Premature failure analysis of forged cold back-up roll in a continuous tandem mill

In this paper, premature failure of a forged back-up roll from a continuous tandem mill was investigated. Microstructural evolutions of the spalled specimen and surface of the roll were characterized by optical microscopy, X-ray diffraction, scanning electron microscopy and ferritscopy, while hardness value of the specimen was measured by Vickers hardness testing. The results revealed that the presence of pore and MnS inclusion with spherical and oval morphologies were the main contributing factors responsible for the poor life of the back-up roll. In addition, metal pick up and subsequently strip welding on the surface of the work roll were found as the major causes of failure in work roll which led to spalling occurrence in the back-up roll. Furthermore, relatively high percentage of retained austenite, say 9%, in outer surface of the back-up roll contributed spalling due to conversion of this meta-stable phase to martensite and creation of volume expansion on the outer surface through work hardening during mill campaign

Carbon nanotubes (CNTs)-reinforced magnesium-based matrix composites: A comprehensive review

In recent years considerable attention has been attracted to magnesium because of its light weight, high specific strength, and ease of recycling. Because of the growing demand for lightweight materials in aerospace, medical and automotive industries, magnesium-based metal matrix nanocomposites (MMNCs) reinforced with ceramic nanometer-sized particles, graphene nanoplatelets (GNPs) or carbon nanotubes (CNTs) were developed. CNTs have excellent material characteristics like low density, high tensile strength, high ratio of surface-to-volume, and high thermal conductivity that makes them attractive to use as reinforcements to fabricate high-performance, and high-strength metal-matrix composites (MMCs). Reinforcing magnesium (Mg) using small amounts of CNTs can improve the mechanical and physical properties in the fabricated lightweight and high-performance nanocomposite. Nevertheless, the incorporation of CNTs into a Mg-based matrix faces some challenges, and a uniform distribution is dependent on the parameters of the fabricating process. The characteristics of a CNTs reinforced composite are related to the uniform distribution, weight percent, and length of the CNTs, as well as the interfacial bonding and alignment between CNTs reinforcement and the Mg-based matrix. In this review article, the recent findings in the fabricating methods, characterization of the composite’s properties, and application of Mg-based composites reinforced with CNTs are studied. These include the strategies of fabricating CNT-reinforced Mg-based composites, mechanical responses, and corrosion behaviors. The present review aims to investigate and conclude the most relevant studies conducted in the field of Mg/CNTs composites. Strategies to conquer complicated challenges are suggested and potential fields of Mg/CNTs composites as upcoming structural material regarding functional requirements in aerospace, medical and automotive industries are particularly presented

A comprehensive review on surface modifications of biodegradable magnesium-based implant alloy: polymer coatings opportunities and challenges

The development of biodegradable implants is certainly intriguing, and magnesium and its alloys are considered significant among the various biodegradable materials. Nevertheless, the fast degradation, the generation of a significant amount of hydrogen gas, and the escalation in the pH value of the body solution are significant barriers to their use as an implant material. The appropriate approach is able to solve this issue, resulting in a decrease the rate of Mg degradation, which can be accomplished by alloying, surface adjustment, and mechanical treatment. Surface modification is a practical option because it not only improves corrosion resistance but also prepares a treated surface to improve bone regeneration and cell attachment. Metal coatings, ceramic coatings, and permanent polymers were shown to minimize degradation rates, but inflammation and foreign body responses were also suggested. In contrast to permanent materials, the bioabsorbable polymers normally show the desired biocompatibility. In order to improve the performance of drugs, they are generally encapsulated in biodegradable polymers. This study summarized the most recent advancements in manufacturing polymeric coatings on Mg alloys. The related corrosion resistance enhancement strategies and future potentials are discussed. Ultimately, the major challenges and difficulties are presented with aim of the development of polymer-coated Mg-based implant materials

MWCNTs-TiO2 incorporated-Mg composites to improve the mechanical, corrosion and biological characteristics for use in biomedical fields

