Machinability aspects of heat-treated Al-(6-11)%Si cast alloys : role of intermetallics and free-cutting elements

Le besoin de combler le vide entre les procédés de coulé et d'usinage donne de bonnes raisons d'examiner les nombreux aspects affectant l'usinabilité des alliages de fonderie Al-Si. Les alliages quasi-eutectiques sont, parmi les alliages Al-Si, les plus difficiles à usiner, puisque les particules de la phase Si sont environ 10 fois plus dures que la matrice d'aluminium, lesquelles expliquent pourquoi les outils de coupe s'usent prématurément. Toutes ces difficultés nécessitent une meilleure compréhension des effets de la microstructure sur l'usinabilité de ces alliages. Ce travail a été mené dans le but d'étudier un nouvel alliage expérimental appartenant au groupe des alliages de fonderie Al-Si quasi-eutectique contenant environ 10,8%Si, à savoir l'alliage 396. Suite à ce qui a été soulevé, l'objectif principal de se travail est de rapporter les changements des critères d'usinage résultant des effets des intermétalliques de fer, à savoir a-Fe, /?-Fe et « sludge »; de deux niveaux de Cu, à savoir 2,25 et 3,5%; et de deux niveaux de Mg, à savoir 0,3 et 0,6%. De plus, les effets des alliages sans Mg et modifiés au Sr ont également été étudiés en plus des effets des éléments de décolletage tels que Sn, Bi et Pb. Le traitement thermique T6 a été sélectionné pour établir le niveau de dureté des alliages étudiés à l'intérieur d'une plage de 110± 10 BHN, conforme à la plupart des niveaux de dureté pour les applications commerciales des alliages d'aluminium. La mesure de la dureté a été faite directement sur les blocs d'usinage pour assurer que les échantillons possèdent le niveau de dureté requis. Tous ces alliages ont également été testés mécaniquement de façon à obtenir une compréhension des effets des additifs sur les propriétés mécaniques de tractions pour les mêmes conditions appliquées aux blocs de test d'usinage. Les tests d'usinage ont été faits sur une machine d'usinage horizontale haute vitesse Makino A88E sous des conditions fixes lesquelles incluent la vitesse de coupe, la vitesse d'avance, la longueur de la coupe, la géométrie de l'outil, le matériau de l'outil ainsi que le liquide de refroidissement. Les critères d'usinage observés sont les forces et les moments total de coupe, la durée de vie de l'outil en termes de nombre de trous percés ou taraudés jusqu'au bris de l'outil, la morphologie des copeaux et l'arête rapporté (BUE). Les résultats démontrent que la présence de « sludge » sous la forme de points durs a un effet sur la force de coupe et la durée de vie de l'outil qui est réduite de moitié par rapport à l'alliage de base. La formation de la phase a-Fe dans l'alliage Ml a un effet bénéfique sur la durée de vie de l'outil, ainsi cet alliage est celui qui donne le plus grand nombre de trous percés comparativement aux alliages contenant le « sludge » ou /?-Fe; ces résultats pourraient être expliqués par le fait que la formation des intermétalliques a-Fe avec leur morphologie de scripts chinois arrondis et de leur présence à l'intérieur des dendrites a-Al améliore l'homogénéité de la matrice par un durcissement des dendrites. L'augmentation du fer de 0,5% à 1% dans l'alliage 396-T6 contenant 0,5% Mn produit une amélioration distincte de l'usinabilité en termes de force de coupe et de durée de vie de l'outil. Lors des tests de taraudage, il a été trouvé que les outils en acier rapide sont considérablement plus sensibles aux phases intermétalliques de fer que les outils en carbure11 utilisés pour le perçage L'ajout de Fe ou de Mn semblent avoir aucun effet sur l'arête rapporté (BUE) et sur la morphologie des copaux comparativement à l'alliage de base. L'augmentation des niveaux de Cu ou de Mg dans l'alliage 396-T6 ont tous des effets nuisibles sur la durée de vie du foret. Cette réduction de la durée de vie du foret pourrait être attribuée à la formation d'une grande quantité de blocs de la phase A^Cu et à la formation de plaques épaisses de la phase Al-Si-Cu-Mg. L'alliage expérimental sans Mg affiche les plus faibles force et moment de coupe en plus de produire le plus grand nombre de trous de tous les alliages étudiés. Cette observation pourrait être expliquée par une précipitation combinée des phases durcissantes AI2CU, Mg2Si, Al2CuMg et AlsSiôQ^Mgg dans les alliages contenant du Mg lesquels confèrent une plus grande résistance à l'alliage que la précipitation seule de la phase AI2CU de l'alliage sans Mg. Une comparaison entre l'alliage modifié et non-modifié (contenant les mêmes niveaux de Mg et de Cu) en terme de nombre de trous percés, révèle que la morphologie des particules de Si a un effet sur la durée de vie de l'outil. L'ajout de petites, mais efficaces, quantités d'éléments de décolletages aux alliages de fonderie Al-Si améliore considérablement l'usinabilité de ces derniers. L'alliage contenant du Sn a un effet sur la durée de vie des forets en carbure et des tarauds en acier rapide. D'un autre côté, les alliages contenant du Bi mènent à un grossissement des particules de Si eutectique résultant à une détérioration de la durée de vie de l'outil. L'ajout simultané d'une petite quantité de deux ou de plusieurs éléments insolubles dans l'aluminium a un plus grand effet sur l'usinabilité en termes de réduction de la force et du moment de coupe que les ajouts individuels de chaque élément. L'ajout de Pb, Bi et Sn semble n'avoir aucun effet sur la formation de l'arête rapporté (BUE) ou sur la morphologie des copeaux excepté que l'alliage contenant du Bi montre un légère tendance à réduire la formation de l'arête rapporté (BUE) et il produit également des copeaux en forme d'éventail plus petit que ceux observés pour les alliage sans Bi. Un examen visuel des copeaux révèle que la forme d'éventail est de loin la forme prédominante pendant le perçage, de plus elle est considérée comme la forme idéale pour beaucoup d'application de perçage. La fragmentation des copeaux des alliages contenant la phase AbCu était supérieure à celle des alliages contenant la phase Mg2Si. Ainsi, l'addition combinée de Cu et de Mg devrait raffiner davantage la taille des copeaux produits. L'examen des forets usés a montré que le maximum d'usure prend place au coin extérieur de l'arête du foret, alors qu'un minimum d'usure se produit à, ou près de, la pointe du foret. Lorsque les coins du foret sont arrondis, le foret colle à la pièce et se brise si le procédé de coupe n'est pas arrêté à temps. Pour les tests de taraudage, le principal mécanisme d'usure observé est l'adhésion, même si une certaine abrasion pourrait se produire lors du taraudage des alliages contenant le « sludge » et le Bi. La rupture se produit fréquemment dans la patrie chanfreinée du taraud puisqu'elle génère une majeure partie de la force résultante, en raison de la plus grande section de copeaux apparentée aux dents du chanfrein

