Effect of Alloying Elements on the Mechanical Properties & Thermal Expansion of LM 13 Piston Alloy.

Transcription

1 METAL , Hradec nad Moravicí Effect of Alloying Elements on the Mechanical Properties & Thermal Expansion of LM 13 Piston Alloy. Eng.M.Tadayon saidi 1,Dr.N.Varahram 2, Eng.N.Baghersaiee 3 1) Dept. of Metallurgy-Karaj Azad University-Karaj,Iran. 2) Associate professor-dept.of Met.Science & Eng.-Sharif University of Technology- Tehran,Iran. 3) Engineering Research Institute, Tehran,Iran. Abstract: The effect of alloying elements on the mechanical properties &thermal expansion of LM13 casting alloy- usually used as piston alloy-, which has low thermal expansion &very good high temperature mechanical properties and good wear resistance,was investigated. The mechanical properties & thermal expansion characteristic of samples with different combination of alloying elements, which enhance to reduce the cost of products, have been studies. The results show remarkable increase in the mechanical properties with increasing the amount of Cu & Mg,while are reduced with increasing amount of Ni. On the other hand,the use of eutectic aluminum alloy as a piston alloy-with higher percent of Cu & Mg and lower percent of Ni are recommended for industry,due to economical viewpoint and satisfactory properties. Introduction: E.Mail: Mehrantadayonsaidi@ yahoo.com 2 - E.Mail: naser sharif.edu. 3 - E.Mail:n_saie@yahoo.com.

2 METAL , Hradec nad Moravicí Aluminum silicon casting alloys are the most versatile of all common foundry cast alloys in the production of pistons for automotive engines. Aluminum, alloyed with copper, mangnesium, nickel and silicon are common piston alloys which is used today. Silicon is the major element added to the aluminum. Aluminum silicon alloys used as pistons alloy fall into three major categories: Eutectic,hypoeutectic & hypereutectic.the point of saturation of silicon in the aluminum is known as a eutectic and occurs when silicon level reaches 12%.aluminum with silicon levels below 12% are known as a hypoeutectic and aluminum with higher silicon levels are know as hypereutectic. However, commercial application for hypereutectic alloys are relatively limited because of the difficulties for casting and machining due to the high Si contents. On the other hand, the usage of hypoeutectic and eutectic alloys are very popular for the industry, because they are more economical to produce by casting, simpler to control the cast parameter easier to machining than hypereutectic alloys, however most of them are not suitable for application,such as automotive piston due to their low mechanical properties at desired temperature range of º. Hypoeutectic and eutectic alloys are intended for application at temperature not higher than about 24 ºc. The major alloying elements that are added to Al piston alloys are copper, nickel & magnesium. Copper. The enhanced high strength at high temperature will be affected if the copper wt% level is not suitable, copper also forms Ǿ intermetallic phase (Al 2 Cu). Magnesium: Using proper addition of magnesium relative to copper and silicon cause the simultaneous formation both of the Ǿ & δ intermetallic phase. Nickel. Nickel improves tensile strength at elevated temperature by forming Al-Cu-Ni intermetallic compounds. The pistons are subjected to extreme forces, frictional wear, and high temperature. Moreover the shape and the size of the piston greatly affects the performance of engines, thus the piston should satisfy following properties: The piston must have high fatigue strength, high proof strength and high hardness at high temperatures and constructed form a material which has a low density and thermal expansion. Further, the piston material must provide high strength and hardness, and must be yieldable if the piston is to be forged. Heat treatment. Heat treatment is a process in which metals are alternately heated & cooled according to a selected time and temperature to improve the characteristics of the metal such as machinability, hardenability, stabilizing casting dimensions and relieves stress. The general heat treatment of aluminum alloys is precipitation strengthening of supersaturated solid solution which involves the formation of finely dispersed precipitates during aging heat treatment, the general procedure of heat treatment is: 1. Solution. 2. Quenching. 3. Age hardening. 2

