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Grain Refining and Direct Chill Casting of Non Ferrous Metals

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Chair of Casting Research

 Montanuniversität Leoben

Univ. Prof. Dipl.-Ing. Dr. Peter Schumacher

Franz-Josef-Strasse 18, 8700 Leoben, Austria

 

GRAIN REFINING AND DIRECT CHILL CASTING OF NON FERROUS METALS

Report on:

Grain refinement of aluminium alloy

Author: Jure Banovšek

Supervisor: Jiehua Li, Priv.-Doz. MSc. PhD

Date: 17. 1. 2018

Table of contents

1. Introduction        3

2. Experimental work        4

2. 1 Alcan TP-1 Test        4

2. 2 Intercept method        6

3. Results        7

4. Conclusion        9

5. References        10


1. Introduction

Grain refinement plays a crucial role in improving characteristics and properties of cast and wrought aluminium alloys. Control of grain size is very critical in maintaining the consistency of the supreme productivity and quality as well as in reducing the costs. Grain refinement of aluminium alloys is most often achieved by chemical inoculation. Commonly used master alloys Al – Ti – B and Al – Ti – C are added to the melt before casting, introducing the inoculant particles to the melt. These particles act as nucleation points for α-Al grains, resulting in a uniformly fine, equiaxed microstructure. Fine grain structure and avoidance of columnar crystals results in many positive properties:

  • Reduced ingot cracking
  • Better homogeneity
  • Reduced porosity
  • Improved feeding
  • Extended furnace refractory life and improved productivity
  • Improved mechanical properties
  • Reduced costs
  • Enhanced response the subsequent heat treatment

Most commonly used master alloys are based on Al – Ti – B system containing phases TiB2 and Al3Ti in α-aluminium matrix. Other common master alloy systems include Al – Ti, Al – Ti – C, in addition research is also continuing into Al – Sc, Al – Zr and Al – Cr grain refiners. All aforementioned systems are capable of grain refinement, whereby last three are rarely used.


2. Experimental work

2. 1 Alcan TP-1 Test

Standard test procedure for aluminium alloy grain refiners, also known as Alcan TP-1, is standardised test for determination of grain sizes. It determines the ability of grain refiners to reduce grain size during solidification of aluminium alloys under standardized conditions, and for examining the structure of the grain refiner for uniformity.

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Figure 2: Alcan TP - 1 ladle measurements

Experimental work started with putting two ladles in the furnace and heating them up to 720 º - 750 ºC. Material in ladles was high purity aluminium (99.7 %) that melted after few minutes. After successfully obtaining the melt, we set up TP-1 apparatus with constant water flow of 4 . In the next step, we created a time dependant schedule for casting samples after 2, 5, 10 and 30 minutes. After adding specific grain refiner, 0.2 % of TiCAl or TiBAl, in individual ladle, we stirred the melt for 30 seconds and started casting the samples. We preheated the coated conical shaped steel tool, dip it into the melt and put it in the opening of TP-1 apparatus. The samples were quenched at approximate cooling rate of 4 . The process was repeated with every sample, the only changing parameter was time. After completing the series, samples were horizontally sectioned, polished and etched. [pic 4][pic 5]


2. 2 Intercept method

Intercept method is a technique used to quantify the grain size for a given material by drawing a set of linear line segments on the micrograph, in longitudinal, transverse or diagonal direction. Counting the number of times each line segment intersects a grain boundary introduces us a ratio of intercepts to line length.

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Figure 5: Intercept method

To measure the grain size, microstructure pictures were needed. We took a screenshot of every sample’s microstructure and then use the intercept method.

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