| Abstract | Sitne čestice lebdećeg pepela koje nastaju kao nusprodukt pri izgaranju ugljena u
termoelektranama mogu negativno utjecati na zdravlje ljudi pa zbrinjavanje pepela predstavlja
ozbiljan zdravstveni, ekološki i ekonomski problem. Jedan od načina rješavanja ovog problema
je ugradnja čestica pepela u aluminijsku matricu čime se dobije uporabiv kompozitni materijal.
U doktorskom radu istražena je mogućnost proizvodnje kompozita s matricom od Al-legure
ojačanog sa što većim udjelom čestica lebdećeg pepela te mogućnost poboljšanja njegovih
svojstava postupkom kutnoga kanalnoga istiskivanja. Metodom rendgenske difrakcijske
analize određen je fazni sastav pepela, a metodom optičke emisijske spektrometrije s induktivno
spregnutom plazmom njegov kemijski sastav. Morfološka analiza čestica pepela provedena je
na elektronskom mikroskopu, a metodom difrakcije laserske svjetlosti određena je njihova
veličina. Kako bi se čestice pepela što ravnomjernije umiješale u matricu, uzorci kompozita
pripravljeni su postupkom lijevanja s miješanjem u poluskrućenom stanju, a zbog usporedbe,
na isti način su reološki obrađene i legure. Odliveni su uzorci kompozita s matricom od
aluminijskih legura oznake AlCu4Mg1 i AlSi7Mg0,3 s masenim udjelom od 4 % i 6 %
lebdećega pepela. Čiste legure i kompoziti podvrgnuti su višestrukom broju prolaza kutnoga
istiskivanja, sa zakretanjem oko uzdužne osi za 90° nakon svakog prolaza. Kompozit na bazi
legure AlCu4Mg1 nije bio prikladan za kutno istiskivanje te se od njega odustalo, a uzorci
legure AlSi7Mg0,3 te kompozita s 4 % i 6 % pepela uspješno su protisnuti do maksimalno tri
puta. Na lijevanim i istisnutim uzorcima legure i kompozita analizirana je mikrostruktura,
određena otpornost na eroziju krutim česticama pri upadnim kutovima od 30° i 90° te izmjerena
tvrdoća pri različitim opterećenjima. Na temelju dobivenih rezultata vrednovan je utjecaj
dodatka pepela i kutnoga kanalnoga istiskivanja na svojstva kompozita. Također je utvrđena
korelacija između tvrdoće i otpornosti na erozijsko trošenje za oba upadna kuta erodenta, kao i
utjecaj opterećenja na vrijednost mikrotvrdoće. Dobiveni rezultati potvrdili su hipotezu da se
metodom kutnoga istiskivanja mogu poboljšati svojstva kompozita. |
| Abstract (english) | As a reinforcement in composites with an aluminium matrix, it is possible to incorporate
particles of Fly Ash (FA), which is appearing as a side-product during the combustion of coal
in thermal power plants. Small particles of fly ash can affect people's health, and its disposal
represents a serious ecological and economic problem, which can be solved by finding
opportunities for its useful application. The properties of the composite material can be further
improved by intensive plastic deformation, such as that of Equal Channel Angular Pressing
(ECAP). The main goal of the research was to produce a composite based on aluminium alloy
reinforced with as much FA as possible, suitable for ECAP, and to improve its properties by
applying this process.
The scientific research is presented through six chapters. In Chapter 1 of the doctoral
dissertation, the motivation and main goals are given, as well as an explanation of the methods
and research phases. The hypothesis was defined and given in this chapter also.
In the Chapter 2, an overview of the literature related to ALuminium + Fly Ash (ALFA)
composites are given. The basic characteristics of the materials used for production of the
composites are described: aluminium and its alloys (AlCu4Mg1 and AlSi7Mg0.3), as well as
fly ash. Various methods for the preparation and characterization of ALFA composites are also
described.
Chapter 3 describes the influence of plastic deformation on the change in the microstructure
of the material. According to the available literature, the principle, and dynamics of the ECAP,
as well as the influencing parameters of this process, are described in detail.
The fourth chapter provides theoretical considerations on tribology, a multidisciplinary
scientific discipline that studies phenomena and processes on the surfaces of elements that are
in direct or indirect contact and in relative motion. The basic principles of wear, especially solid
particle erosion wear, are explained.
