Proceedings of the 5th International Conference on Integrity-Reliability-Failure, Porto/Portugal 24-28 July 2016 Editors J.F. Silva Gomes and S.A. Meguid Publ. INEGI/FEUP (2016)

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PAPER REF: 6289

FRACTURE ANALYSIS OF THE ZK60A MAGNESIUM ALLOY DUE

TO HIGH DEFORMATION CAUSED BY EXTRUSION Gilmar Cordeiro

(*), Pedro Paiva Brito, Thais Campos, Fernanda Machado

Polytechnic Institute of PUC-Minas (IPUC), Pontifical Catholic Univ. of Minas Gerais Belo Horizonte, Brazil (*)Email: gilmarcordeiro@pucminas.br

ABSTRACT

This study aims the analysis of fractures arising from magnesium ZK60A alloy extrusion process, when subjected to deformation of 20 to 40%. It is intended to demonstrate the result of such process with samples, expressed in the brittle-type fracture presentation, called cleavage, being necessary to clarify that they were analyzed with the aid of scanning electron microscopy (SEM), featuring his formation.

Keywords: Fracture, deformation, extrusion, magnesium alloy, cleavage.

INTRODUCTION

Magnesium and its alloys are used in industry in view of the importance of their characteristics, such as low density (1.8 g/cm3 compared to 2.7 g/cm3 aluminum). Furthermore, it is emphasized that magnesium-based alloys present good machinability and can be easily merged to obtain parts with complex geometries (FROES et al, 1998).

On the other hand, magnesium alloys have limitations on their application because of low plasticity at room temperature cause by the Hexagonal Close-Packed (HCP) structure, which is characterized by having compact planes, i.e. juxtaposed planes of atoms, wherein each of the atoms of a plan fits into the depression left by another subsequent atoms in the stacking order, so that every atom of a plane supports itself and touches in three atoms of the adjacent plane (BUTTON 2011).

The HCP structure presents four independent slip systems, composed of basal and prismatic planes. However, slip systems of pyramidal plans are not considered independent, since they are crystallographically equivalent to slips with deviation (cross-slip) generated by combining a basal with prismatic plane (ZARANDI, et al. 2007). Thus, the limited number of independent slip systems restricts the plastic deformation of Mg-alloys, especially in cold deformation, because they do not satisfy the criteria of Von Misses, which emphasizes that the homogeneous deformations without fracture may happen if each crystal work with a minimum of five independent slip systems, activating and facilitating the slippage of the crystal planes (Yoo, 2002).

Recent studies were performed, which aim at the improvement of mechanical properties of the magnesium alloys through the use of conventional forming processes, such as drawing and extrusion at room temperature followed by, in some cases, recrystallization annealing between 250ºC and 300C (CHAO et al, 2011; LIANG et al, 2008). In this sense, the increase

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of mechanical properties is justified by the plastic deformation that the material undergoes because of these processes and the resultant structural changes.

Extrusion is one of the most widely applied forming processes, which consists in forcing the passage of a workpiece through a die by using a piercing punch, causing material to flow and producing a reduction in diameter. The plastic deformation associated with this process causes an increase in mechanical strength due to strain-hardening.

In the present work, ZK60A Mg-alloys were submitted elevated deformation rates (between 20 and 40%) by direct extrusion at room temperature, and the tested materials were submitted to fractography analysis with the objective of identifying deformation and failure mechanisms during the extrusion process.

METHODOLOGY

The materials used in the extrusion tests were cylindrical bars of Mg-alloy ZK60A-T5 with an initial diameter of 25.6 mm. The chemical composition of the alloys, as determined by optical emission spectroscopy, is presented in Table 1.

Table 1 - Chemical composition of the ZK60A alloy used in the present study (wt.%)

Ag Mn Ca Ni Zr Zn Na Ce Mg P

0.002 0.011 0.007 0.003 0.691 5.774 0.001 0.026 Bal. 0.001

Source: The author

The direct extrusion procedures were carried out using an extrusion die developed in-house which consist of five components: casing, main bushing, punch, interchangeable die and interchangeable bushing. The material used for building the die was 1045 carbon steel; its geometry and main dimensions is presented in Figure 1.

The interchangeable dies were produced in 4340 alloy steel according to the desired reductions: 25.6 to 23 mm, 25.6 to 22 mm and 25.6 to 21 mm, which correspond, respectively, to 19.28, 26.15 and 32.71% cold work (CW). The surface of the dies was ground with 1200 grit SiC paper in or der to reduce friction between the steel and Mg-alloy surfaces. The geometry and dimensions of one of the dies is presented in Figure 2.