This study attempts to synthesize MgZn/TiO2-MWCNTs composites with varying TiO2-MWCNT concentrations using mechanical alloying and a semi-powder metallurgy process coupled with spark plasma sintering. It also aims to investigate the mechanical, corrosion, and antibacterial properties of these composites. When compared to the MgZn composite, the microhardness and compressive strength of the MgZn/TiO2-MWCNTs composites were enhanced to 79 HV and 269 MPa, respectively. The results of cell culture and viability experiments revealed that incorporating TiO2-MWCNTs increased osteoblast proliferation and attachment and enhanced the biocompatibility of the TiO2-MWCNTs nanocomposite. It was observed that the corrosion resistance of the Mg-based composite was improved and the corrosion rate was reduced to about 2.1 mm/y with the addition of 10 wt% TiO2-1 wt% MWCNTs. In vitro testing for up to 14 days revealed a reduced degradation rate following the incorporation of TiO2-MWCNTs reinforcement into a MgZn matrix alloy. Antibacterial evaluations revealed that the composite had antibacterial activity, with an inhibition zone of 3.7 mm against Staphylococcus aureus. The MgZn/TiO2-MWCNTs composite structure has great potential for use in orthopedic fracture fixation devices

Recent trends in three-dimensional bioinks based on alginate for biomedical applications

Three-dimensional (3D) bioprinting is an appealing and revolutionary manufacturing approach for the accurate placement of biologics, such as living cells and extracellular matrix (ECM) components, in the form of a 3D hierarchical structure to fabricate synthetic multicellular tissues. Many synthetic and natural polymers are applied as cell printing bioinks. One of them, alginate (Alg), is an inexpensive biomaterial that is among the most examined hydrogel materials intended for vascular, cartilage, and bone tissue printing. It has also been studied pertaining to the liver, kidney, and skin, due to its excellent cell response and flexible gelation preparation through divalent ions including calcium. Nevertheless, Alg hydrogels possess certain negative aspects, including weak mechanical characteristics, poor printability, poor structural stability, and poor cell attachment, which may restrict its usage along with the 3D printing approach to prepare artificial tissue. In this review paper, we prepare the accessible materials to be able to encourage and boost new Alg-based bioink formulations with superior characteristics for upcoming purposes in drug delivery systems. Moreover, the major outcomes are discussed, and the outstanding concerns regarding this area and the scope for upcoming examination are outlined

Enhancement of corrosion resistance of magnesium by alloying, fluoride treatment and nano-hydroxyapatite coating for biomedical applications

Compared to the traditional metallic implant materials such as stainless steels, titanium alloys and cobalt chromium alloys, magnesium (Mg) has received great attention as biodegradable medical implants as it does not require second surgical procedure for removal. Mg and its alloys also possess suitable mechanical properties for orthopaedic and cardiovascular applications. However, clinical applications of Mg have been limited due to its relatively poor corrosion resistance, rapid degradation rate and hydrogen gas evolution in human body fluid. This research is aimed at decreasing the Mg degradation and corrosion rate by alloying with calcium (Ca) and zinc (Zn), surface treatment by hydrofluoric acid and coating with nanosized hydroxyapatite (HA) and brushite (DCPD) using electrodeposition method. The first stage of the research is to enhance the corrosion resistance of pure Mg by the addition of Ca (0.5 to 10 wt.%). In the second stage, Zn at different percentages (0.5 to 9 wt.%) was added to the binary Mg-Ca alloy to further enhance the corrosion properties. Both strategies were found to enhance the corrosion resistance of the alloy, however, the effect was not significant. To further enhance the corrosion resistance fluoride treatment by using different concentrations of hydrofluoric acid (35 and 40%) for the duration of 6 to 24 hrs were employed on binary Mg-Ca and ternary Mg-Ca-Zn alloys. Finally, nano-HA and Brushite were coated on the fluoride-treated specimens via electrochemical deposition (ED) method at different voltages (0.15 to 0.8 mA/cm2) and deposition times (10 to 60 min). Microstructural evolutions were characterized by XRD, AFM, FTIR, SEM, and TEM. Corrosion resistance was examined by potentiodynamic polarization and immersion test in Kokubo solution at room temperature. The results revealed that the grain size and dendrite cell size decreased with the addition of Ca and Zn contents into the binary and ternary alloys respectively. The addition of 0.5 wt.% Ca content was found to produce the lowest dissolution rate and the highest corrosion resistance. However, further addition of Ca led to an increased dissolution rate and pH value. The corrosion resistance of Mg-0.5Ca alloy was enhanced with the addition of up to 1 wt.% Zn, but further addition produced the reverse effect. Mg-0.5Ca-lZn alloy, which has ll-Mg+Ca2Mg6Zn3+M~Ca phases showed lower corrosion rate than those alloys with Zn/Ca atomic ratio higher than 1.23. After fluoride treatment the degradation rates of the alloys were significantly reduced compared to the untreated alloys. Electrochemical tests showed a significant decline in corrosion current density from 365.2 to 5.23 !!A1cm2 on Mg-0.5Ca-lZn alloys coated with composite nano-HAlMgF2. The application of composite coating of nano-Ha/Mgfe, on Mg-CaZn alloys could be used to reduce the corrosion rates ofMg alloys for biodegradable medical applications