Chapters 5 and 6 are related to the experimental part of the work.
Materials and methods used in the research are described in Chapter 5. In this research, FA
from the thermal power plant "Kolubara", Republic of Serbia, was used for the preparation of
composites. FA was sieved (w = 45 µm, d = 63 µm) and only this fraction was characterized.
The phase composition was determined by X-ray diffraction analysis (XRD), chemical
composition by Inductively Coupled Plasma - Optical Emission Spectroscopy (ICP-OES),
particle size by laser diffraction, and morphological analysis by Scanning Electron Microscope
(SEM).
This chapter also describes the casting of composites in the semi-solid state and the ECAP
of the samples. Composite samples with aluminium alloy matrices reinforced with 4 % FA and
6 % FA were cast. The composite AlCu4Mg1 + 4 % FA was not suitable for ECAP and was
abandoned. The samples of AlSi7Mg0.3 alloy and composites with 4 % FA and 6 % FA were
subjected to multiple passes of ECAP with rotation around the longitudinal axis by 90° after
each pass (route BC). They have been successfully pressed a maximum of three times.
In the final phase, the samples were characterized. The microstructure, hardness and solid
particle erosion resistance were analysed. The microstructure was analysed on an optical and
scanning electron microscope, and when necessary, Energy Dispersive Spectroscopy (EDS
analyse) and Computer Tomography (CT) were also used. The micro and macro hardness of
the samples were tested using the Vickers method, from HV0.02 to HV1. Meyer's model was
used to interpret the results of microhardness measurements. When solid particle erosion
resistance was tested the impact angle of the particles was varied (30° and 90°), and silicon
carbide (SiC) particles were used as erodent. Erosion resistance was determined based on
volume loss ΔV.
The obtained results are given and discussed in Chapter 6. The results of the FA
characterization show that a used FA fraction consists of metal oxides such as SiO2, Al2O3,
Fe2O3 and others in the amount of 95.8 %. Morphologically FA particles are mostly spheric and
precipitator type. Approximately 90 % of all used FA particles are smaller than 68.3 µm.
The microstructure of the cast alloy consists of primary (α-Al) crystals and eutectic (Al-Si).
Due to the applied casting process, dendritic formations of α-crystals take the form of primary
and mature rosettes, and spheroidal forms are also visible. Larger or smaller agglomerations of
fly ash are present in the microstructure of the cast composite. About 1 % of pores were
observed by CT analysis in the cast composite.
Applying the ECAP procedure causes the collapse of the rosettes, and FA is additionally
distributed in the matrix. The material becomes homogenized and the pores disappear. After
one ECAP pass CT scans show a certain orientation of the microstructure in the direction of the
shear planes, while after the second pass, there is no difference in the CT scans in the horizontal
and vertical planes.
The addition of fly ash particles reduces the hardness and resistance to erosion compared to
the cast, rheologically treated alloy. ECAP increases the hardness of the alloy, and especially
the hardness of the composite. After the second extrusion, the hardness of the alloy and
composite is equal. Whit very high coefficients of determination (R
2 ≈ 1), a significant influence of load on the measured values of microhardness of the samples was confirmed.The hardness
of the alloy and composite with 6 % FA in the as-cast condition, decreases with the increase of
the test load, which is defined as the normal Indentation Size Effect (ISE). Angularly extruded
specimens have a Reverse Indentation Size Effect (RISE).
Applying ECAP generally increases solid particle erosion resistance. On the SEM images,
signs of wear characteristic of certain impact erodent angles were observed. For an angle of
90°, cavities were observed due to the "falling out" of particles and slight indentations caused
by plastic deformation, which indicates surface fatigue. For an angle of 30°, micro-ploughing,
and volume displacement were observed in the direction of the erodent movement, which
indicates an abrasive wear mechanism.
Linear regression analyses showed that there was a very strong negative linear relationship
between hardness and volume loss at the solid particle erosion wear at an impact angle of 30°,
where the dominant wear mechanism is abrasion. In the case of composites with 6 % FA, the
mutual dependence of hardness and resistance to impact erosion was also established (impact
90°), while in the case of non-reinforced alloy and composites with 4 % FA, the observed values
are not correlated.
Considering all the results, it can be concluded that the main goals of the work have been
achieved and the hypothesis was confirmed. It is possible to prepare a composite with an
aluminium matrix reinforced with FA and to improve its properties by multiple ECAP passes. |
