42 research outputs found
Microstructural, texture, plastic anisotropy and superplasticity development of ZK60 alloy during equal channel angular extrusion processing
No full textIn this study, equal channel angular pressing (ECAP) was exploited to refine the grain size of a ZK60 magnesium alloy in multi-processing steps, namely at temperatures of 250 ̊C, 200 ̊C and 150 ̊C, producing an ultrafine-grained (UFG) structure. The microstructural development and texture evolution during ECAP were systemically investigated by electron backscattered diffraction (EBSD) analysis. The microstructure of the ECAP processed alloy was remarkably refined to an average grain size of 600 nm. During ECAP process the original fiber texture of the as-extruded alloy was gradually weakened and eventually replaced by a stronger texture component coinciding with ECAP shear plane. The ECAP processed material showed a proper balance of tensile as well as compression strength and tensile ductility at room temperature. Yield strength of 273 and 253 MPa in tension and compression, respectively, ultimate tensile strength of 298 MPa and fracture elongation of about 30% were obtained in the UFG alloy. A transition from ductile–brittle to ductile fracture consisting of very fine and equiaxed dimples was also found in the ECAP processed material. Compared to the as-received alloy, a combination of grain refinement and texture development in the UFG alloy gave rise to a notable reduction in mechanical asymmetric behavior at room temperature. The superplastic behavior of the as-extruded and ECAP processed alloy was also investigated at 200 ̊C with strain rate of 1.0×10-3 s-1.
The concurrent effect of grain boundary sliding and favorable basal texture in the UFG alloy led to an achievement of elongation value of about 300% while, under similar testing conditions, the elongation of about 140% was obtained in the as-extruded alloy
Ultra-fine grained degradable magnesium for biomedical applications
No full textProperties of commercially available purity magnesium and wrought ZM21 Mg alloy were investigated in view of their
biodegradable applications. In particular, the opportunities offered by grain size refinement down to the ultra-fine scale achieved by
equal channel angular pressing (ECAP) and warm extrusion were discussed and material properties were analyzed. Results show that
the grain refinement will lead to a significant improvement in compression strength. The tension strength of the coarse grained alloy
is always significantly higher than that measured in compression due to the sharp texture of the starting wrought alloy. ECAP also
causes an attenuation of the above texture effects, promoting marked changes in plastic flow behavior. The corrosion behavior of the
investigated materials are affected by a combination of microstructural effects such as chemistry, grain size and the extent of lattice
distortion inherited from previous processing stages. ECAP leads to refinement of grain size and to increased lattice defect density
which apparently produce counterbalancing effects on corrosion performance. The improved dispersion of second-phase particles
brings positive effects on development of pitting
Formability enhancement of Al 6060 sheets through fiber laser heat treatment
No full textDue to the continuous weight reduction effort in the automotive sector, formability enhancement of aluminum alloys in forming and hydroforming processes is gathering much attention from research institutes and industries. During sheet forming processes, large deformations are desired to obtain complex shapes but these are limited by the appearance of defects such as wrinkling and cracks. To avoid these issues, intermediate annealing heat treatments are often applied as a possible solution. Nevertheless in large components where small details have to be created, local heat treatment through lasers can be cost effective over the furnace treatment of the whole part and it would limit possible geometrical distortion in large components. The following article presents fiber laser process parameters definition on deformed sheets made of Al6060 alloy. Grain structure variation and hardness decrease were studied to correctly select process parameters (laser power, feeding speed and overlapping among subsequent passes) to increase material formability. In addition, a systematic comparison between fiber laser and furnace heat treatment was assessed proving the equivalence of the two methods in terms of achieved mechanical proprieties
Grain size and texture dependence on mechanical properties, asymmetric behavior and low temperature superplasticity of ZK60 Mg alloy
