Basınçlı Döküm Yöntemiyle Üretilen Zn-al Alaşımlarının Mikroyapıları İle Mekanik Ve Korozyon Özelliklerine Alaşım Elementlerinin Etkisi

Abstract

Bu çalışmada basınçlı döküm yöntemiyle üretilen Çinko-alüminyum esaslı alaşımların mekanik ve korozyon özellikleriyle mikroyapılarına alüminyum, bakır, magnezyum ve manganez elementlerinin etkisi incelenmiştir. İncelenen alaşımlarda alüminyum %3.5- 9.6, bakır %0.05-2.7, magnezyum %0.02-1.9 ve manganez %0-l aralığında değiştirilmiştir. Alaşımların mekanik özellikleri sertlik, çekme ve darbe deneyleri ile belirlenmiştir. Alaşımların korozyon özellikleri kenar uzunluğu 7.5 mm olan küp şeklindeki numunelerin 1M HC1 ve 1N H2SO4 çözeltileri içinde 6 saat süreyle tutulmasıyla meydana gelen ağırlık kaybının ölçülmesiyle belirlenmiştir. Alaşımlarda alüminyum miktarının artmasıyla mikroyapı incelmekte, dendritlerin hacim oranı artmaktadır. Ayrıca artan alüminyum ile sertlik değeri artmakta, darbe direnci azalmaktadır. Alüminyum miktarının artışı alaşımların korozyon direncini olumsuz yönde etkilemiştir. Cu elementinin ilavesiyle alaşım sertliği artmakta olup %1.6'dan yüksek Cu içeriğine sahip alaşımların mikroyapılarında çinko-alüminyumca zengin fazların yanısıra CuZn4 bileşiğine rastlanılmaktadır. %1.6 Cu içeriğine kadar artan Cu miktarı ile tokluk düşmektedir. Ayrıca alaşımların korozyon direnci artan bakır içeriği ile azalmaktadır. Magnezyum elementinin ilavesi ile mikroyapıda Mg2Zn11 fazı oluşmaktadır. Bu fazın varlığı alaşımın sertlik değerini arttırırken tokluk ve süneklik değerlerini azaltmaktadır. Alaşımların magnezyum miktarının artması 1M HCl solüsyonu içersindeki korozyon davranışı üzerinde bir etki göstermemektedir, fakat aynı bileşimdeki magnezyum alaşımları ise 1N H2SO4 solüsyonu içersinde artan magnezyum miktarı ile korozyon direnci azalmaktadır. Çinko-alüminyum alaşımlarında manganez miktarının artmasıyla sertlik değeri fazla değişmemekte; tokluk ise düşmektedir. Korozyon direnci açısından manganez alaşımının 1M HC1 ve 1N H2SO4 solüsyonları içersindeki korozyon davranışı incelendiğinde artan manganez miktarı ile korozyon direncinin düştüğü görülmüştür.

