CERAMIC BASED COMPOSITE MATERIALS FOR WELDING PROCESSES-RRM-2009 (1).pdf

188 Revista Romn de Materiale / Romanian Journal of Materials 2009, 39 (3), 188 - 195

MATERIALE COMPOZITE CERAMICE PENTRU PROCESUL DE SUDARE. PREPARARE, CARACTERIZARE SI EVALUAREA CALITATII SUDURII

CERAMIC BASED COMPOSITE MATERIALS FOR WELDING PROCESSES. PREPARATION, CHARACTERIZATIONS AND THE WELD QUALLITY

EVALUATION

CHRISTU RDEI1, BOGDAN FLORENTINA1, MARIANA OCHEEL2 1Institutul National de Cercetare-Dezvoltatre pentru Inginerie Electrica ICPE-CA, Splaiul Unirii 313, Bucureti

2DAMEN SHIPYARD, Str. Moruzzi 132, 800223, Galai

Lucrarea prezint rezultatele experimentale

referitoare la elaborarea ceramicii, caracteristicilor termomecanice i testelor experimentale n procesul de sudare cu materiale ceramice suport. O atenie deosebit este acordat controlului calitii sudurii prin evaluri nedistructive. Sistemele suport pe baz de materiale ceramice constituie poteniale soluii. Ceramica cordieritic i cele pe baz de cordierit datorit caracteristicilor de rezisten ridicat la oc termic constituie o categorie promitatoare de materiale. Compozite mulit-cordieritice cu microstructuri diferite i coninut variabil de faza mulitic au fost elaborate din pulberi ceramice prin procedee convenionale, astfel amestecul de pulberi ceramice a fost omogenizat n mori, iar produsele presate semiuscat au fost apoi sinterizate le temperaturi adecvate. Compuii ceramici mulit i cordierit au fost preparai independent i apoi amestecai pentru obinerea compozitelor dorite. Toate amestecurile au fost apoi sinteriazate la 1350C timp de 2 ore, cu excepia compuilor puri de cordierit i mulit care au fost sinterizai timp de 3 ore la 1300C i respectiv 1600C. n toate procesele de sudare (sudare tip MAG) ansamblele au fost sudate cap la cap. Numai probele sudate prezentnd un cordon de sudur acceptabil dup inspecia vizual au fost radiografiate. Controlul calitatii sudurii s-a realizat prin evaluri nedistructive (vizual, cu lichide penetrante, cu particule magnetice i cu ultrasunete). Deasemenea, au fost determinate, pe produsele ceramice, caracteristicile de sinterizare i principalele caracteristici termomecanice.

This paper present experimental results related

to ceramic development, some thermomechanical pro-perties and experimental testing methods. Special emphasis is given to nondestructive evaluation for the quality control of the welds.

Ceramic-type backing systems offer a potential solution. Cordierite and cordierite-based ceramics are promising materials for high thermal shock resistance specially compared to alumina or mullite.

Mullite-cordierite composites with various mullite contents and microstructures were prepared by a conventional powder processing, i.e. mechanical mixing, forming of green bodies by dry pressing, and sintering at appropriate temperatures.

Cordierite and mullite powders were prepared separately and mixed to get the required compositions. All were sintered for 2h at 1350C, except for the pure cordierite and mullite which were soaked for 3h at 1300C and 1600C, respectively.

In all welding processes (MAG welding) test assemblies were made by butt welding. Test assemblies with visually acceptable beads were radiographed.

The quality control of the weld are verified by a nondestructive evaluation (visual, liquid penetrant, magnetic particles and ultrasonic).

Sintering characteristics, physical and the main thermo-mechanical properties of ceramic tiles were also determined.

Keywords: ceramics, welding, cordierite, mullite, nondestructive testing 1. Introducere

Piaa produselor ceramice tehnice avansate

este n continu cretere i diversificare. Cu precdere, ceramicile tehnice sunt adesea supuse la sarcini termice i mecanice mari care implic tensiuni multiaxiale semnificative. Principalele caracteristici pentru aceste materiale sunt: punct de topire ridicat, inerie chimic la temperaturi nalte i o rezisten excelent la oc termic datorat unei combinaii de proprieti precum, coeficient mic de dilatare termic i conductivitate termic ridicat. Pentru procesele industriale de sudare, utilizarea suporilor ceramici (CBM) ca produse auxiliare, constituie o schimbare nou i revoluionar, cu aplicaii n industria constructoare de vapoare, pentru instalaiile marine i industriale, pentru