No full textThe mechanical properties of ZK60 alloy were characterized as function of texture and grain size. An experimental methodology was designed by exploiting ECAP and annealing treatments on an extruded alloy, through which the individual effects of the mentioned parameters could be investigated. Microstructural observations revealed a significant grain refinement through ECAP processing, leading to an equiaxed ultrafine grain (UFG) structure with an average size of 500 nm. The initial extrusion fiber texture was gradually evolved into a new and more intense one featuring the preferential alignment of the basal planes along the ECAP shear planes with considerably higher intensity and Schmid factor. After annealing, the UFG structure was replaced by a structure close to that of the extruded condition. However, the basal texture orientation was only slightly modified. Tensile and compression tests at room temperature showed that regardless of grain size, fracture elongation and yield asymmetry were strongly influenced by basal texture orientation. Nevertheless, tensile tests at 200 degrees C showed that the flow stress was considerably texture dependent, whereas the fracture elongation was mainly dictated by the grain size
Microstructure, mechanical behavior and low temperature superplasticity of ECAP processed ZM21 Mg alloy
No full textIn this study, ultra-fine grained ZM21 Mg alloy was obtained through two-stage equal channel angular pressing process (ECAP) at temperatures of 200 and 150 degrees C. For each stage four passes were used. Plastic behavior, mechanical asymmetry and low temperature superplasticity of ultra-fine grained ZM21 alloy were investigated as a function of processing condition with particular attention to microstructural and texture evolution. Microstructural observations showed that after the first stage of ECAP an equiaxed ultra-fine grain (UFG) structure with average size of 700 nm was obtained. Additional stage did not cause any further grain refinement. However, Electron Backscattered Diffraction analysis showed that the original extrusion fiber texture evolved into a new one featuring a favorable alignment of the basal planes along ECAP shear planes. Such a preferential alignment provided a considerably higher Schmid factor value of 0.32, resulting in a remarkable loss in tensile yield stress, from 212 to 110 MPa and an improvement of the tensile fracture elongation, from 24% to 40%. Tensile and compression tests at room temperature revealed that yielding asymmetry could be alleviated by either weakening of basal plane fiber texture or by grain refinement. Tensile tests at 150 degrees C showed that texture supplies a significant contribution to plastic flow and elongation, making dislocation slip the dominant mechanism for deformation, while grain boundary sliding was not actively operated at this temperature. However, at 200 degrees C the effect of texture on fracture elongation of UFG alloys was subtle and the impact of grain size became more important. Hence, UFG samples exhibited maximum elongation values exceeding 370% at a strain rate of 5.0 x 10(-4) s(-1), confirming that the flow stress has notable texture dependence, while superplastic ductility was strongly influenced by grain size, being detectable only in UFG samples
Influence of ECAP process on mechanical and corrosion properties of pure Mg and ZK60 magnesium alloy for biodegradable stent applications
No full textEqual channel angular pressing (ECAP) was performed on ZK60 alloy and pure Mg in the temperature range 150-250 °C. A significant grain refinement was detected after ECAP, leading to an ultrafine grain size (UFG) and enhanced formability during extrusion process. Comparing to conventional coarse grained samples, fracture elongation of pure Mg and ZK60 alloy were significantly improved by 130% and 100%, respectively, while the tensile strength remained at high level. Extrusion was performed on ECAP processed billets to produce small tubes (with outer/inner diameter of 4/2.5 mm) as precursors for biodegradable stents. Studies on extruded tubes revealed that even after extrusion the microstructure and microhardness of the UFG ZK60 alloy were almost stable. Furthermore, pure Mg tubes showed an additional improvement in terms of grain refining and mechanical properties after extrusion. Electrochemical analyses and microstructural assessments after corrosion tests demonstrated two major influential factors in corrosion behavior of the investigated materials. The presence of Zn and Zr as alloying elements simultaneously increases the nobility by formation of a protective film and increase the local corrosion damage by amplifying the pitting development. ECAP treatment decreases the size of the second phase particles thus improving microstructure homogeneity, thereby decreasing the localized corrosion effects
Mechanical Properties of Extruded and ECAP Processed Magnesium Alloy AZ91 at Elevated Temperature
No full textIn this paper tensile properties at elevated temperature of extruded AZ91 magnesium
alloy and the same alloy further processed by ECAP (exECAP) are compared. The tensile tests were
performed at room temperature and for the temperature range of 100 to 300 °C. Loading speed
2 mm/min was used for the tests. At room temperature mechanical properties except elongation
were slightly higher for extruded material yet still very similar to properties of exECAPed material.
