the functional performance of magnesium alloys, including corrosion/wear resistance and fatigue life. In this study, the influence of dry and cryogenic machining (liquid nitrogen was sprayed on the machined surface during machining) using different cutting edge radius tools on surface integrity was investigated. Compared with the initial material, cryogenic machining when using a large edge radius tool led to enhanced surface integrity in terms of the following: (1) improved surface finish; (2) significant grain refinement from 12 μm to 31 nm in the featureless surface layer; (3) large intensity of (0002) basal plane on the machined surface; (4) 10 times larger compressive areas in residual stress profiles; these changes should notably improve the functional performance of machined AZ31B Mg alloy. In addition to the frequently reported benefits on tool life, this study suggests that cryogenic machining may also enhance the surface integrity of the workpiece and improve the performance of machined components.

Enhanced surface integrity of AZ31B Mg alloy by cryogenic machining towards improved functional performance of machined components

Magnesium-Calcium (Mg-Ca) alloy has received considerable attention as an emerging biodegradable implant material in orthopedic fixation applications. The biodegradable Mg-Ca alloys avoid stress shielding and secondary surgery inherent with permanent metallic implant materials. They also provide sufficient mechanical strength in load carrying applications as opposed to biopolymers. However, the key issue facing a biodegradable Mg-Ca implant is the fast corrosion in the human body environment. The ability to adjust degradation rate of Mg-Ca alloys is critical for the successful development of biodegradable orthopedic implants. This paper focuses on the functions and requirements of bone implants and critical issues of current implant biomaterials. Microstructures and mechanical properties of Mg-Ca alloys, and the unique properties of novel magnesium-calcium implant materials have been reviewed. Various manufacturing techniques to process Mg-Ca based alloys have been analyzed regarding their impacts on implant performance. Corrosion performance of Mg-Ca alloys processed by different manufacturing techniques was compared. In addition, the societal and economical impacts of developing biodegradable orthopedic implants have been emphasized.

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Biodegradable Orthopedic Magnesium-Calcium (MgCa) Alloys, Processing, and Corrosion Performance

Magnesium is the lightest metal used in construction and therefore offers the greatest potential for weight reduction. The automotive industry in particular has an upcoming interest in the use of magnesium alloys. To observe the interactions between workpiece material and tool material and coating, respectively, turning experiments were carried out machining the alloy AZ91 HP. When machining magnesium dry, flank build-up due to adhesion between cutting tool and workpiece can occur at cutting speeds of vc = 900 m/min and more for uncoated and TiN-coated cemented carbide tools. Tools with either polycrystalline diamond (PCD) insert or coating can be used to reduce friction and adhesion in the tool-workpiece contact resulting in low machining forces as well as a superior surface quality even at high cutting speeds of vc = 2400 m/min. PCD coatings can also be applied to complex tool geometries. As tool wear can hardly be observed when machining magnesium alloys, a magnesium-based metal matrix composite (MMC) is machined to combine the adhesive effects of magnesium with the abrasive load caused by a reinforcement component. Excessive tool wear can be observed for TiN-coated carbides even at low cutting speeds of fvc = 100 m/min. PCD coatings show better results but cannot compete with tools having a PCD insert if a low film thickness is chosen.

The influence of tool coatings in machining of magnesium

Diamond coatings are mainly used in cutting processes due to their tribological characteristics. They show a high hardness, a low friction coefficient, high wear resistance and a good chemical inertness. In relation to polycrystalline diamond (PCD)-tipped cutting inserts especially the advantageous chemical stability of diamond coatings is superior as no binder phases between diamond grains are used. However, the deposition of adherent high quality diamond coatings has been found difficult. Thus, substrate pretreatment is utilized to improve film adhesion. This investigation is based on water peening of the substrate material before coating. The investigation revealed best results for diamond film adhesion on pretreated substrates compared to conventional diamond coatings on cemented carbide tools applied with the CVD hot-filament process. In final cutting tests with increased film adhesion water-peened cutting tools an improved wear behavior was detected.

Surface modification of cemented carbide cutting tools for improved adhesion of diamond coatings

WITHDRAWN: Mechanical pretreatment for improved adhesion of diamond coatings

Improving the Characteristics of Magnesium Workpieces by Burnishing Operations

Diamond coatings are mainly used in cutting processes due to their tribological characteristics. They show a high hardness, low friction coefficient, high wear resistance and good chemical inertness. In relation to polycrystalline diamond (PCD)-tipped cutting inserts, especially the advantageous chemical stability of diamond coatings is superior as no binder phases between diamond grains are used. However, the deposition of adherent high-quality diamond coatings has been found difficult. Thus, substrate pretreatment is utilised to improve film adhesion. This investigation is based on water peening of the substrate material before coating. The investigation revealed best results for diamond film adhesion on pretreated substrates compared to conventional diamond coatings on cemented carbide tools applied with the CVD hot-filament process. In final cutting tests with increased film adhesion trough water peened cutting tools an improved wear behavior was detected.

J. Winkler

Papers

The influence of tool coatings in machining of magnesium

Surface modification of cemented carbide cutting tools for improved adhesion of diamond coatings

WITHDRAWN: Mechanical pretreatment for improved adhesion of diamond coatings

Improving the Characteristics of Magnesium Workpieces by Burnishing Operations

Mechanical pretreatment for improved adhesion of diamond coatings