Candida albicans and Napkin Dermatitis: Relationship and Lesion Severity Correlation

Introduction: Napkin Dermatitis (ND) is a common problem in infancy that affects almost every child during the early months and years of their lifetime. It is a skin disease that becomes a challenge for both parents and physicians because of its frequency and difficulty in eliminating all of the causative factors in diapered infants. Usually Napkin dermatitis is self-limiting but when associated with Candida albicans (C. albicans) seems to be moderate to severe.Aim: The aim of the present study was to determine the colonization of C. albicans in children with Napkin dermatitis and to correlate between intensity of C. albicans colonization and the severity of napkin rash.Patients and Methods: This case-controlled study was conducted at Qassim University pediatric outpatient clinics, during the period from August 2014 to July 2015. Sixty patients with diaper dermatitis and 33 healthy controls were enrolled to this study. Sociodemographic and clinical data were obtained from the parents of each participant using questionnaires Paired (stool and skin) samples were collected from all cases and healthy control children. The samples were cultured on differential and selective chromogenic medium for isolation and initial identification of candida species. Identification confirmation of the isolates was determined by the Vitek 2 compact automated system.Results: Diaper dermatitis shows significant outcome to washing diaper area (per day) (P=0.001), History of diarrhea last 7 Days (P˂0.001), skin lab results (+/-) for Candida albicans, (P˂0.001), skin colony count, (P˂0.001), However, there is no correlation to age (P=0.828), gender (P=0.368) and feeding style (P=0.401).Conclusion: The severity score of napkin dermatitis was significantly observed among cases with diaper dermatitis (p-value<0.001) and control children (p-value<0.001) respectively.Keywords: Candida albicans; Napkin dermatitis; Diaper dermatitis; Vitek 2 compact system; Qassim

