Patent Application: US-201214358726-A

Abstract:
the present invention describes an effective grain refining practice for aluminum foundry alloys . the method described herein relies on the control of the titanium level of the alloy to be grain refined and the addition of boron once it is melted . boron addition can be made via al — b master alloys as well as with boron compounds such as kbf4 salt . the boron added into the melt dissolves first and then forms the alb2 particles that act as potent substrates for the nucleation of aluminum once solidification process starts . the ti concentration of the alloy must be controlled below 100 ppm for this method to offer effective grain refinement . the boron becomes ineffective when the ti concentration in the alloy is higher .

Description:
the grain structures of the grain refined al — si alloys are illustrated in fig1 while the average grain sizes of the alloys before and after the grain refiner addition are shown in fig2 . it is seen that commercial purity aluminium cannot be grain refined with boron addition . al — si alloys with up to 3 wt % si cannot be grain refined with boron either . however , the improvement in grain structure upon the addition of boron in al — si alloys with higher levels of si is evident . the grain size of these hypo - eutectic al — si alloys decrease with increasing silicon with the addition of as much as 200 ppm boron . this range of silicon levels cover the entire series of al — si based foundry alloys . the majority of aluminium foundry alloys contain at least 5 wt % si . the only condition for refining the grain structure with this method is the formation of alb2 particles before the aluminium starts to solidify . the liquidus temperature at which alb2 starts to crystallize from the melt is estimated from the al — si — b ternary system to be 639 centigrade at a boron concentration of 0 . 02 wt %. hence , al — si , al — cu and al — mg alloys that start to solidify below approximately 639 centigrade can be grain refined with boron at an addition rate of 0 . 02 wt %. in summary , grain refinement of al — si foundry alloys upon the addition of boron occurs through the effective heterogeneous nucleation of aluminium on alb2 particles . alb2 is not a stable compound in al — si melts at typical boron addition rates of 0 . 02 wt %. this feature of boron addition is different from that of tib2 particles introduced with the al — ti — b based grain refiners . alb2 particles form in the melt only when the solidification process starts and provide the potent substrates for the nucleation of aluminium . hence , alb2 is an effective substrate for all alloys where the solidification of aluminium follows alb2 formation . this condition is readily satisfied in al — si with approximately 4 wt % si . this si level covers more or less the composition of the entire set of al — si foundry alloys . 356 and 357 aluminium foundry alloys with less than 0 . 01 wt % ti , could be effectively grain refined with 0 . 02 wt % boron addition , offering an average grain size after solidification of approximately 100 microns . this grain size is at least two times smaller than the average grain size obtained in aluminium foundry alloys with the present art and provides in foundry alloys that are normally typical of wrought alloys . the method described in this invention involves the control of titanium below 0 . 01 wt % in the alloy to be grain refined and the addition of 0 . 02 wt % boron into the alloy melt shortly before casting . boron addition could be achieved with al — b based master alloys regardless of boron content as well as with boron compounds such as kbf4 salt , boron oxide , borax as long as the final boron level in the melt is 0 . 02 wt %. since , the solidification of aluminium must follow the formation of alb2 particles for effective grain refinement in this practice , al — cu and al — mg alloys that start to solidify below approximately 639 centigrade can also be grained with this method . the grain size after solidification is extremely fine and the average grain size is invariably below 200 microns once these conditions are met . today , al — ti — b based grain refiners are employed in the grain refinement of aluminium foundry alloys . grain refinement is achieved in the present invention with the addition of boron into the aluminium alloy . the control of titanium level in the alloy to be grain refined is just as important as boron addition for an effective grain refinement . the effectiveness of boron addition is severely impaired when titanium control is ignored and titanium in the alloy exceeds 0 . 01 wt %. grain refinement experiments were performed with the commercial and ti - free alsi7mg alloys . the ti - free alsi7mg alloy was prepared in an electric resistance furnace by melting commercial purity aluminium ( 99 . 7 wt % al ) and adding elemental silicon and finally maintaining the temperature of the melt at 720 centigrade . the alloy melts thus obtained were inoculated with al - 5ti - 1b and boron additions . boron addition was made with an al - 3b master alloy as well as with kbf4 salt . reference samples were taken before boron additions in every test . alti5b1 and al - 3b master alloy and kbf4 additions were made so as to bring the boron concentration of the melt to 200 ppm boron . the melt was stirred with a graphite rod for 20 seconds after these additions and the inoculated melt was sampled 2 , 5 , 10 , 15 , 30 and 60 minutes later . these samples were solidified inside copper based permanent moulds with a diameter of 25 mm and a height of 50 mm . measures were taken to maintain the temperature of the melt at 720 ± 10 centigrade throughout these experiments . these samples were sectioned 20 mm from the bottom surface and were prepared with standard metallography practices for grain size measurements . they were etched with poulton &# 39 ; s reagent and were subsequently examined with an optical microscope . these samples were also anodized in barker &# 39 ; s solution ( 5 ml hbf4 ( 48 %) and 200 ml distilled water ) and were examined with an optical microscope under polarized light . the grain size measurements were performed with the linear intercept method according to the astm e112 - 88 standard . the grain structures before and after grain refiner additions are shown in fig3 . a modest grain refinement effect is observed in samples inoculated with the standard a1ti5b1 grain refiner ( fig3 a ). the grain refining effect of the al - 3b grain refiner in the case of the commercial alsi7mg alloy is very similar ( fig3 b ). however , the performance of the al - 3b grain refiner with the ti - free alsi7mg alloy is markedly better ( fig3 c ). grain size measurements from these experiments are illustrated in fig3 d and show the remarkable improvement in grain refinement with 200 ppm boron addition in the ti - free aluminium foundry alloys . this performance is much better than that obtained with the present art . fig3 — grain structures of alsi7mg alloys before and after the addition of grain refiner master alloys 1 . crossley , p . b . and mondolfo , l . f ., mechanism of grain refinement in aluminium alloys , transactions aime , 191 , 1143 - 1148 ( 1951 ) 2 . maxwell , i . and hellawell , a ., a simple model for grain refinement during solidification , acta metall ., 23 , 229 - 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