Overall trend of properties evolution with increasing temperature was also similar but the decrease
of strength or the increase of elongation and reduction of area respectively is more intensive for
exECAPed material. Elongation of exECAPed material exceeded elongation of extruded material
more than twice at 300 °C and with value of ~260% this alloy exhibited pseudosuperplastic
behavior
In Vitro Degradation of Absorbable Zinc Alloys in Artificial Urine
No full textAbsorbable metals have potential for making in-demand rigid temporary stents for the treatment of urinary tract obstruction, where polymers have reached their limits. In this work, in vitro degradation behavior of absorbable zinc alloys in artificial urine was studied using electrochemical methods and advanced surface characterization techniques with a comparison to a magnesium alloy. The results showed that pure zinc and its alloys (Zn–0.5Mg, Zn–1Mg, Zn–0.5Al) exhibited slower corrosion than pure magnesium and an Mg–2Zn–1Mn alloy. The corrosion layer was composed mostly of hydroxide, carbonate, and phosphate, without calcium content for the zinc group. Among all tested metals, the Zn–0.5Al alloy exhibited a uniform corrosion layer with low affinity with the ions in artificial urine
Zinc-based alloys for degradable vascular stent applications
No full textThe search for biodegradable metals with mechanical properties equal or higher to those of currently used permanent biomaterials, such as stainless steels, cobalt chromium and titanium alloys, desirable in vivo degradation rate and uniform corrosion is still an open challenge. Magnesium (Mg), iron (Fe) and zinc (Zn)-based alloys have been proposed as biodegradable metals for medical applications. Over the last two decades, extensive research has been done on Mg and Fe. Fe-based alloys show appropriate mechanical properties, but their degradation rate is an order of magnitude below the benchmark value. In comparison, alongside the insufficient mechanical performance of most of its alloys, Mg degradation rate has proven to be too high in a physiological environment and corrosion is rarely uniform. During the last few years, Zn alloys have been explored by the biomedical community as potential materials for bioabsorbable vascular stents due to their tolerable corrosion rates and tunable mechanical properties. This review summarizes recent progress made in developing Zn alloys for vascular stenting application. Novel Zn alloys are discussed regarding their microstructural characteristics, mechanical properties, corrosion behavior and in vivo performance
Processability of pure Zn and pure Fe by SLM for biodegradable metallic implant manufacturing
No full textPurpose
This work investigates the processability by selective laser melting of materials of potential interest for innovative biodegradable implants, pure Fe and pure Zn. The processability of these materials is evaluated with a more established counterpart in permanent implants, stainless steel. In particular, the processing conditions were studied to reduce porosity due to incomplete fusion of the powder.
Design/methodology/approach
In the first phase of the experiments, SLM of AISI 316L was studied through design of experiments method. The study was used to identify the significant parameters in the experimental range and estimate the fluence ranges for pure Fe and pure Zn using the lumped heat capacity model. In the second phase, SLM of pure Fe and pure Zn were studied using estimated fluence ranges. In the final phase, best conditions were characterized for mechanical properties.
Findings
The results showed that complete melting of AISI 316L and pure Fe could be readily achieved, whereas laser melting generated a foam-like porous structure in Zn samples. The mechanical properties of laser melt implant materials were compared to as-cast and rolled counterparts. Laser melted AISI 316L showed superior mechanical performance compared to as-cast and rolled material whereas Fe showed mechanical performance similar to rolled mild steel. Despite 12% apparent porosity, laser melted Zn exhibited superior mechanical properties compared to as cast and wrought material due to reduced grain size.
Originality/value
The paper provides key processing knowledge on the SLM processability of new biodegradable metals namely pure Fe, which has been studied sparingly; and on pure Zn, on which no previous work is available. The results prefigure the production of new biodegradable metallic implants with superior mechanical properties compared to their polymeric counterparts and with improved degradation rates compared to magnesium alloys, the reference material for biodegradable metals