The zinc pressure die casting alloys are mostly discovered in the 1920's. Alloying of zinc with aluminium, copper and other metals to improve the mechanical properties of alloys began early in this century. The older die-casting alloys with considerable amount of tin have been replaced with aluminium and copper alloys (ZA alloys), because of the poor mechanical properties of tin alloys. The alloys produced under the name of Zamak by the New Jersey Zinc Co. contain between 3.5-4. 1% aluminum Alloying of zinc with aluminium, copper and other metals is made for economical, decorative, functional and lower casting cost purposes. The first major alloys developed for the zinc industry were the pressure die-casting alloys Zamak 3 and Zamak 5. They have used for many application; which require the combination of mechanical and physical properties overall other types of pressure die-casting alloys. These alloys are used in a vast number of applications, ranging from the simplest drawer handle to the most complex precision automotive' parts. Their castability, fluidity casting specialities and having good mechanical properties are successfully used on many areas and hard working industries. Common usage areas of zinc- aluminum alloys are automobile parts, sewing machines and engine parts. Table 1. 1 shows the compositions of some Zamak alloys. In the recent years, the plastics-vs-zinc struggle was taking place so the zinc industry was developing a series of gravity cast alloys which could be produced in sand and permanent mould foundries to compete with aluminium, copper and ferrous metals. This case effects on production of zinc alloys family such as Zamak 3, Zamak 5, ZA 8, ZA 12, ZA 27, etc. ZA 27 alloy is being successfully produced by the cold chamber die cast process to produce components with high strength and high wear resistance. However, due to its low aluminium content, the other mechanical properties of these alloys are inferior to those of the conventional alloys, and they cannot be cast by the traditional hot chamber die-casting process, which severely limits their use on the market. The new developments of this sector are due to the contents of aluminium in zinc alloys (known as ZA foundry alloys ZA-8, ZA-12 and ZA-27) and the evolution of this new zinc-aluminium alloy family has a great importance in the industrial market. Their excellent mechanical and casting properties have led the ZA alloys being specified for and industrial applications and on many areas of castings.. This applies particularly to the die-casting sector where ZA alloys are being selected over traditional zinc and aluminium alloys to meet more stringent property requirements. Although originally developed as sand and gravity die-casting alloys, they can all be readily die cast in cold Xll chamber machines, and also being routinely pressure die-cast by the traditional hot chamber die casting process. In ZA alloy family, ZA 8 was the most popular alloy which has a lower melting point, and lower aluminium content. ZA 27 alloy has the highest strength and hardness followed by ZA-12 and ZA-8. These alloys (ZA 12, ZA 8)have also excellent tensile properties. In die casting condition, these mechanical properties of alloys raised over 50 % with respect to other casting methods. On the physical properties of die casting products are 30-45% higher than those of other casting methods. It is believed that these improvements are brought about principally by microstructural refinement resulting from cooling rate characteristic of the casting process. Table 1.1 Chemical compositions of commercial pressure die casting alloys according to theASTMBandB791. Elements Zamak 3 Zamak 5 ILZRO 16 Zamak 8 Zamak 12 Zamak 27 % % compounds Al 3.5-4.3 3.5-4.3 0.01-0.04 8.0-8.8 10.5-11.5 25-28 Cu 0.25 max. 0.75-1.25 0.8-1.3 0.5-1.2 2.0-2.5 Mg 0.02-0.05 0.03-0.08 0.02 0.015- 0.015-0.30 0.30 0.010-0.020 Ti 0.15-0.25 Cr 0.1-0.2 Xlll The aim of this study was to investigate the effects of alloying elements; (aluminium, copper, magnesium and manganese) on the mechanical, and corrosion behaviours of die casting zinc-aluminum alloys. The following tests were performed to determine the mechanical properties of the investigated alloys ;. Tensile Test. Hardness Test. Impact Test(Unnotched) In order to determine the tensile strength of the alloys tensile test specimens were prepared according to ASTM E 8. The dimensions of the specimens were of 16mm length and 4mm diameter. Tensile tests were performed on INSTRON 1 195 universal strength machine at the rate of 0.5mm/min. Hardness tests were performed on the HARDESS TESTER MISAWA SEIKI SEISOKUSHO All test were done under the weight of 60kg load, with a 1/16 steel ball indenter and measured under the scala of Rockwell F. Impacts tests are done according to of ASTM E 23. Impact tests specimens were unnotched with dimensions of 10x7.5x55mm. Experiments were performed on the Mohr V Feder Hoff AG MANNHEIM-Charpy Impact Test Equipment at the room temperature. Beside mechanical tests, microstructure of the cast alloys were examined by;. X-Ray Diffraction Technique. Metallgraphic Techique After grinding and polishing all the alloys were etched with %2 nital Microstructural examined were performed by utilizing OLYMPUS Optical Microscope X rays diffraction examinations were done in order to reveal the intermetalics that they have in their microstructure. All experiments were done on X ray machine Rigaku D-Max 100 with Coa radiation.. Corrosion Resistance Tests By cubic shape specimens, corrosion test specimens were prepared from the impact test specimens after impact tests having dimension 7.5x7.5x7.5mm. Tests were performed in the solutions of İM HC1 and IN H2S04 Specimens were divided into two groups as a cast and tempered. Tempering was done at 320 C for 6 hours. These two groups were immersed in the solutions of İM HC1 and İN H2SO4 for 6 hours and weight loss of specimens were determined. Among the Zn-Al alloys investigated, aluminium content varied between 3.5% Al to 9.6 % Al. It has been found that increase of aluminium content, increases the xiv hardness of the alloy. Among the aluminium series alloys 7.8 % Al alloy contains, has the highest ultimate tensile strength as 205.9 MPa. The microstructure of zinc-aluminum alloys consists of solids surrounded by eutectic areas. Increasing of aluminium content refines the microstructure. X-ray diffractions analysis revealed that aluminum series alloys contain zinc-aluminum rich phases in the microstructure. However in the alloy containing 4.7% aluminium and 0.15 % manganese, AlsMns phase is present. The addition copper to the zinc-aluniinum alloy in the range 0.05 % to 2.7% also effects the mechanical properties and corrosion resistance. The hardness increased and impact resistance decreases with increasing copper content. Microstructural examinations copper series revealed that dendrites were surrounded with eutectic areas. X-ray diffractions analysis showed that microstructure contain CuZn4 intermetallic phase which has a great effects on mechanical properties besides zinc and aluminum rich phases. In this study magnesium content of Zn-Al alloys were varied between 0.02%-1.9%. In this magnesium series alloys microstructures is consists of solids and eutectic areas. Increasing of magnesium content increases the dendrites sizes. X-ray diffraction analysis showed that addition of magnesium to zinc-aluminum alloys causes formation Mg2Zn11 in the microstructure. The addition of manganese in the ranges between 0 - 1 % did not have significant influence and effect on hardness. However, impact resistance of manganese content on zinc-aluminum alloys decreases with increased manganese content. In these manganese series alloys microstructures consists of dendrites and eutectic areas. Increase of manganese, decreases the eutectic areas. X-ray diffraction analysis revealed that AteMn or AteMns intermetallic phases are present in the microstructure. The corrosion behavior of investigating zinc-aluminum alloys were examined by immersing the cubic specimens (7.5 mm length) to 1M HC1 and 1N H2S04 for 6 hours. Corrosion tests were performed on as cast and heat treated( 320°C and 6 hours) specimens. In aluminum series alloys corrosion resistance decreases with increasing aluminum content. Applying heat treatment to the aluminum series of zinc-aluminium alloys increases the corrosion rates. In 1N H2S04 weight loss is greater than 1 M HC1. Among the zinc-aluminum alloys containing copper the corrosion rate of alloy in 1 M HC1 is higher than in 1N H2SO4 solution. Increasing of copper increases the corrosion rates of zinc- aluminum alloys similar behaviors has also been observed in tempered conditions. In magnesium series of zinc-aluminum alloys, corrosion rate is almost independent from magnesium content of the magnesium alloy. Corrosion rate was doubled in tempered specimens with respect to as cast specimens in both 1M HC1 and 1N H2SO4 solutions. However in the alloy contains 0.5 % magnesium, tempering decreases the weight loss with respect to as cast conditions in 1N H2SO4. xv As a last alloying element manganese, it is found that the increase of manganese content of as cast zinc-aluminum alloys increase the corrosion rates in both İM HC1 and İN H2SO4 solutions. Similar effect have been observed in tempered condition.