1. Introduction The market for advanced technical

ceramics is large and growing. Engineering ceramics are often subject of high thermal and high mechanical loadings including severe multiaxial stressing. The main material properties of these are: high melting point, chemically inert at high temperature, and excellent resistance to thermal shock achieved by a combination of high thermal conductivity and low thermal expansion coefficient. A new and revolutionary change in the welding industry is realized, especially by using ceramic welding backing materials (CBM) as welding supplement in shipbuilding marine plant, plant, and various steel, steel alloys and aluminum structures. Ceramic-type backing systems offer a potential

Autor corespondent/Corresponding author, Tel.: 0040 21 345.82.97, e-mail: [email protected]

C. rdei, F. Bogdan, M. Ocheel / Materiale compozite ceramice pentru procesul de sudare. Preparare, caracterizare i 189 evaluarea calitii sudurii

structurile din oel sau aliaje de oel, dar i din aluminiu. Pentru toate sectoarele industriale, de la centrale i uniti cu profil metalurgic i pn la domeniul microelectronicii sau cel al producerii de dispozitive medicale, procedeele de mbinare i sudare reprezint o soluie cheie pentru obinerea de produse performante. Tot n domeniul construciilor navale, dar i pentru alte domenii industriale, au fost utilizate i bare metalice (din cupru sau oel), ca supori pentru sudur, pentru eliminarea cantitii de cldur acumulate n timpul procesului [1-2]. n realitate, aceste bare metalice (fuzibile) se topesc, lipindu-se de prile metalice de asamblare. Evitarea acestor neajunsuri s-a realizat prin folosirea suporilor ceramici. O alt soluie alternativ, practic, const n acoperirea barelor metalice cu un strat subire de material ceramic, din alumin sau spinel [3].

Pe de alt parte, nici un alt proces de sudare nu produce suduri pe o singur parte lucioase i cu un contur uniform, cu o penetrare complet, n principal datorit ratei sczute de umplere, de aceea, ceramica este adesea utilizat i datorit unor proprieti favorabile la temperaturi ridicate i n medii oxidante. Pentru astfel de aplicaii, materialele ceramice refractere utilizate trebuie s prezinte rezisten relativ mare la oc termic, din aceast categorie reprezentative sunt ceramicile pe baz de cordierit, alumin sau SiC. Utiliznd ca materiale suport materiale ceramice cu anumite compoziii specifice pot fi realizate principalele cerine de mbuntire a caracteristicilor de izolaie termic, n scopul reducerii consumului de energie electric, dar i a timpului efectiv de sudare. Aceste produse sunt utilizate ca materiale auxiliare (consumabile), n procedeele de sudare pe o singur parte, automate sau semiautomate, n mediul de gaz protector, CO2 sau amestec cu argon.

Ceramica preponderent cordieritic (2MgO2Al2O35SiO2), datorit principalelor proprieti precum conductivitate termic i constant dielectric relativ scazute, bun stabilitate termic, este utilizat n diverse aplicaii din domeniul electronicii i al aplicaiilor termomecanice. Totui, datorit unor proprieti mecanice mai sczute, utilizarea lor este limitat. Dintre celelalte materiale ceramice oxidice, pe baz de MgO, SiO2, mulit (3Al2O32SiO2) i spinel (MgOAl2O3), numai mulitul prezint dilatare termic i constant dielectric sczute, compatibilitate chimic [4], dar i proprieti mecanice rezonabile. Prezena n compozite a particulelor cu modul de elasticitate ridicat va determina i o cretere a rezistenei compusului, concomitent cu mrirea valorii modulului pentru produse. Compozitele din sistemul cordierit-mulit ofer proprieti promitoare prin creterea gradului de sinterabilitate al mulitului, ct i prin mbuntirea proprietilor mecanice. Adaosul de faz mulitic are rol i de mrire a temperaturii maxime de utilizare.