Synthesis and kinetic study of (Mo,W)Si ²-WSi ² nanocomposite by mechanical alloying

In this study, nanocomposite of (Mo,W)Si2–WSi2 was synthesized via mechanical alloying (MA) and heat treatment. The phase transformation of the powders after various milling durations and annealing was investigated by X-ray diffraction (XRD) and differential thermal analysis (DTA). Microstructural evolutions were characterized by scanning electron microscopy and transmission electron microscopy (TEM). Increasing the milling time to 80 h caused the formation of (Mo, W, Si) solid solution, t-(Mo,W)Si2, h-WSi2 phase, and a trace amount of unreacted raw material. However the post-annealing at 1000 °C caused the complete formation of (Mo,W)Si2–WSi2 nanocomposite. The values of the grain growth exponent of t-(Mo,W)Si2 phase for the powders milled for 40 and 80 h were 0.3 and 0.8, respectively, at 1000 °C. The grain growth activation energy of t-(Mo,W)Si2 phase for the 80 h milled powders (97.19 KJ/mol) was lower than that for the 40 h sample (120.83 KJ/mol). The crystallite size of t-(Mo,W)Si2 decreased to 32 nm (40 h) and 24 nm (80 h) with increasing milling time. However, the crystallite size of the milled samples increased to 60 and 87 nm after annealing at 1000 °C for 90 min. The DTA results of the as-milled specimens showed two exothermic peaks at around 600 and 900 °C relating to the formation of t-(Mo,W)Si2 and h-WSi2, respectively. The formation activation energy of t-(Mo,W)Si2 was higher (144.58 KJ/mol) for the 80 h milled sample compared to the 40 h milled sample (131.61 KJ/mol). The microhardness of (Mo,W)Si2–WSi2 nanocomposite increased with increasing milling time to 1020 Hv but decreased with escalating annealing temperature to 726 Hv

In-vitro biocompatibility, bioactivity, and mechanical strength of PMMA-PCL polymer containing fluorapatite and graphene oxide bone cements

In this study, a bone cement consisting of poly methyl methacrylate (PMMA)–poly caprolactone (PCL)–fluorapatite (FA)–graphene oxide (GO) was synthesized as bone filler for application in orthopedic surgeries. The FA and GO particulates were homogenously distributed in the PMMA-PCL polymer matrix and no defects and agglomeration were found in the PMMA-PCL/FA/GO bone cement. The in-vitro bioactivity result exhibited that addition of FA and GO to the polymer cement (PMMA-PCL) improved the apatite formation ability on the surface of polymer. The results also showed that addition of FA to the polymer bone cement escalated the compressive strength and elastic modulus while reducing elongation to 8 ± 2%. However, after addition of GO into the PMMA-PCL/FA bone cement, both compressive strength and elongation considerably increased to 101 ± 5 MPa and 35 ± 6%, respectively. Furthermore, tensile tests exhibited that inclusion of GO was favorable in improving the tensile modulus, UTS and elongation of the PMMA-PCL/FA bone cement. The cytotoxicity test pointed out that MG63 osteoblast cells viability increased to 279 ± 15% after addition of FA and GO to the PMMA-PCL polymer bone cement. The DAPI (4′,6-diamidino-2-phenylindole) staining demonstrated better spreading and attachment of MG63 cells on PMMA-PCL/FA/GO surface compared to the PMMA-PCL bone cements. These results confirm the suitable mechanical properties and favorable bioactivity along with high cells viability of PMMA-PCL/FA/GO bone cement, indicating its potentials for orthopedic applications

