Investigation of the influence of alloying elements in Zn-Al alloy on thermal and mechanical properties.

Kostolny, Igor ; Kolenak, Roman

1. Introduction

As a result of EU directives of avoiding the use of lead [1] in solder joints, the attention of soldering at higher application temperatures is especially focused to trying to find most appropriate alloy as a substitute for Pb-Sn solder. The research is focused mainly on Zn based solders with the addition of other elements to lower the melting temperature and improve their properties [2, 3, 4, 5]. Because of the low price of zinc alloys, they are very attractive nowadays. The use of these solders is oriented not only to die-attaching but also for attaching optical components, circuit modules, in the automotive, aerospace industry and many others. Effect of the addition of rare earth (Er, Nd) on the structure and properties of Sn-Zn solder dealt the authors of [6, 7]. The results showed improvement in wettability, physical and strength properties. The investigation of properties of Zn-Al based solders has been studied by the authors [8, 9, 10]. The addition of Cu resulted in the creation of intermetallic phase CuZn4. In the structure of eutectic solder, Zn-Al phase and basic Zn matrix were observed. Increasing the amount of Cu growing volume fraction of intermetallic phases CuZn4. Increasing the amount of Al resulted in increasing the formation of eutectic phase of the Zn-Al and increasing micro hardness and tensile strength. The addition of Mg and Ga to Zn-Al solder and investigation of its properties is dedicated by authors [11]. Solder was tested for die-attaching applications. The melting temperature of solder was lowered to 347[degrees]C. Decreasing of micro-hardness with increasing temperature was observed. Authors [12] studied the fluxless soldering. The addition of Ag to the structure of Zn-6Al solder resulted witch creating of intermetallic phase AgZn3. The melting temperature of solder was 380[degrees]C. The addition of In to the basic Zn solder and analyze its properties are dealt by authors [13, 14]. A small amount of In to Zn-Al solder decreases the melting temperature. It also resulted in an increase of electrical resistance compared to Zn-Al eutectic solder. The contact angle of the Al and Cu substrate is reduced with increasing of In content. The overall analysis of influence of adding alloying elements Al, Cu and Mg to Zn-Al solder is still not completely implemented. This paper therefore focuses on the study of changes in melting temperature, hardness and tensile strength depending of alloying elements.

2. Experimental

As the input component for the production of the solder alloy, materials of high purity of 3N to 5N are used. Manufacture of solder alloy was carried out in a vacuum oven. Alloys were cast into ingots. As a basis for the creation of ternary solders Zn-Al5 alloy was used due to higher concentrations of Al. The chemical composition of each alloy is shown in Tab. 1

The alloys were prepared by normalized sizes and thoroughly cleaned of impurities to perform the static test of the tensile strength. Illustration of the test specimen in mm is shown in Fig. 1.

To measure the tensile strength, the LABORATECH LabTest 5250 SP1 equipment was used. Measurement of microhardness indentation of diamond consisted of regular tetrahedral pyramid (Vickers hardness). The actual measurements were carried out on the device BEUHLER indentata Met 1100. To carry out the DSC analysis, test samples weighing 3-8 grams were prepared.

3. Experimental results

DSC analysis was performed for understanding the thermal reactions of investigated solders during heating. The melting temperatures of Zn-Al (1-5) solders are significantly changing depending on the quantity of Al. After reaching the composition of Zn-Al4, the melting point had decreasing character. This composition is characterized as a eutectic melting temperature and reached 378.6[degrees]C. After adding of next 1% Al, the temperature decrease to 380.5[degrees]C. In the case of solders Zn-Cu-Al5 (1-3) was also observed decreasing character. The Lowest point reached Zn-Al5-Cu3 solder with value376.2[degrees]C. The opposite was observed in the case of Zn-Al5-Mg (1-3) alloy. In this case was characterized increasing character and the highest melting temperature reached the Zn-Al5-Mg3 solder with value of 342.9[degrees]C.

The melting temperatures of examined solders are shown in tab. 2.

From Fig. 3 it can be seen that the tensile strength ranged from 160 to 200 MPa. The highest value of the ultimate strength of 196 MPa reached Zn-Al4 solder and the lowest Rm reached ZnAl1 solder. The results show that the gradual addition of Al to Zn increases its strength. When reaching 4 wt. % of Al, which is an alloy of nearly eutectic composition, its strength is highest.

Adding of next 1 wt. % of Al, the strength of alloy decreases again. From the measured values, the lowest hardness reached Zn-Al1 solder with an average hardness 47.5 HV and vice versa highest hardness was observed in ZnAl5 solder with an average hardness 60.27 HV. This means an increasing the hardness with adding of next wt. % of Al into the Zn-Al1 alloy. The Increasing of the hardness of solder was caused because of greater hardness of Al compared to Zn.