solution. Heavy industry such as shipbuilding, marine industry, and plant industry has mostly employed ceramic materials as welding supplements. Welding and joining are key enabling technologies in all sectors of industries involved in products made with engineering materials-from heavy and power plant, to microelectronic and medical devices. In shipbuilding and many other industries, metal (copper, steel) weld backing bars are used to draw the heat out of the weld [1-2]. The problem is that these bars actually melt and become welded to the weld plates. to avoid this fusion between the backing bars and the weld plate, ceramic backing material are very indicated. Another possible solution currently being tested is to coat the metallic bars with a thin layer of ceramic coating (alumina or spinel) [3]. Apart from the low deposition rate of the majority of processes, no others will consistently produce full penetration one-side welds with the smooth, controlled back-side contour. Ceramics are often used because of their favourable properties at high temperatures and under oxidizing conditions. The refractory ceramic materials for this application should have relatively high heat shock resistance, and examples of such materials include cordierite, compositions consisting largely of alumina, silicon carbide, s.o. So, the main purpose is to improve a heat insulating characteristics and to decrease a weld heat input as well as to induce the electric power consumption and welding time by using the backing material made of sintered ceramics consisting of specific compositions. They are widely used in one-side welding methods such as CO2 semiautomatic or automatic welding. Cordierite (2MgO2Al2O35SiO2) and cordierite based ceramics are utilized in various electronic and thermo-mechanical applications due to their relatively low thermal conductivity and low dielectric constant, coupled with high chemical and thermal stability. Their use is limited by relatively poor mechanical properties. Comparing other possible oxide materials, e.g. MgO, SiO2, mullite (3Al2O32SiO2), and spinel (MgOAl2O3), only mullite offers low thermal and dielectric constants, chemical compatibility [4], and reasonable mechanical properties. The presence of higher modulus particles increases the strength proportional to the increase of Youngs modulus, E of the system. Composites in the system cordierite-mullite offer promising properties in both enhancing the sinterability of mullite and improving mechanical properties of cordierite. Mullite is added to the body to extend the maximum temperature of use.

The paper has as main objectives the preparation of composite ceramic materials based cordierite and mulit for ceramic applications as auxiliary in the process of welding metal plates, and assessment of quality welded joints by non-destructive evaluation techniques. For this work,

190 C. rdei, F. Bogdan, M. Ocheel / Ceramic based composite materials for welding processes. Preparation, characterizations and the weld quality evaluation

Lucrarea are ca principale obiective elaborarea de materiale ceramice compozite, pe baz de cordierit i mulit, pentru aplicaii ca auxiliare ceramice n procesul de sudare a tablelor metalice, ct i evaluarea calitii mbinrilor sudate prin tehnici de evaluare nedistructive. Pentru aceasta lucrare, imbinrile sudate realizate la o prim "trecere" au fost completate i evaluate n continuare cu radiaii penetrante. 2. Materiale i procedee experimentale 2.1. Sinteza compozitelor ceramice

Compozitele cordierit-mulitice pot fi realizate plecnd de la diverse materii prime: pulberi oxidice, amestecuri de cordiert i mulit, frite i chiar de la precursori organo-metalici [5-7]. Fr adaosuri de sinterizare cordieritul este dificil de obinut. In acest sens, pentru experimentri, drept adaos de sinterizare s-a folosit feldspatul (1-5%). Pentru procedeele de sudare a barelor din oel, compoziia oxidic a suportului ceramic poate fi: 60-75% SiO2, 15-25% Al2O3 i 10-20% MgO. Materiile prime i proporiile lor pentru elaborarea suporilor ceramici sunt: argil (30-45%), talc (20-35%) i respectiv alumin (20-40%). Datorit simplicitii procesului, presarea uniaxiala a pulberilor a fost aleas ca modalitate de fasonare. Experimentrile au urmrit i o mbuntire a proprietilor ceramicii cordieritice, prin adaosul de faz mulitic. Formula stoechiometric a cordieritului a fost aleas ca baz de calcul, iar materiile prime utilizate au fost: alumin , caolin i oxid de magneziu pentru masele ceramice cordieritice tip A, alumin, caolin i talc pentru ceramicile tip B. Diferitele compoziii au fost omogenizate umed, uscate, presate i n final arse la diferite temperaturi. Compozite cordiert-mulitice au fost realizate din materii prime elaborate, sinter cordieritic i sinter mulitic (electrotopit), n proporiile 3:1; 1:1; 1:3, definite n continuare mase C2, C3, C4, pentru amota A i respectiv, mase C2, C3, C4, pentru amota B. Amestecurile celor doi componeni au fost omogenizate umed i apoi presate semiuscat (cu 12% soluie APV, 5%) la ~350 daN/cm2, sub form de bare prismatice cu dimensiunile 6,5x6,5x57mm. Probele pentru testri au fost iniial calcinate la ~1100C, dup care au fost sinterizate la trei temperaturi diferite (1300C, 1330C, 1360C), cu palier de 2 ore.