Reduced graphene oxide (RGO) reinforced Mg biocomposites for use as orthopedic applications: Mechanical properties, cytocompatibility and antibacterial activity

Magnesium (Mg) has attracted wide interest in orthopedic applications as they exhibit great biodegradability and strong biocompatibility, while corrosion is the main concern for Mg that should be addressed prior to biomedical applications. In this work, ZM31 (Mg-3Zn-1Mn)/xRGO (x = 0, 0.5, 1 and 1.5 wt%) biocomposites were synthesized by semi-powder metallurgy method. The results showed that the RGO acting as an effective reinforcing filler to prevent deformation and showed better compressive strength (282.3 ± 9 MPa) and revealed enhancement in failure Strain (7.8 ± 2.1%) at 1 wt% RGO concentration compared to Mg alloy (244.5 ± 9 MPa and 7.1 ± 1.5% respectively). Moreover, fracture analysis indicated a more ductile fracture of the nanocomposites after the incorporation of RGO. Crack bridging, crack deflection and crack branching are dominant mechanisms for reinforcement of Mg-based containing RGO. Mg composites containing 0.5 wt% RGO showed a low corrosion rate (2.75 mm/year), while more incorporation of RGO resulted in an increased corrosion rate (4.38 mm/year). In addition, the degradation rate of ZM31 alloy (2.57 mg·cm−2·d−1) obviously decreased with the addition of 0.5 wt% RGO (1.84 mg·cm−2·d−1) in the SBF. Besides, continuous apatite layers were created on the composites in the SBF solution. Also, the cell culture examinations showed good cell viability and adhesion on composites with 0.5 and 1 wt% RGO, which was demonstrated by the SEM and MTT assay The alkaline phosphatase (ALP) activity of the ZM3–0.5RGO composite was considerably higher than that of ZM31 matrix alloy in 24 h and 48 h, implying higher osteoblastic differentiation. The antibacterial behavior toward both bacteria (E. coli and S. aureus) exhibited that escalating RGO concentration in Mg-matrix composites leads to further inhibition of bacteria growth. These findings suggested that ZM31–0.5RGO biocomposite could be a more promising candidate for orthopedic implants

Effect of fluoride conversion coating on corrosion of behavior of mg-ca-zn alloy

Magnesium and its alloys have been received huge attention as new kind of degradable biomaterials. However its application hindered by poor carrion resistance fluoride conversion coating was performed due to improve the corrosion resistance of Mg-Ca-Zn alloy. In the present work ccorrosion of behaviour and degradation bahaviour of fluoride treated Mg-Ca-Zn alloy were investigated. Microstructural evolutions were characterized by scanning electron microscopy and energy dispersive x-ray spectroscopy. The corrosion resistance was examined in vitro by potentiodynamic polarization and immersion test in Kokubo solution at room temperature. The coating characterization indicated that the dense and uniform film with 6 µm thickness consists of MgO and MgF2 formed on the alloy. Polarization tests recorded a significant reduction in the corrosion current density from 188 µAcm-2 in bare Mg-Ca-Zn to 6.11 µAcm-2 in fluoride treated alloy as a result of formation MgF2 protective layer. The in vitro degradation tests showed that the average weight loss of the untreated specimens significantly higher than that of fluoride treated Mg-Ca-Zn alloy. The results revealed that the fluoride conversion coating noticeably improve the corrosion resistance of Mg-Ca-Zn alloy resistance of Mg in Kokubo solution