Three test samples from each solder was prepared to analysing tensile strength (fig. 4) and then subsequently calculated the total average strength. From fig. 4 can be deduced that with a higher percentage of Cu, alloys decreasing their average tensile strength. Cu and Zn creates a brittle intermetallic phase CuZns. By increasing of the content of Cu in solder, the content of the intermetallic phase is increasing as well. Therefore, the greatest tensile strength reached solder with the lowest content of Cu. The lowest average tensile strength was observed at Zn-Al-Cu3 solder. The results of measuring hardness showing that the HV hardness increased with increasing percentage composition of Cu in the Zn-Al5 alloy. The highest average hardness reached a ternary alloy Zn-A15-Cu3 due to the formation of intermetallic phases.

Fig. 5 is showing ultimate tensile strength of ZnAl5Mg (1-3) solder. Three test pieces was used and then from the measured values was calculated average value. From the fig. 5 it can be seen that how tensile strength decreasing with increasing of content of Mg in Zn-Al5 solder. It means that the highest average tensile strength reached Zn-Al5-Mg1 alloy. This is due to the presence of intermetallic compound Mg2Zn11, which is at the higher content of Mg (3%) dispersed in about 48% of the volume of the solder.

4. Conclusions

The present work dealt with the investigation of the influence of alloying elements in Zn-Al alloy on thermal and mechanical properties. Subject of study consisted in the study of effect of Al, Cu and Mg addition on hardness, thermal and mechanical properties. Zn-Al (1-5), Zn-5Al-Cu(1-3) and Zn-Al5-Mg(1 -3) solders were used in experiments.

Based on achieved results, the following was observed:

* addition of alloying elements to Zn decreased melting temperatures,

* the tensile strength of Zn-Al(1-5) had increasing character, in case of Zn -Al5-Cu(1-3) and Zn-Al5Mg(1-3) was observed opposite character due to presentence of intermetallic phases,

* the hardness of examined solders had increasing character. The highest values reached Zn-Al5 (60.27 HV), ZnAl5Cu3 (108.1 HV) and Zn-Al5-Mg3 (94.5HV)

It was found that the addition of alloying elements to the Zn-Al solder significantly changing the thermal and mechanical properties. In further research, determination of the strength of solder joints is necessary.

DOI: 10.2507/26th.daaam.proceedings.096

5. Acknowledgement

The contribution was prepared with the support of APVV-0023-12: Research of new soldering alloys for fluxless soldering with application of beam technologies and ultrasound and VEGA 1/0455/14: Research of modified solders for fluxless soldering of metallic and ceramic materials.

6. References

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Igor Kostolny, Roman Kolenak

Slovak University of Technology in Bratilsava, Faculty of Materials Science and Technology in Trnava, Institute of Production Technologies Paulinska 16, 91724 Trnava, Slovak Republic

Caption: Fig. 1. Schematic of the specimen

Caption: Fig. 2. DSC Analysis of examined solders Table 1. Chemical composition of used solders Chemical Zn [wt. %] Al [wt. %] Cu [wt. %] Mg [wt. %] composition ZnAl1 99 1 -- -- ZnAl2 98 2 -- -- ZnAl3 97 3 -- -- ZnAl4 96 4 -- -- ZnAl5 95 5 -- -- ZnAl5Cu1 94 5 1 -- ZnAl5Cu2 93 5 2 -- ZnAl5Cu3 92 5 3 -- ZnAl5Mg1 94 5 -- 1 ZnAl5Mg2 92 5 -- 2 ZnAl5Mg3 92 5 -- 3 Tab. 2. Melting temperature of examined solders Solder Melting temperature [[degrees]C] ZnAl1 400,8 ZnAl2 379,3 ZnAl3 378,3 ZnAl4 378,6 ZnAl5 380,5 Solder Melting temperature [[degrees]C] ZnAl5Cu1 380.9 ZnAl5Cu2 376.4 ZnAl5Cu3 376.2 ZnAl5Mg1 339,1 ZnAl5Mg2 340,3 ZnAl5Mg3 342,9 Fig. 3. Tensile strength and Vickers hardness of Zn-Al(1-5) solders ZnAl1 47,05 ZnAl2 50,3 ZnAl3 51,4 ZnAl4 52,68 ZnAl5 60,27 Note: Table made from bar graph. Fig. 4. Tensile strength and Vickers hardness of Zn-Al-Cu(1-3) solders ZnAl5Cu1 72,4 ZnAl5Cu2 94,7 ZnAl5Cu3 108,1 Note: Table made from bar graph. Fig. 5. Tensile strength and Vickers hardness of Zn-Al-Mg(1-3) solders ZnAl5Mg1 80,5 ZnAl5Mg2 84,9 ZnAl5Mg3 84,5 Note: Table made from bar graph.