2.2. Msurtori ceramice

Pe epruvete prismatice au fost efectuate, conform STAS, msurtori de parametri specifici de ardere, densitate aparent, absorbie de ap i contracie. Msurtorile pentru coeficientul de dilatare termic au fost efectuate pe epruvete cu dimensiunea 55x10x7mm i calculate pentru intervalul de temperatur 25-800C. Compoziia mineralogic pentru produsele sinterizate a fost determinat prin difracie de raze X, i prin evaluri semicantitative au fost identificai principalii

welded joints made at the first "crossing" was completed and evaluated by further penetrating radiation. 2. Materials and experimental procedures

2.1. Ceramic composites synthesis

Composites of cordierite and mullite can be produce from a variety of starting materials: oxide powders, cordierite and mullite powders, glass frits, and even organo-metallic precursors [5-7]. However, cordierite ceramics are difficult to sinter without any sintering aids. So, in our experimental work we used 15% of feldspar as sintering aid. The ceramic backing materials for welding of steel bars could be made of the sintering ceramic consisting of 100 parts total of 60-75% SiO2, 15-25% Al2O3, and 10-20% MgO. Ceramic products may have the following raw materials composition: 30-45% clay, 20-35% talc and 20-40% alumina. Uniaxially dry pressing of milled powders were chosen as the processing route for its simplicity. The intention of this work is to study the possibility of improving some properties of cordierite ceramics by forming a cordierite-mullite composite material The formula of pure cordierite 2MgO2Al2O35SiO2 was taken as the basis of calculation, and the following raw materials were employed: alumina, kaolin, magnesium oxide, for the cordierite ceramic marked A, and alumina, kaolin, talc, for cordierite B, respectively. The materials for each batch were milled in a small ball mill, dried and cold pressed, and finally sintered at different temperatures. In order to obtain composite materials, from cordierite sinter and electro-melted mullite fine ground were prepare samples in the three massic ratios 3:1, 1:1 and 1:3, defined masses below C2, C3, C4, for cordierite A, and masses C'2, C'3, C'4, for cordierite B, respectively. The mixtures of two constituents were homogenized, semi-dray (12% aqueous solution 5% of PVA) pressed at 350 daN/cm2 as prismatic sticks (6.5x6.5x57mm). The best specimens were pre-calcinated at the temperature of~1100C, then they were been subjected to three different thermal treatments for 2 hours, at 1300C, 1330C, and 1360C respectively.

2.2. Ceramic measurements

Bulk density, water absorption and shrinkage of the bodies were determined according to the standards. Thermal expansion measurements were made on rectangular test specimen of 55107mm, and coefficients of thermal expansion between 25-800C were calculated. The mineralogical composition of the synthetic products was investigated by X-ray diffraction, and a semi-quantitative estimation of the mineral phases detected in the two synthesized powders at the different temperatures was performed. Specimens were machined into


componeni mineralogici. Probele pentru msurtori termomecanice au fost prelucrate la dimensiunile de 6 mm x 6 mm x 55 mm i supuse apoi testelor de ncovoiere n trei puncte, la o vitez de culisare de 0.5 mm/min, i cu distana dintre reazeme de 40mm. Pentru fiecare msurtoare, i pentru determinarea valorilor medii i a parametrilor statistici, au fost folosite 5 probe. Evalurile microstructurale au fost realizate prin microscopie electronic SEM. Produsele ceramice au fost realizate n forma i la dimensiunile specifice procesului de sudare pe o singur parte. 2.3. Construcia sistemului de sudur cu suport

ceramic Ansamblul de sudur este constituit din plcue ceramice cu lungimea/limea de ~25mm, conform figurii 1, montate pe o folie din aluminiu printr-un adeziv refractar.

the dimension of 6 mm x 6 mm x 55 mm for the threepoint bending test with a cross-head speed of 0.5 mm/min and a support distance of 40 mm. Five specimens were tested to obtain the average strength and standard deviation. Microstructures were observed by SEM. Ceramic backing material was design in form and at the main dimensions specific to the one-side welding process. 2.3. Ceramic backing construction

The ceramic backing tiles consisted of ~25mm square tiles, as shown in Figure 1, were mounted on to adhesive coated aluminium foil. So, the product is composed of ceramic tiles attached to aluminium tape on witch specific heat-resistant adhesive are spread. 2.4. Welding process

All test assemblies were made by butt welding. The design of this process, shape and

Fig. 1 - Sistem de sudare cu suport ceramic Welding system with ceramic backing.

