The Effect of Processing Parameters and Alloy Composition on the Microstructure Formation and Quality of DC Cast Aluminium Alloys Majed M. R. Jaradeh Doctoral Thesis In Materials Processing Department of Material Science and Engineering School of Industrial Engineering and Management KTH-Royal Institute of Technology SE-10044 Stockholm, Sweden Department of Engineering, Physics and Mathematics Mid Sweden University SE-851 70 Sundsvall, Sweden Akademisk avhandling som med tillstånd av Kungliga Tekniska Högskolan (KTH) framlägges till offentlig granskning för avläggande av Teknologie Doktorsexamen, Todsdagen den 14 Dec 2006, kl 10.15, sal F3, Lindstedtsvägen 26, KTH, Stockholm. i The Effect of Processing Parameters and Alloy Composition on the Microstructure Formation and Quality of DC Cast Aluminium Alloys Majed Jaradeh Department of Material Science, Royal Institute of Technology SE-100 44 Stockholm, Sweden Abstract The objective of this research is to increase the understanding of the solidification behaviour of some industrially important wrought aluminium alloys. The investigation methods range from direct investigations of as-cast ingots to laboratory-scale techniques in which ingot casting is simulated. The methods span from directional solidification at different cooling rates to more fundamental and controlled techniques such as DTA and DSC. The microstructure characteristics of the castings have been investigated by optical and Scanning Electron microscopy. Hardness tests were used to evaluate the mechanical properties. The effects of adding alloying elements to 3XXX and 6XXX aluminium alloys have been studied with special focus on the effects of Zn, Cu, Si and Ti. These elements influence the strength and corrosion properties, which are important for the performance of final components of these alloys. Solidification studies of 0-5wt% Zn additions to 3003 alloys showed that the most important effect on the microstructure was noticed at 2.5 wt% Zn, where the structure was fine, and the hardness had a maximum. Si addition to a level of about 2% gave a finer structure, having a relatively large fraction of eutectic structure, however, it also gave a long solidification interval. The addition of small amounts of Cu, 0.35 and 1.0 wt%, showed a beneficial effect on the hardness. Differences have been observed in the ingot surface microstructures of 6xxx billets with different Mg and Si ratios. Excess Si compositions showed a coarser grain structure and more precipitations with possible negative implications for surface defect formation during DC casting. The comparison of alloys of different Ti content showed that the addition of titanium to a level of about 0.15 wt% gave a coarser grain structure than alloys with a normal Ti content for grain refinement, i.e. < 0.02 wt%, although a better corrosion resistance can be obtained at higher Ti contents. The larger grain size results in crack sensitivity during DC casting. A macroscopic etching technique was developed, based on a NaOH solution, and used in inclusion assessment along DC cast billets. Good quantitative data with respect to the size and spatial distribution of inclusions were obtained. The results from studied billets reveal a decreasing number of inclusions going from bottom to top, and the presence of a ring-shaped distribution of a large number of small defects in the beginning of the casting. The present study shows how composition modifications, i.e. additions of certain amounts of alloying elements to the 3xxx and 6xxx Al alloys, significantly change the microstructures of the materials, its castability, and consequently its mechanical properties. Keywords: Aluminium wrought alloys, AA3xxx and 6xxx, Direct Chill Casting, Unidirectional Solidification, Bridgman technique, Process parameters, Microstructure, Composition modification, Thermal analysis, Inclusions, Metallographical investigations. ii Acknowledgments This work has been carried out at Mid Sweden University, at the Department of Engineering, Physics and Mathematics. I would like to express my sincere appreciation to my professors, Torbjörn Carlberg, the main thesis supervisor and Hasse Fredriksson (co-supervisor), for their scientific guidance, encouragement and kindness throughout my graduate studies and the completion of this dissertation. The great inspiration and confidence that they have given me make me believe that I can continue my academic dream. I am also grateful to the staff members of the Sundsvall branch of the Department of Engineering, Physics and Mathematics, at Mid Sweden University and the Department of Materials processing at the Royal Institute of Technology in Stockholm, especially Advenit Makaya. The financial support from EU, Objective 1, has been of great