The influence of the parameters of lost foam process on the quality of aluminum alloys castings

Abstract

This paper presents the research results of application of Lost foam process for aluminum alloys castings of a simple geometry. The process characteristic is that patterns and gating of moulds, made of polymers, stay in the mould until the liquid metal inflow. In contact with the liquid metal, the pattern intensely and in relatively short time decomposes and evaporates, which is accompanied by casting crystallization. As a consequence of polymer pattern decomposition and evaporation, a great quantity of liquid and gaseous products are produced, which is often the cause of different types of casting errors. This paper presents the results of a research with a special consideration given to detecting and analyzing the errors of castings. In most cases the cause of these errors are defects of polymer materials used for evaporable patterns production, as well as defects of materials for refractory coatings production for polymer patterns. The researches have shown that different types of coatings determine the properties of the obtained castings. Also, the critical processing parameters (polymer pattern density, casting temperature, permeability of refractory coating and sand, construction of patterns and gating of moulds) significantly affect on castings quality. During the research a special consideration was given to control and optimization of these parameters with the goal of achieving applicable castings properties. The study of surface and internal error of castings was performed systematically in order to carry out preventive measures to avoid errors and minimize production costs. In order to achieve qualitative and profitable castings production by the method of Lost foam it is necessary to reach the balance in the system: evaporable polymer pattern - liquid metal - refractory coating - sandy cast in the phase of metal inflow, decomposition and evaporation of polymer pattern, formation and solidification of castings. By optimizing the processing parameters castings of required structural and mechanical characteristics could be achieved, casting defects characteristic for this process could be avoided with the result of the production costs saving. The presented results can be useful to define parameters of the Lost foam process and its application in practice with objective to obtain castings within advance specified quality.

U radu su prezentirani rezultati istraživanja primene 'Lost foam' procesa (tj. livenje primenom isparljivih modela) za livenje odlivaka legura aluminijuma. Pokazalo se da se povoljni uslovi livenja ovom metodom postižu primenom polistirenskih modela sa manjom gustinom, 16-20 kg/m3, jer se brže razlažu i isparavaju. Istražena je primena različitih vatrostalnih punioca (talka, cirkona, kordijerita, liskuna, mulita) u sastavu premaza za polistirenske modele. Optimizacija sastava premaza sa kontrolisanim reološkim svojstvima postignuta je primenom različitih komponenti premaza, kao i izmenom postupka izrade premaza. Ocenjeno je da vatrostalni premaz na bazi talka sasvim zadovoljava uslove livenja legura aluminijuma. Premazi istraženog sastava, gustine 2 g/cm3 lako se nanose na polimerne modele, ravnomerno se slivaju pri prelivanju i potapanju, lako se premazuju četkom bez tragova četke, curenja, stvaranja kapi, grudvi. Osušeni slojevi premaza su ravnomerne debljine, bez pojava mehurića, ispucalosti, ljuštenja ili otiranja. Primena ovog tipa premaza na alkoholnoj i vodenoj osnovi, tanjih slojeva (0,5 mm) i polistirenskih modela manje gustine (20 kg/m3) pozitivno utiče na kvalitet površine, strukturna i mehanička svojstva odlivaka. Na strukturu i svojstva odlivaka utiču i drugi procesni parametri koji su istraživani u korelaciji sa gustinom modela, vrstom i debljinom sloja vatrostalnog premaza, a pre svega istraživan je uticaj temperature livenja, vrsta i veličina zrna peska za kalupovanje, odnosno propustljivost peska za kalupovanje, konstrukcija modela i ulivnih sistema. Rezultati istraživanja mogu korisno da posluže za definisanje ove metode i njene primene u praksi za dobijanje odlivaka unapred zadatog kvaliteta.