Fig. 2 - mbinare standard pentru sudura pe o singur parte cu

suport ceramic/ Standard joint for one-side welding with ceramic backing material.

2.4. Procesul de sudare Toate probele (metalul de baz) pentru

sudare au fost realizate prin sudur cap la cap. Desenul unei astfel de mbinri, forma i dimensiunile rostului pentru sudare sunt prezentate n figura 2. Pentru procedeul de sudare utilizat (MAG, cu electrod de srm de 1,2mm, n mediu de gaz protector Ar-20%CO2) probele ceramice au fost testate comparativ cu produsele CBM Dong IL (Korea). S-a utilizat att sudarea orizontal ct i cea vertical. Testele de sudare s-au efectuat pe table din oel naval grad D36, grosime 12mm. 2.5. Controlul calitii sudurii - evaluri

nedistructive Asigurarea unor performane satisfctoare

pentru structurile sudate impune utilizarea unor proceduri de testare adecvate n scopul evalurii calitii sudurilor. Exist teste de mare siguran efectuate n condiii similare sau chiar mai severe dect cele din exploatare. Metodele de evaluare nedistructiv fac posibil verificarea sudurilor la exigenele impuse de standarde, att pentru

dimensions of the welding seam are shown in Figure 2. Our ceramic was compared with CBM Dong Il (Korea), in a MAG welding application with 1,2mm diameter wire electrodes, argon-20%CO2 shielding gas, in horizontal/vertical welding positions. Welding tests were performed on the steel table ship grade D36, 12mm thickness. 2.5. The quality control of the weld, non-destructive evaluations

Providing satisfactory performance of welded structures require the use of appropriate testing procedures to assess the quality of welds. There are great safety tests performed under conditions similar or even more severe than those of exploitation. Non-destructive evaluation methods make it possible to check welds to the requirements imposed by standards, both for surface and the weld and adjacent area of the basic material. Methods of examination the most common, along with visual are the method of penetrating liquids, magnetic particle, ultrasonic, Radiographic (X-ray or ), and are illustrated in


suprafaa i interiorul sudurii, ct i pentru zona adiacent materialului de baz. Metodele de examinare cele mai comune, alturi de examinarea vizual sunt: metoda cu lichide penetrante, cu particule magnetice, cu ultrasunete, radiografice (cu raze X sau ), principial fiind redate n figurile 3- -6. Pentru aceasta lucrare, mbinrile sudate realizate la o prim "trecere" au fost completate i evaluate n continuare cu radiaii penetrante. Controlul radiografic s-a efectuat numai la probele care au fost considerate corespunztoare n ceea ce privete forma, dimensiunile i aspectul exterior al cordonului de sudur realizat pe suportul ceramic.

Pentru viitor, ali factori importani precum grosimea metalului de baz, dimensiunea i/sau caracteristicile electrodului, technica de sudare i ali parametri trebuie evaluai.

figure 3-6. For this work, welded joints made at the first "crossing" was completed and evaluated by further penetrating radiation. Radiographic control was performed only on samples which were considered appropriate in terms of shape, size and appearance of the cord welding performed on a ceramic.

Important factors such as base material thickness, size and/or electrode characteristics, welding technique and other parameters must be evaluated in the future.

3. Results and Discussion 3.1. Sintering behaviour

In all cordierite bodies the development of cordierite proceeds as a time-temperature reaction. Figure 7 show the X-ray diffraction pattern of cordierite compound sintered at 1300C

Fig. 3 - Evaluare optico-vizual a sudurii / Visual evaluation of

the weld.

Fig.5 - Evaluarea sudurii cu particule magnetice / Welding

evaluation with magnetic particles

Fig. 4 - Evaluarea sudurii cu lichide penetrante / Dye penetrant

liquids evaluation of the weld.