Applications of Rare Earth Metals in Al-Si Cast Alloys

The present article reviews a large number of research publications on the effect of mischmetal (MM), rare earth metals (RE), La or Ce, and combinations of La + Ce on the performance of Al-Si cast alloys mainly 319, 356, 380, 413, and 390 alloys. Most of these articles focused on the use of rare earth metals as a substitute for strontium (Sr) as a eutectic silicon (Si) modifier if added in low percentage ( 1 wt.%) is required to achieve a noticeable reduction in grain size, however at the cost of alloy brittleness

Effect of tool geometry and welding speed on mechanical properties of dissimilar AA2198–AA2024 FSWed joint

International audienceDifferent tool geometries were used to investigate the joining of aluminum alloys (AA2198 to AA2024) by friction stir welding (FSW). Three shoulder profiles (flat, raised spiral, and raised fan) and five different pin profiles (cone, half threaded cylindrical, straight cylindrical, tapered cylindrical and square) were selected. Preliminary investigations were conducted by moving the tool into a seamless sheet made of the AA2024-T3 in order to select the tools that produce defect-free joints. Preliminary investigations showed the raised fan shoulder profile helps the material flow from the edge of shoulder to the pin creating a smooth surface finish with no flash in comparison with flat and raised spiral shoulder profiles. Pins with a minimum diameter equal to half the plate thickness produced lack of penetration (LOP) defects, while increasing minimum pin diameter to the plate thickness eliminates the LOP defects. Half threaded cylindrical pin produced tunneling defect, whereas defect free joint made by straight cylindrical, tapered cylindrical and cubic pin profiles. So they were selected for joining AA2024 to AA2198. Fracture locations of different joint variants were observed the vicinity of the thermomechanical affected zone (TMAZ) of AA2198-T3 alloy, and in the nugget on the AA2198-T3 side which have the minimum hardness and highest strain localization as confirmed by hardness maps and digital image coronation (DIC). Higher measured temperature than dissolution temperature of AA2198 main strengthening precipitates could be the reason of low hardness and fracture in TMAZ and center of nugget. Furthermore a raised fan shoulder with a tapered cylindrical pin produced highest elongation and yield strength and it was selected as the best candidate for optimization of the welding parameters. It was found that higher rotational and traverse speeds enhance the formation of tunneling and kissing bond defects, suggesting that longer pins have to be used for higher traverse speeds. Welding speed 750 rpm with 450 mm min−1 could create joint with highest yield strength

The Influence of Tool Geometry on Mechanical Properties of Friction Stir Welded AA-2024 and AA-2198 Joints

International audienceThe tool geometry in friction stir welding (FSW) is a critical parameter to produce reliable and consistent joints, especially when it comes to joining dissimilar alloys with different mechanical and thermal properties. The aim of the present work is to investigate the effect of FSW tool design on the mechanical characteristic of as-welded AA2198(T3)-AA2024(T3) joints. Three shoulder profiles (flat, spiral, and fan) and five different pin profiles (tapered cylindrical, straight cylindrical, threaded cylindrical, cone and square) were tested. A visual inspection and metallographic characterization were conducted to evaluate the sound state of the joint. Tensile tests and scanning electron microscopy combined with energy-dispersive X-ray spectroscopy were used to assess the mechanical properties of the different joints in regard to fractographic analyses and local chemical composition. Significant flash was produced for flat shoulder configurations. FSW tools with fan or with spiral shoulders prevent the formation of the flash defect. Lack of penetration (LOP) and kissing bond defects were avoided by tapering the tip of the pin with a diameter greater than or equal to the plate thickness. The tapered cylindrical pin with a fan shoulder was the optimal tool design configuration for mechanical properties

Effect of heat treatments on microstructural and mechanical characteristics of dissimilar friction stir welded 2198/2024 aluminum alloys

