Effect of titanium addition on fracture toughness behavior of ZL108 alloy

WENG Yong-gang(翁永刚), LI Zi-jing(李姿景), LIU Zhi-yong(刘志勇),

LIU Wen-cai(刘文才), WANG Ming-xing(王明星), SONG Tian-fu(宋天福)

Key Laboratory of Materials Physics, Ministry of Education, Physics Department, Zhengzhou University,

Zhengzhou 450052, China

Received 28 July 2006; accepted 15 September 2006

Abstract: Two different titanium alloying methods were applied to ZL108 alloy for preparing specimens containing titanium. The specimens were tested on the MTS 810 material test system for studying their behavior of the plane strain fracture toughness K_IC. The experimental data were analyzed by the statistical significance tests. The results show that the fracture toughness of the ZL108 alloy containing titanium is superior to that of common ZL108 alloy containing no titanium, but there is no significant difference for different titanium alloying methods. Therefore titanium addition is an effective method for improving the fracture toughness of the alloy ZL108.

Key words: ZL108 alloy; electrolytic titanium alloying method; fracture toughness; significance test

1 Introduction

The ZL108 alloy (ZAlSi12Cu2Mg1) is a brand of casting aluminum alloy of China. It is used as piston alloy widely in China for a long time[1]. Because of the cruel working condition in engine cylinder, the piston should be resistant to the impact fatigue load besides high temperature, wear and tear[2]. In other words, the ZL108 alloy as the piston alloy should be superior in its fracture toughness. It is well known that a little of titanium addition will refine the grains of aluminum alloy and improve its mechanical properties[3]. People have studied the effects of titanium addition on the mechanical properties of ZL108 alloy, but they were focused mainly on the high temperature tensile, wear and tear properties[4]. There were little studies on the fracture toughness of ZL108 alloy. It is believed that the fracture toughness is a very important engineering parameter for the application of metals in the view of modern fracture mechanics.

According to the standard GB8063 of China, it is not necessary for ZL108 alloy to contain titanium, which is treated as impurity element. Therefore, it is an attractive question to answer if titanium addition could improve the fracture toughness of ZL108 alloy. The plane-strain fracture toughness K_IC defined in fracture mechanics is an important engineering parameter for judging the cracking resistance of materials[5]. We adopted K_IC as the mechanical parameter of ZL108 alloy for being tested and studied.

Two different titanium alloying methods are applied to ZL108 alloy for preparing specimens containing low content titanium respectively. One is the common fusion alloying method for titanium addition; the other is the electrolytic titanium alloying method. In the latter method, the electrolytic low titanium aluminum base alloy is produced in the industrial electrolytic aluminum cell directly, which is called electrolytic titanium alloying method in situ, and is the patent of our laboratory[6]. Our objective is to investigate the effects of titanium addition and different titanium alloying methods on the fracture toughness of ZL108 alloy, and explore their mechanism by microstructure analysis. This is of practical significance for improving the fracture toughness of ZL108 alloy.

2 Experimental

2.1 Producing electrolytic low-content titanium aluminum base alloy

Our laboratory proposed a new titanium alloying method of aluminum alloy by electrolytic processing. The main idea is to keep the producing technology of electrolytic aluminum unchanged. The primary raw material is still aluminum oxide, and only the titanium dioxide powder is needed for adding in the aluminum cell during electrolytic process. While keeping the production efficiency of the cell, the aluminum base alloy with low-content titanium(＜0.30%) is produced from the cell directly, which is referred as ELTA alloy in abridged notation. It can be used for adding low-content titanium into aluminum alloy, in place of common Al-Ti master alloy[7]. Its chemical compositions are listed in Table 1.

Table 1 Chemical compositions of ELTA alloy (mass fraction, %)

2.2 Preparing ZL108 alloys for contrast test

In order to carry out the contrast tests for investigating our subject, three kinds of ZL108 alloys were prepared. Their chemical compositions and contents are listed in Table 2.