3 METAL , Hradec nad Moravicí Experimental procedure: LM 13 alloy with different percent of alloying element were cast into the cylindrical permanent mold with diameter of 2 mm. Chemical composition of the cast samples is shown in table 1.Samples were heat treated by solution treatment at 55ºc for ten hours and after quenching in hot water at 7-8 ºc were subjected to aging process at 2 for 6 hours and then air cooled. For determining the expansion coefficient of samples,round samples with diameters of 1 mm and height of 2 mm were machined and both end were carefully grinded. The specimen was fixed into a G.F dilatometer with capability of measuring the expansion of the sample with accuracy of.1 mm. The specimens were hold at 5,1,15,2 & 3 ºc and correspondence liner expansion was measured directly via dial gauge of instrument and coefficient of expansion was calculated as α = L L.tensile test specimens were prepared according to ASTM A 37. Results and discussions: On the basis of chemical composition, cast samples could be categorized in three groups which differs in percentage of copper (alloys 2,3,4), magnesium (alloys 5,6,7) and nickel(alloys 8,9,1).Mechanical properties of all sample are listed in table 2 and as a nomograph in fig1. The ultimate tensile strength of sample increased almost 15% with increasing the percent of copper from.96% up to 1.55% (fig 1). The relation between thermal expansion coefficient with different percent of copper before and after heat treatment is shown in fig 2.As can be seen copper does not effect thermal expansion coefficient in different temperature. Thermal expansions of sample at different temperatures are shown in fig 3.Before and after heat treatment respectively. As can be seen thermal expansion increase linearly by increasing the temperature from 5ºc up to 3 ºc. The effect of magnesium on mechanical properties is increasing ultimate tensile strength up to 17.5 % by increasing the amount of magnesium from.28 % up to 1.26.The effect of magnesium on thermal expansion coefficient is shown in fig 4,5.For non-heat treated samples there is no effect on thermal properties, but for heat treated samples, sample with lower percentage of magnesium show lower thermal expansion at 25 ºc The effect of nickel on mechanical is almost negligible (almost 4 % increase in U.T.S by increasing the amount of nickel from.4% up to 1.42%). Thermal expansion properties of specimens is shown in fig 6,7. As can be seen there is no major varition in thermal expansion properties by changing the percent of nickel. Conclusion: The results of present work could be summarized as followings: 1- Addition of copper improves mechanical properties of LM 13 alloy but has no major effect on the thermal expansion properties. 2- Magnesium has similar effect as copper both on mechanical and thermal expansion properties. 3- Nickel has not major effect on mechanical properties and although the lowest thermal expansion is associated with samples containing nickel, however the difference in minor. 3

4 METAL , Hradec nad Moravicí References: 1. Stephen T.robison, Maximizing the use of aluminum casting in transportation Industries, American Foundry Society,June 4, Aluminum Alloys for Pistons, Performance pistons, October 15, M.m. Haque and m.a. maleque, Effect of process variables on structure and properties of Aluminum Silicon piston alloy, journal of materials processing technology, 1998, vol.77, p J.a.taylor, Metal-related Cast ability effects in the aluminum foundry alloys,cast Metals, 1995,vol.8,no.4,p R.kovacheva, Metallogeraphic Investigation of Multicomponent intermetallic Compound in compound in complex AI-SI alloys,aluminum,1993,vol.69,no.12,p M.harun and A.R.Daud, Effect of element on wear properties of eutectic aluminum silicon alloy,wear,1996,vol.194,p Jonathan A.Lee, Aluminum silicon alloy having Improved properties at elevated temperatures and article cast thereform,united States Patent,Appl.no.63992,October 11,2. 8. M.djudjevic and T.Stockwell, The effect of strontium on the microstructure of the AL-SI and AL-CU eutectics in the 319 AL alloys, International journal of cast metals, 1999, no. 12,p Kurita, Piston for Internal Combination Engine and process of making same, United States Patent, Appl.no ,April.3,21. J.Nete and S.Mielk, Thermal expansion and dimensional stability of aluminum fiber 1. reinforced aluminum alloys, Material science and engineering, 1991,vol. A148, p Valtierra-Gallardo, Method and apparatus for simplified production of heattreatable aluminum alloy casting, United State Patent, Appl.no , July 13,1999. S.Shabestari & S.Saeidinia. Investigation on the Effect of Heat Treatment on the 12. Microstructure and Mechanical Properties of 319 Aluminum Alloy, Rikhtegary,22 nd. Yr, no.68,spr.2. Table 1.Chemical composition of specimens (wt%) 4

5 METAL , Hradec nad Moravicí Alloy %Si %Cu %Ni %Mg 1(LM 13) Table 2.Mechanical properties of specimens. Alloy %Si %Cu %Ni %Mg Y.S U.T.S Elongation U.T.S Mechanical Properties Sample Fig.1 Mechanical properties of specimens 5

6 METAL , Hradec nad Moravicí.7 After heat treatmen oc 1 oc 15 oc 2 oc 25 oc 3 oc Series7 Termal expantion oc 1 oc 15 oc 2 oc 25 oc 3 oc percent of Cu percent of Cu Fig.2 Effect of different percent of Cu on the thermal expansion coefficient. After heat treatment %cu 1.1 %cu 1.55 %cu tempereture tempereture.14 %cu 1.1 %cu 1.55 %cu Fig.3 relationship between thermal expansion coefficient and temperature in different percent of Cu. Fig.4 Effect of different percent of Mg on the thermal expansion coefficient. After heat treatment oc 1 oc 15 oc 2 oc 25 oc 3 oc Termal expantion oc 1 oc 15 oc 2 oc 25 oc 3 oc percent of Mg.5 1 percent of Mg 6

7 METAL , Hradec nad Moravicí After heat treatme Thermal eapantion Temperature.28% Mg.66% Mg Fig.5 relationship between coefficient thermal expansion and temperature in different percent of Mg. Thermal eapantion Temperatur.28% Mg.66% Mg After heat treatment Termal Expantion oc 1 oc 15 oc 2 oc 25 oc 3 oc Termal Expantion oc 1 oc 15 oc 2 oc 25 oc 3 oc percent of Ni percent of Ni Fig.6 Effect of different percent of Ni on the coefficient thermal expansion. Afterheat treatment Temperature.4 % Ni.63 % Ni 1.42 % Ni Temperature.4 % Ni.63 % Ni 1.42 % Ni Fig.7 relationship between coefficient thermal expansion and temperature in different percent of Ni. 7