International audienceThe 3rd generation of aluminum–lithium (Al–Li) alloys provides a desirable combination of high mechanical properties and low density compared to their traditional counterparts, exempt of lithium. Therefore, providing a reliable joint between new and conventional aluminum alloys is crucial for making hybrid structures. The present study focuses on improving the mechanical properties of dissimilar AA2198/AA2024 joints using different heat treatments before and after welding. Tensile tests paired with digital image correlation (DIC) techniques and micro-hardness maps were performed to document the macro-scale and local mechanical behavior of the joints. As-welded joints demonstrated a similar yield strength, 30% lower than that of the base metals in T3 and T8 metallurgical states. As-welded joints failed at the AA2198 side in the heat affected zone (HAZ), parallel to the thermo-mechanically affected zone (TMAZ), region experiencing intense strain concentration and minimal hardness values. Post welding-heat treatments (PWHT) was found to successfully strengthen HAZ on the AA2198 side, without abnormal grain growth in the nugget and impairing the hardness properties on AA2024 side. This improvement in local mechanical properties on the AA2198 side was related to the re-precipitation of dissolved T1 (Al2CuLi) and θ (Al2Cu) during welding as characterized by differential scanning calorimetry (DSC) and microscopy analyses. However, PWHT joint variants demonstrated a reduction in total elongation and ultimate tensile strength due to intense strain localization on the AA2024 retreating side compared to a much more homogeneous strain distribution in the as-welded joints

Analysis of AA2XXX/AA7XXX friction stir welds

International audienceComparable microstructures and mechanical properties are obtained for the different AA2139-T8/AA7020-T651 joints welded in the range 600–1000 rev. min−1 / 250–550 mm.min−1 as rotation and advance speeds. Joints exhibit tensile properties as good as 77%–79% of the yield strength and 88%–96% of the ultimate tensile strength of the AA7020 base material. Except for the high speed conditions that failed in the nugget, all joint variants failed in the heat affected zone (HAZ), which is also consistent with lower hardness properties. This drop in hardness in the HAZ is produced by reaching and keeping for a short time a critical temperature during welding between 225 and 260 °C. Such a combination of dissimilar alloys offers conservative mechanical properties for a large range of welding parameters suitable for the assembly purpose of high-performance structural components

Effect of Aging Treatment on the Strength and Microstructure of 7075-Based Alloys Containing 2% Li and/or 0.12% Sc

The present study was performed on three versions of 7075 alloy to which Sc or Sc + Li was added. The alloys were subjected to various aging treatments. The microhardness results show that the highest value of hardness was achieved when the alloy containing Li + Sc was aged at 120 °C for 24 h whereas the minimum level was exhibited by the base alloy aged at 280 °C. The results were interpreted in terms of the size and distribution of the main hardening phase (η′(MgZn2)), and the role of the presence of Al and Cu in the used alloy. Precipitation of Al3(Sc, Zr, Ti) phase particles during solidification of the Sc-containing ingots was also discussed. The coarsening and spheroidi-zation of η-phase particles take place through the Ostwald ripening mechanism while smaller par-ticles in solution dissolve and deposit on larger particles. In Sc-containing alloys, star phase particle consists of different layers. The change in the brightness from layer to layer indicates that the Zr and Sc concentrations are varied within the star phase, since the atomic number of Zr (40) is higher than the atomic number of Sc (21). The addition of Sc, as well, leads to marked decrease in the grain size of the as-cast alloys i.e., 300 µm and 45 µm, respectively. The interaction between Li and Sc would reduce the effectiveness of the grain refining effect of Sc. The results of the refining effect of Sc were confirmed using the EBSD technique

Assessment of the Influence of Additives on the Mechanical Properties and Machinability of Al-11%Si Cast Alloys: Application of DOE and ANOVA Methods

In the present study, the statistical design of experiments (DOE) method was applied to study and control the properties of near-eutectic Al-11%Si alloys. In this study, we developed regression equations between response variables, including hardness, yield stress, ultimate tensile stress, elongation, total cutting force, cutting power, and tool life, and varying factors which included the percentage of the alloying element in the composition and the modification level. These equations may be analyzed quantitatively to acquire an understating of the effects of the main variables and their interactions on the mechanical behavior and the machinability of the alloy under investigation. Analysis of variance (ANOVA) was performed to verify the fit and adequacy of the developed mathematical models. The results show that increasing the levels of Cu and Fe results in an increase in hardness, yield stress and ultimate tensile strength in both modified and non-modified alloys. On the other hand, both Cu and Fe appear to affect the elongation adversely, whereas the Sr level shows a positive effect on the elongation percentage. We found that the Sr level had the most significant effect on the cutting forces and cutting power, followed by Fe and Cu contents