Table 2 Chemical compositions of specimens (mass fraction, %)

Alloy No.1 is the common ZL108 alloy containing no titanium; the other two alloys have nearly the same chemical composition as that of ZL108 alloy, except a little titanium added. If the ELTA alloy is used as master alloy for titanium addition, the resultant ZL108 alloy is referred as the electrolytic low titanium alloying ZL108, or in abridged notation of ZL108 (ETA). If the common Al-Ti master alloy is used for fusion alloying, then the resultant ZL108 alloy is referred as the fusion titanium alloying ZL108, or in abridged notation of ZL108 (FTA). For simplicity in this paper, the common ZL108 alloy is called alloy No.1, the ZL108 (ETA) alloy is called alloy No.2, and the ZL108 (FTA) alloy is called alloy No.3.

2.3 Smelting, heat treating and machining of specimens

Three batches of SE(B) specimens for fracture toughness tests were made of the above three different ZL108 alloys respectively. For the sake of contrast, all the specimens should be processed under the same technology of smelting, heat treating and machining.

In smelting process, aluminum-strontium master alloy is adopted for modification of aluminum alloy[8], and gas argon is used for dehydrogenation. In heat treatment, the temperature and time of solid solution are 515 ℃ and 7 h respectively; the quenching medium is water with temperature of 60 ℃; and the temperature and time of artificial aging are 175 ℃ and 16 h respectively. All the specimens are machined in compliance with national standard GB4161-84 or ASTM E399-90. The SE(B) specimen’s geometric parameters are: width 32.0 mm, thickness 16.0 mm, notch height 1.6 mm, and primary notch length 12.0 mm.

2.4 Test method

The plane-strain fracture toughness K_IC of SE(B) specimens were tested on MTS 810 material testing system. All specimens should be precracked in fatigue method before test. In execution of test, the crack opening displacement(COD) and load raw data were collected automatically. The crack length can be obtained by the compliance method. The expression of stress intensity factor K for SE(B) specimen can be taken from ASTM E399-90.The critical stress intensity factor K_q can be calculated from the raw data. If it passes the validity check, then the candidate K_q will become K_IC. The fracture toughness K_IC obtained from this test is in accordance with ASTM E399-90.

3 Results and data analyses

Three batches of SE(B) specimens were prepared for contrast tests. There are five effective specimens in each batch. The plane-strain fracture toughness K_IC of the three alloys is obtained and the results are listed in Table 3. The data were analyzed by the statistical significance tests, which were carried out in two steps.

From the significance test of F distribution, it was deduced that there are not obvious difference among the standard deviations of three alloys at the given significance level α=10%. Then the second significance test of t distribution was carried out and the results are listed in Table 4, where ν is the degree of freedom of t distribution; α is the given significance level; t_α is the t value corresponding to α; t_ν is the value calculated from Table 3.

From the results in Table 4, it could be deduced that at the given significance level α=1%, the difference of K_IC between alloys No.1 and No.2 (or No.3) is obvious; but the difference of K_IC between alloys No.2 and No.3 is negligible. Therefore, it is confirmed that titanium addition can improve the fracture toughness of ZL108 alloy at significance level of 1%, and the effects of two different titanium alloying methods are nearly equal.

Table 3 Fracture toughness test results of three alloys

Table 4 Significance test of t distribution

4 Microstructure analyses

4.1 Metallographic analysis of α(Al) phase

The metallographic images of three alloys are shown in Fig.1. It can be seen from the images that the dendrites of alloy No.1 seem larger than those of alloys No.2 and No.3. The grains of primary phase α(Al) of alloys No.2 and No.3 were refined by the titanium element added.

Fig.1 Primary α(Al) phase images of three alloys: (a) Alloy No.1; (b) Alloy No.2; (c) Alloy No.3

It is well known that the finer grains will increase the grain boundary, thus increasing the material micro cracking resistance[9]. In other words, it will improve the fracture toughness of ZL108 alloy. The data in Table 5 are the measured second dendrite spacings of three alloys, which are in conformity with the above conclusion for Fig.1.

Table 5 Second dendrite spacing of alloys (μm)

4.2 Microscopic morphology analysis of eutectic silicon particles

The morphological images of eutectic silicon particles of three alloys are shown in Fig.2.

Two main alloying elements in ZL108 alloy are aluminum and silicon. In the view of microstructure analysis, α(Al) phase and eutectic silicon are two main phases in ZL108 alloy, which play an important role in the mechanical properties[10]. The morphological parameters of eutectic silicon particles were measured and the results are listed in Table 6.