From the as-received ZK60A Mg-alloy (25.6 mm diameter), four 50 mm long specimens were sectioned. The face of the specimens was prepared by conventional turning in order to guarantee parallelism between the sample cross section and the piercing punch, thus assuring homogeneity in extrusion forces. The samples were extruded in the as-received (T5) heat treated condition.

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Fig. 1 - Die and punch set used for the extrusion process (Source: The author)

Fig. 2 - Interchangeable die used for the extrusion operations ((Source: The author)

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The direct extrusion operations were performed at room temperature in a 2-stage hydraulic press (LIMA, QUIRINO and FARIA 2012; ARAÚJO, PERES and PETRUCCI, 2012). The extrusion load (F) was determined by block method, following the equations below (HELMAN and CETLIN, 2005):

� � � �� �� ���

(1)

� � 1 � �� �1 �� �����

(2)

Where Y is the average yield strength materials (kgf / mm2), do initial diameter (mm), and the d final diameter (mm) and B is a extruder factor calculated according to equation (3 )

B = µ cotg α (3) After extrusion, the fracture surface of the tested specimens was analyzed by Scanning Electron Microscopy (SEM).

RESULTS AND DISCUSSION

The general appearence of the Mg-alloy materials submitted to elevated strains (between 20 and 40%) is presented in Figure 3. The results are characteristic for all tested specimens (19.28, 26.15 and 32.71%CW).

Fig. 3 - Macroscopic fracture surface of the ZK60A alloy after extrusion , 26,15%CW (Source: The author)

By analyzing Figure 3, it can be noticed that the fracture surfaces show cleavage signs, which denote brittle fracture. Brittle fracture occurs normally in high strength metals and alloys, but also in low ductility materials, such as the present Mg-alloy, which exhibits a limited number of slip systems. Generally, brittle fractures are caused by impact but not overload. Thus, when cleavage fracture occurs, the surface of each fractured grain is smooth and distinctly oriented, revealing a crystal-like appearance (ASKELAND, 2008).

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Another feature of brittle fracture is the V-pattern, produced by different crack propagation paths. A characteristic surface pattern spreads outwards from the crack initiation site, as exemplified in Figures 4 and 5. In the specific case of Figure 5, the crack initiation site took place at the center of the extruded bar. This is an interesting observation because, supposedly, the surface regions should be subject to a higher Von Mises stress state due to the contact with the die surface, which causes local shear as well as compressive (normal to extrusion direction) and tensile stresses (parallel to extrusion direction) (CETLIN et al., 2003).

Fig. 4 - Characteristic V-pattern indicating brittle fracture with the crack initiation site at the center of the

extruded bar. (Source: The author)

Fig. 5 - Schematic V-pattern showing crack initiation site and propagation direction. (Source: The author)

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From a microstructure perspective, brittle fracture is characterized by the presence of surface features known as river marks, which originate from confluence of cleavage facets, which develop along parallel plains. During crack propagation, these facets are generated at the interception of a spiral dislocation. In a crystalline material, dislocations can be organized in low energy structures known as sub-grain boundaries (tilt or twist boundaries). When a cleavage crack meets such a boundary, the nucleation of facets may take place. Subsequently, with the progression of the fracture process, the facets coalesce generating the river-like pattern. This effect could be identified by SEM analysis, as seen in Figure 6. Another microstructural feature present in the extruded ZK60A alloy is the formation of shell-like surface patterns, with smooth facets, as seen in Figure 7.

Fig. 6 - SEM micrograph showing river marks characteristic of brittle fracture of the extruded ZK60A Mg-alloy (x 30 magnifications). (Source: The author)

Fig. 7 - SEM micrograph showing brittle fracture of the extruded ZK60A alloy (x 30 magnifications). (Source: The author)

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CONCLUSIONS

In the present work the failure modes associated with elevated deformation values (20 to 40%) in the extrusion of ZK60A Mg-alloys was evaluated. Due to their low ductility, the materials exhibited brittle fracture during extrusion, which could be identified both by macroscopic and microscopic examination. From a macroscopic perspective, the main identifiable brittle fracture characteristic was the formation of the V-pattern indicating crack initiation site and propagation directions. Microscopic examination of the fractured surfaces further corroborated brittle fracture, with the presence river-marks. The fractography analysis indicated that cracks were initiated at the center of the extruded bars and propagated outwards radially to the surface

ACKNOWLEDGMENTS

The present research project was funded by the Pontifical Catholic University of Minas Gerais, Project number 152235/2015-9. The authors also acknowledge funding from CNPQ–Conselho Nacional de Desenvolvimento Científico e Tecnológico.

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