Fig. 6 - Evaluarea sudurii cu ultrasunete / Ultrasonic evaluation

of the weld.

3. Rezultate i discuii

3.1. Comportarea la sinterizare Pentru toate masele ceramice cordieritice,

reaciile de formare a fazei cordieritice sunt dependente de temperatur i timp. Figura 7 prezint difractograma compusului cordierit, sinterizat la 1300C timp de 3 ore. O cantitate nsemnat de cordierit se formeaz la temperaturi mai mari de 1200C, cu creterea temperaturii i a timpului de meninere la temperatura maxim,

for 3 hours. Significantly large amounts of cordierite are formed at temperatures above 1200C. At any rate, as time and temperature increase, the amount of cordierite increases. This causes a decrease in the thermal expansion with a corresponding increase in the resistance to thermal shock. Beside the nature of raw materials, the synthesis of cordierite is influenced by initial structural state of the reactants, by their grain size, by the presence of the impurities which have a role of mineralizes [8-9].


Fig.7 - Difracie de raze X pentru cordieritul sinterizat la 1300C-3h / X-ray diffraction patterns of cordierite sintered at 1300 C-3h.

cantitatea de faz cordieritic crete. Aceasta va determina o reducere a coeficientului de dilatare termic i implicit, o cretere a rezistenei la oc termic. Formarea fazei cordieritice este de asemenea dependent de natura materiilor prime, de dimensiunea particulelor, dar i de prezena impuritilor cu rol de mineralizator [8-9].

3.2. Caracteristici termomecanice pentru compozitele ceramice

Pentru compoziiile n care mulitul este faza

majoritar, valoarea coeficientului de dilatare este determinat n principal de fracia de volum de mulit din amestec (tabelul 1). Pentru amestecurile cu cantitate redus de mulit (>70-75% cordierit) valoarea coeficientului de dilatare termic pentru compozit, este dat n principal de faza cu coeficient mai mic (tabelul 2).

3.2. Thermomechanical characteristics of the composite ceramics The thermal expansion coefficient of the

composites seems to be determined by the volume fraction and distribution of the mullit, when mullit is the major phase (table 1). At lower mullit concentration (>70-75% cordierite) the low thermal expansion phase controls the thermal expansion of the composites. The presence of higher modulus particles increases the strength of the composites proportional to the increase of Youngs modulus of the system (table 2).

3.3. Mechanism of the welding process

When welding is started the heat of the arc melts a portion of the ceramic backing simultaneously with electrode and base material;

Tabelul 1

Variaia coeficientului de dilatare termic cu temperatura i coninutul de mulit Dependence of thermal expansion coefficient, , on mullite content and temperature

20800C 10-6/C 20800C 10-6/C Temperaturi de sinterizare Firing temperatures

C2 C3 C4 C2 C3 C4 Temperaturi de sinterizare Firing temperatures

1300C 3.3 3.8 4.3 3.1 3.4 4.0 1330C 1330C 3.2 3.7 4.3 2.9 3.5 3.9 1360C 1360C 3.3 3.8 4.3 2.8 3.5 4.1 1400C

Tabelul 2

Variaia rezistenei cu temperatura de ardere i coninutul de mulit

Dependence of strength, , on mullite content and temperature Rezistena la ncovoiere, , [Mpa]

Bending strength, , [Mpa] Rezistena la ncovoiere, , [Mpa]

Bending strength, , [Mpa] Temperaturi de sinterizare Firing temperatures C2 C3 C4 C2 C3 C4

Temperaturi de sinterizare Firing temperatures

1300C 120 102 78 70 65.8 60 1330C 1330C 115 103 70 77 76.5 68.5 1360C 1360C 130 123 87 85 83 78 1400C


3.3. Mecanismul procesului de sudare La nceputul procesului de sudare, cldura

arcului va topi concomitent o parte din suportul ceramic, electrodul i metalul de baz, n timp, o anumit cantitate de zgur se va amesteca cu aceast topitut n vecintatea cordonului de sudur. Acest mecanism de topire sub cordonul de sudur este corelat cu natura refractar a ceramicii. Fluxul de cldur de la bara de metal topit este mult mai redus la trecerea prin ceramic dect prin suportul metalic. Aceast cldur concentrat va topi marginile metalului de baz din vecintatea ceramicii. Ceramica va pstra baia de metal topit pn cnd aceasta se va rci i va produce un cordon de sudur lucios i cu un profil rotunjit. Topitura metalic este bine meninut ceea ce va facilita ptrunderea sudurii n toate punctele de mbinare.