importance for my investigation, and my co- workers and I would also like to thank Kubikenborg Aluminium AB for their support and provision of the sample materials. Last, I would like to thank my family and friends who have motivated and supported me through the completion of this dissertation. Sweden, December 2006 Majed M. R. Jaradeh iii List of papers The thesis includes the following supplements: Supplement 1 Analysis of solidification in a Bridgman furnace as a simulation of DC casting of aluminium alloy slabs. Majed Jaradeh and Torbjörn Carlberg. Accepted for publication in Materials Science and Technology, 2006 Supplement 2 Solidification studies of 6xxx alloys with different Mg and Si contents. Hu Jin, Majed Jaradeh and Torbjörn Carlberg. Proceedings, Light Metals. TMS, San Francisco, California, USA, Feb. 2005, pp. 1039-1044. Supplement 3 Directional solidification of type AA3003 alloys with addition of Zn and Si. Majed Jaradeh and Torbjörn Carlberg. Proceedings, Solidification of Aluminum Alloys. TMS, Charlotte, North Carolina, USA. March 2004, pp. 53-62. Supplement 4 DTA and DSC Studies of aluminium 3003 alloys with Zn and Cu additions. Majed Jaradeh and Torbjörn Carlberg. Submitted to Metallurgical and Materials Transactions A. 2006 Supplement 5 Effect of titanium additions on the microstructure of DC-cast aluminium alloys. Majed Jaradeh and Torbjörn Carlberg. Materials Science and Engineering A. Vol. 413-414, 15 December 2005, pp. 277-282. Supplement 6 Method Developed for Quantitative Assessment of Inclusions in Aluminium Billets. Majed Jaradeh and Torbjörn Carlberg. To be published in Proceedings, Light Metals, TMS. Orlando, Florida, USA. Feb. 2007. Supplement 7 Solidification Studies of Automotive Heat Exchanger Materials. Torbjörn Carlberg, Majed Jaradeh and H. Kamgou Kamga. JOM. Vol. 58, no. 11. Nov, 2006.pp 56-61 iv TABLE OF CONTENTS Abstract..........................................................................................................................ii Acknowledgments ........................................................................................................iii List of papers................................................................................................................ iv TABLE OF CONTENTS ............................................................................................. v Chapter 1 .......................................................................................................................6 1 Introduction and Background of Thesis ............................................................ 6 1.1 Direct Chill Casting of Aluminium Alloys...................................................... 6 1.2 Aluminium Alloys and Effects of Alloying Elements..................................... 9 1.3 Aluminium Melts Cleanliness and Treatment ............................................... 11 1.4 The Scope of the Thesis................................................................................. 14 1.5 References...................................................................................................... 15 Chapter 2 ..................................................................................................................... 19 2 Experimental methods....................................................................................... 19 2.1 Materials......................................................................................................... 19 2.2 Experimental Techniques............................................................................... 19 Chapter 3 ..................................................................................................................... 28 3 Results and discussion........................................................................................ 28 3.1 Results of thermal conditions in the Bridgman furnace................................. 28 3.2 Results of different Mg and Si contents on solidification of 6XXX alloys... 32 3.3 Results of additions of Zn and Si on the solidification of 3003 alloys.......... 35 3.4 Results of DTA and DSC studies of 3003 alloys containing Zn and Cu....... 37 3.5 Results of additions of excess titanium.......................................................... 41 3.6 Results of assessment of inclusions in aluminium billets.............................. 43 4 General Conclusions of Thesis.......................................................................... 46 v Chapter 1 1 Introduction and Background of Thesis Aluminium and its alloys have excellent properties: they are light in weight, corrosion- resistant, good electrical and thermal conductors and relatively easy to produce. They find their main uses in the transportation, construction,