Table 6 Morphological parameters of eutectic silicon

The aluminum-strontium master alloy is adopted for modification of ZL108 alloy[11]. The strontium modification plays an important role for improving the morphosis of eutectic silicon particles in ZL108 alloy. It is believed reasonably that titanium addition should play a supplementary role for strengthening the effects of strontium modification[12]. It can be seen in Table 6 that the average diameter, area and slenderness ratio of eutectic silicon particles are decreased with titanium addition, but the circularity of eutectic silicon particles is increased. These results favor the microscopic cracking resistance, and improve the fracture toughness of ZL108 alloy.

Fig.2 Morphological images of eutectic silicon particles: (a) Alloy No.1; (b) Alloy No.2; (c) Alloy No.3

5 Conclusions

Titanium addition is an effective method of improving the fracture toughness of ZL108 alloy. The microstructure analyses reveal that titanium addition of ZL108 alloy can refine the grains of α(Al) phase, and play a supplementary role for strontium modification to improve the morphosis of eutectic silicon particles. These functions in microstructure favor the microscopic cracking resistance, and improve the fracture toughness of ZL108 alloy.

References

[1] LIU Zhi-yong, WANG Xue-qing, WANG Yun-tao. Influences of micro-alloy and heat-treatment procedure on the hardening of ZL108 alloy [J]. Acta Metallurgica Sinica(English letter), 1999, 12(5): 856-860.

[2] HAQUE M M, MALEQUE M A. Effect of process variables on structure and properties of aluminum-silicon piston alloy [J]. Journal of Materials Processing Technology, 1998, 77: 122-128.

[3] HU Bang-hong, LI Hang. Grain refinement of DIN226S alloy at lower titanium and boron addition levels [J]. Journal of Materials Processing Technology, 1998, 74: 56-60.

[4] SAHEB N, LAOUI T, DAUD A R, et al. Influence of Ti addition on wear properties of Al-Si eutectic alloys [J]. Wear, 2001, 249: 656-662.

[5] RICE J R, ROSENGREN G R. Plane strain deformation near a crack tip in a power-law hardening materials [J]. Journal Mech Phys Solids, 1968, 16(1): 1-12.

[6] LIU Zhi-yong, WANG Ming-xing, WENG Yong-gang, SONG Tian-fu, XIE Jing-pei, HUO Yu-ping. Grain refinement effects of Al based alloys with low titanium content produced by electrolysis [J]. Trans Nonferrous Met Soc China, 2002, 12(6): 1121-1126.

[7] WANG Ming-xing, WANG San-jun, LIU Zhi-yong, LIU Zhong-xia, SONG Tian-fu. ZUO Xiu-rong. Effect of B/Ti mass ratio on grain refining of low-titanium aluminum produced by electrolysis [J]. Materials Science and Engineering A, 2006, 416(1/2): 312-316.

[8] HESHMATPOUR B. Modification of silicon in eutectic and hypereutectic Al-Si alloys [J]. Light Metals, 1997, 3: 801-808.

[9] LIAO Heng-cheng, SUN Yu, SUN Guo-xiong. Correlation between mechanical properties and amount of dendritic α-Al phase in as-cast near-eutectic Al-11.6% Si alloys modified with strontium [J]. Materials Science and Engineering A, 2002, A335: 62-66.

[10] OGRIS E, WAHLEN A, LUCHINGER H, UGGOWITZER P J. On the silicon spheroidization in Al-Si alloys [J]. Journal of Light Metals, 2002, 2: 263-269.

[11] ZU Shu, HELLWELL A. Modification of Al-Si alloys: microstructure, thermal analysis and mechanisms [J]. JOM, 1995, 2: 38-40.

[12] LIU Zhi-yong, WANG Ming-xing, WENG Yong-gang, SONG Tian-fu, HUO Yu-ping, XIE Jing-pei. Influence of silicon content on grain refinement and nucleation growth of Al-based alloys produced by electrolysis [J]. Materials Transactions, JOM, 2003, 44(10): 2157-2162.

(Edited by HE Xue-feng)

Foundation item: Project(03220600) supported by the Science and Technology Foundation of Henan Province, China

Corresponding author: WANG Yong-gang; Tel: +86-371-67767776; E-mail: wengyg@zzu.edu.cn