3.4. Controlul calitii sudurii Calitatea unei mbinri sudate, reprezentat

n figura 8, este reprezentat de un profil al cordonului de sudur lucios, rotunjit i fr incluziuni gazoase. Calitatea acceptabil a mbinrii este remarcat printr-o prima evaluare vizual. Dup aceast prim evaluare proba este supus la multiple i complexe evaluri nedistructive. Pentru fiecare din cele dou mase ceramice cel puin o prob a ndeplinit cerinele de calitate impuse: admise dup inspecia vizual i fr defecte n cordonul de sudur, dup testarea radiografic. Probele care nu au corespuns n urma acestor teste au avut ca principale nereguli aspecte legate de forma i dimensiunea cordonului de sudur (margini neregulate i supranlare), dar i fisuri i urme vizibile de pori.

as welding proceeds, some slag from the process mixes with the molten ceramic beneath the weld pool. The mechanism for melt-back involves the refractory nature of the ceramics. Heat flow from the puddle (i.e. molten pool) is much slower through ceramic backing than through metal backing. This concentrated heat melts the edges of the base material adjacent to the ceramic, as illustrated in figure 9. The ceramic supports the molten pool until it cools and helps to produce a smooth, rounded profile on the back. The molten pool is well supported and this facilitates penetration welding in all positions. 3.4. The weld quality control

The quality of the weld-joint, represented in figure 8, illustrates the aspect of the welding seam with smooth and rounded profile, with no gas inclusions. The first visual inspection denotes an acceptable weld-joint. Next step will consist in multiple and complex non-destructive evaluations. For each of the two masses at least a ceramic sample has met the quality requirements imposed: accepted after visual inspection and non-cord defects in welding after radiographic testing. Samples were not matched in these tests were irregularities as the main issues shape and size of welding root (irregular margins and overload), cracks and traces of visible pores.

3.5. Ceramic damage The ceramic tiles suffered various degree

of damage due to weld penetration and fission to the ceramic, and became blackened and roughened. The use of ceramic backing with flux

Fig.8 - Proba sudat cu suport ceramic pentru msurtori

nedistructive / Welded sample with ceramic backing for non-destructive testing.

Fig. 9 - Micrografie digital a mbinrii sudate i microstructura metalului de baz pentru cordonul de sudur /Digital micrograph of the weld zone and base metal microstructure of weld seam. Magnification is shown.

3.5. Distrugerea ceramicii suport

Datorit topirii ceramicii ct i a penetrrii sudurii, plcuele din ceramic vor suferi diferite grade de deteriorare, devenind nchise la culoare i rugoase. Procedeele de sudare ce utilizeaz srm tubular (cu miez de flux) au uneori inconvenientul apariiei unei porozitti interne, diferit ca form i

cored wire was occasionally burdened by internal porosity with different shape and arrangement. Several sources of porosity causing gas may be unique to ceramic backing and may be a factor with the larger defects. Moisture absorption by the ceramic due to high atmospheric humidity is a possibility. Moisture adsorption was found to be much higher in cordierite-based materials.


poziionare. Unele surse de porozitate, generate de gaze, specifice produselor ceramice, pot constitui un mare neajuns figura 9. Absorbia de umiditate din atmosfera de ctre ceramic constituie o astfel de posibilitate. Pentru ceramicile cordieritice adsorbia de umiditate este de regul mai mare.

4. Concluzii

Compozitele cordierit-mulitice ofer proprieti promitoare, att prin mbuntirea sinterabilitii ct i prin mbuntirea caracteristicilor mecanice ale produselor cordieritice. Pentru masele ceramice elaborate msurtorile difractometrice au pus n eviden prezena fazelor cordierit i mulit, ca principali constitueni. Proprietile fizice, termice i mecanice sunt n principal influenate de compoziie, de natura materiilor prime - dimensiunea particulelor i prezena impuritilor, cu rol de mineralizator, i depind n mare msur de condiiile specifice de procesare. Datorit caracteristicilor termomecanice excelente astfel de produse se utilizeaz cu succes n procesele de sudur, pe o singur parte, ca materiale suport.

