Anagnostis Toulfatzis

Fracture Behavior and Characterization of Lead-Free Brass Alloys for Machining Applications

The stricter environmental, health, and safety regulations address the harmful effects of lead and provide the driving force for the development of lead-free brass alloys. Conventional leaded brass rods are widely used in several manufacturing sectors (i.e., fabrication of hydraulic components, fittings, valves, etc.) due to their superior workability in extrusion and drawing as well as their superior machinability. As machinability performance involves shear and dynamic fracture processes evolved under high strain-rate conditions, the understanding of the mechanical behavior/microstructure interaction is critical in order to successfully tailor candidate lead-free alloys for improved machinability without compromising the reliability of manufactured components. In this work, the mechanical behavior under static and dynamic loading of three lead-free brass alloys (CW510L-CW511L-C27450) in comparison to a conventional leaded brass alloy (CW614N) was studied. The fractographic evaluation of the texture of conjugate fracture surfaces was performed to identify the involved fracture mechanisms and their relation to the alloy microstructure. It was shown that the CW510L lead-free brass alloy is a potential candidate in replacing conventional CW614N leaded brass, combining high tensile strength and fracture toughness, due to the prevalence of the β-intermetallic phase in the alloy microstructure.

Fracture Modes and Mechanical Characteristics of Machinable Brass Rods

Machinable brasses are a broad class of high strength copper–zinc alloys mainly containing lead to improve machinability. Conventional leaded brasses are widely used in several manufacturing sectors (i.e., fabrication of hydraulic components, fittings, valves, etc.) due to their superior workability in extrusion and drawing, together with their superior machinability for high efficiency production of final components in high speed/high precision machining centers. In addition to machinability, the mechanical behavior and general fracture mechanisms of these alloys are also important, due to their impact on the overall reliability and safety of brass components. In this study, the main fracture modes and mechanical characteristics of two industrial copper alloys, namely, CuZn39Pb3 and CuZn36Pb2As, are presented in relation to their microstructure. Optical metallography, macro- and microfractography, together with static and dynamic mechanical testing, were used as the principal analytical techniques for the present investigation.

Microstructure and properties of lead-free brasses using post-processing heat treatment cycles

We report the tailoring of the microstructure of lead-free brasses with a view to improving their machinability via the controlled augmentation of the β-phase. This should be performed while keeping the mechanical properties within the EN standard, as β-phase augmentation can lead to embrittlement. Twenty seven heat treatment protocols were performed in order to transform the microstructure and consequently modify the properties of three lead-free brasses. The most successful heat treatment was chosen for each studied alloy according to the above criteria. The selected protocols led to a combination of β-phase augmentation with mechanical properties conforming to standards providing a promising ground for improved machinability performance. This paper is part of a Themed Issue on Brass Alloys.

Analysis of the Degradation Process of Structural Steel Component Subjected to Prolonged Thermal Exposure

A steel frame installed in a metallurgical plant, subjected to prolonged temperature exposure, presented distortion, cracking, and oxidation at areas that could compromise its structural integrity. Further assessment of mechanical properties indicated an acute reduction of tensile strength up to 33%. Metallurgical examination revealed significant microstructural degradation involving cementite spheroidization and graphite formation due to carbon diffusion and coalescence that caused softening. Fractographic evaluation addressed the fracture mechanisms in damaged and unaffected areas, which supported the previously stated differences in mechanical properties, as a result of thermal deterioration processes occurred during service in industrial furnace conditions.

Mechanical behaviour and microstructure of heat-treated Cu–Ni–Si alloy

ABSTRACT Cu–Ni–Si alloys exhibit a good combination of strength and electrical conductivity and may be a potential candidate for utilisation in electrotechnical applications. In this work, the mechanical behaviour and its relation to the microstructure of a Cu–Ni–Si alloy, subjected to different solution heat treatment cycles, were investigated. Tensile, bend and hardness testing, in addition to Optical and Scanning Electron Microscopy, were employed, as the main analytical techniques, in the context of the present investigation. This paper is part of a Thematic Issue on Copper and its Alloys.

Failure and Fracture Analysis of Al-alloy Wheel Rim of a Vehicle

An Al-alloy wheel rim, showing a transverse and axial fracture was submitted for failure analysis investigation. A localized dent advocating the occurrence of intense plastic deformation was observed adjacent to the fracture area. In the frame of the present study, macro- and micro-fractographic analysis along with microstructural examination and mechanical testing were employed as the principal analytical methods for the investigation. Fractographic assessment indicated the presence of rough fracture surface topography, without signs of any subcritical or progressive cracking region, as suggestive evidence of fatigue. Conversely, the transgranular quasi-cleavage fracture together with minor shallow dimple terraces/tear ridges is indicative of low ductility overload fracture. Metallographic examination using SEM/EDS analysis revealed a typical as-cast structure with shrinkage cavities and intermetallic phases, pertaining to the Al-Fe-Si and Mg-Si constituents. No significant macro-defects were found that could be inculpated as the causative factors of the overload fracture of the wheel rim, rather than an impact incident that might have induced a high strain-rate loading crack propagation having resulted in the ultimate failure.

Effect of Natural Aging of 6xxx Series Extrusions on the Energy Absorbance Capacity

In this study the influence of natural aging in energy absorbance capacity of 6xxx series extruded profiles after artificial aging is examined by means of optical and scanning electron microscopy, quasi-static compression and tensile tests. Mobile quenched-in vacancies are found to play an important role in aging kinetics and formation of precipitate free zones (PFZs) which govern both the observed fracture modes and the resulting strength of the material signifying their important effect when maximum energy absorbance is desired. It is shown that fracture toughness of naturally aged samples is equivalent or higher compared to that of directly aged samples due to restriction of intergranular cracking.

Fundamental Aspects of Rolled Zn Alloy Sheet Formability: Structure-Property and Failure Mode Relationships

In the present work, critical testing methods are employed in order to assess the formability of a Zn-Ti-Cu alloy, evaluating, therefore, the anisotropic properties of the produced sheet. The determination of plastic strain ratios and the induced combined mathematical expressions, utilizing bi-axial strain measurements for the various test directions (0, 45 and 90 degrees towards the RD), together with the performance of cupping tests are compiled, aiming to rank and interpret the bending and sheet metal roll-forming capability. Moreover, the microstructural characterization is realized to address the influence of grain and phase structure on the sheet metal formability and identify potential optimization routes. Fracture analysis approach elucidated the micro-mechanisms prevailed in damage evolution and accumulation during monotonic loading, signifying the importance of microstructure development during thermomechanical process history.

Accelerated carbide tool wear failure during machining of hot work hardened tool steel

Purpose – The purpose of this paper is to outline and document the failure root cause of a carbide cutting tool during machining of a hardened tool steel under automatic machining conditions. Design/methodology/approach – Optical metallography and SEM/energy dispersive spectroscopy analysis, together with optical profilometry were employed for failure investigation. The use of an alternative cutting tool and modification of machining conditions are proposed as a failure preventive action. Findings – Severe abrasive wear and adhesion of machining chips are observed in the flank zone, causing blunting of the cutting edge. The revision of cutting conditions, together with the use CBN-based tool insert leads to an overall improvement of the stability of the process and tool lifetime. Originality/value – This paper places emphasis on a failure analysis case history following a structured approach in industrial machining problem solving, highlighting suggestions for process improvement.