Avantaje precum reducerea consumului de energie dar i a timpului de sudare apar i datorit calitii suporilor ceramici de bun izolator termic. Astfel, prin folosirea suporilor ceramici principalele avantaje ale procesului constau n reducerea costurilor, mrirea ratei de depunere ct i prevenirea contaminrii cordonului de sudur de atmosfera oxidant.

Prin posibilitatea de a menine (susine) topitura metalic se permite o penetrare foarte bun a sudurii n toate poziiile. Dezvoltarea suporilor ceramici pentru astfel de aplicaii mbuntete calitatea sudurii, produsele fiind acceptate ca auxiliare utile n mbinrile sudate din domeniul construciilor navale, instalaiilor industriale i construciei de maini. Sunt folosite pe scar larg n procedeele de sudare pe o singur parte (automate sau semiautomate), n mediu de gaz protector.

Evaluarea suporilor ceramici trebuie corelat cu calitatea sudurii. Factori precum tehnica de sudare, materialul de baz, caracteristicile electrozilor din srm (neevaluai n aceast lucrare) joac un important rol. Evaluarea n continuare a suporilor ceramici trebuie s se axeze pe determinarea unei combinaii optime ntre parametrii procesului de sudare i proiectarea mbinrii sudate cu suport ceramic.

Mulumiri Autorul este profund recunosctor i multumete Prof. Sanda Mihai, Conductorul Laboratorului de Msurtori Nedistructive, Universitii POLITEHNICA Bucureti, pentru msurtorile (teste nedistructive) efectuate.

REFERENCES

1. I. Shunji, and U. Masayoshi, U.S.P., 6,386,427 B2, One-side

welding method for steel structure, May 14, 2002.

4. Conclusion

Composites in the system mullite-cordierite offer promising properties in both enhancing the sinterability and improving mechanical properties of cordierite. The XRD patterns of the powders showed that cordierite and mullite exist as two main different phases. Mechanical and thermal properties is mainly influenced by composition, by the nature of raw materials-their grain size, and the presence of the impurities which have a role of mineralisers, and depend on the specifically conditions of preparation. Due to their excellent thermo-mechanical properties these materials could be utilized as temporary auxiliaries backing materials in the welding processes. Since the heat insulating characteristic is good the weld heat input is decreased and the welding time is reduced. So, significant cost savings and another advantage is the use of the higher deposition rates, possible with a refractory ceramic material to hold the molten weld pool in the root and prevent atmospheric contamination of the back weld. The development of ceramic backing improved the welding result and it is recognized as a high-quality and reliable welding aid in such heavy industry as shipbuilding and automobiles. It is widely used in one-side welding methods such as CO2 semiautomatic welding. Ceramic backing evaluation should centre on the quality of weld. Factors such welding technique, base material, thickness and size and/or electrode characteristics may have an important effect and not identified by this work. Continuation of ceramic backing evaluation should concentrate on determining the optimum combination of welding parameters and ceramic/weld-joint design.

Acknowledgments The author wishes to express deep appreciation to Prof. dr. Sanda Mihai, Head of NDT Laboratory, POLITEHNICA University of Bucharest, for the nondestructive evaluations ************************************************************************ 2. M. Egashira, T. Konno, and M. Kobayashi, Evaluation of contact welding and strengthening of joints by non-contact discharge, Science and Technology of Advanced Materials, 2006, 7, 745. 3. B.R. Eggleton, Development of ceramic-coated weld backing bars, DOE Information Bridge Document, 1994. 4. M. Smart, and F.P.Glasser, J. Mat. Sci., 1976, 11(7), 1459. 5. B.H. Mussler, and M.W. Shafer, Am. Ceram. Soc. Bull., 1984, 63 (5), 705. 6. P. Buduer, H. Liebmann, H. Rump, C. Schneider, Silikattechnik, 1985, 36 (10), 307. 7. M.G.M.U. Ismail, H. Tsunatori, and Z. Nakai, J. Amer. Ceram. Soc., 1990, 73 (3), 537. 8. V. Burghelea, Ch. rdei, A. Melinescu, Synthesis and characterization of cordierit-mullite composites, Key Engineering Materials Journal, Elsevier, 2004, 264-68, 1689. 9. Ch.rdei, G.Gavriliu, M. Hagiopol, Ceramic backing materials used in one-side welding processes, Key Engineering Materials Journal, Elsevier, 2004, 264-68, 671.


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