MegsNotebook < Brunelleschi

FINAL PRESENTATION: BRONZE CASTING PROBLEMS

Notes to go with poster -- 7 June, 2005 (see BronzeCasting for background and more refs) %MRC% Useful discussion of the common problems and some good links. Some minor notes/additions from me below. Look for stuff in square brackets [like this] -mrc

Porosity

Casting porosity [often results in] depressions on the surface of a caste bronze object. It is caused either by premature cooling or air bubbles that become trapped and act as part of the mold when the caste [cast?] is forming. In the lost wax casting process, air bubbles are caught when the molten bronze is being poured into the mold. Porosity is a problem for aesthetic reasons when the bubbles are trapped on the surface and for strength reasons if they are trapped within the bronze, which causes weakness. Porosity due to premature cooling can be avoided through maintaining the molten bronze at a temperature well above the melting point, which will maintain fluidity of the metal while pouring. %MRC% (Porosity is especially a problem for functional parts like automobile wheels that need to be airtight)

Shrinkage

Shrinkage in the casting process is due to the thermal properties of the metal itself. Bronze, like most metals, is naturally less dense as a liquid than in its solid state. Therefore, molten bronze takes up more space than hardened bronze. As the liquid metal cools and solidifies in the mold, it shrinks, causing the finished casting to be smaller than the mold it was created from. The technical problem with skrinkage is that it can leave cavities in the casting, which could weaken the structure. To avoid shrinkage, oversized molds are used to create a final casting in the desired dimensions. %MRC% Bronze shrinks something like up to 10 percent as a I recall - a significant amount. The real problems occur when the casting has combinations of thick and thin cross sections. The thin parts cool faster and solidfy, then the thick parts cool. Because the part cools unevenly it can't pull away from the mold evenly and therefore local depressions and gaps result. See notes by Gutowski (MIT slides) for design suggestions to avoid shrinkage problems.

Hydrogen Embrittlement

Hydrogen embrittlement occurs in bronze casting when hydrogen molecules fail to dissipate from the bronze. It is also possible for hydrogen to enter into a casting during corrosion reactions. The hydrogen combines with carbon, forming methane gas that collects in voids in the metal, building up pressure that can initiate cracks. Therefore, the presence of hydrogen in the casting reduces the ductility and load-bearing capacity of the metal, which can result in fractures due to stresses below the standard yield. To avoid the inclusion of hydrogen in bronze castings, degasifiers are now added to the molten metal to combine with the hydrogen molecules and pull them out of the casting.

Liquid Metal Embrittlement

Another possible form of embrittlement is that caused by the addition of molten bronze over a layer of already cooled bronze. It is for this reason that a bronze casting must be completely poured the first time, as the addition of a second layer of bronze will cause liquid metal embrittlement, decreasing the ductility of the structure. When liquid bronze comes in contact with hardened bronze, decohesion can occur, resulting in a decrease in the mechanical properties of the metal. In short, adding liquid bronze over hardened bronze decreases the tensile strength and fractures can occur at much lower levels of stress. The only way to avoid this form of embrittlement is to caste a bronze piece all at once, thereby maintaining a homogeneous temperature throughout the cooling process.

BIBLIOGRAPHY

Today we will be discussing three prominent figures of the Renaissance with the intention of demonstrating the emergence and subsequent specification of engineering and scientific professions. At the start of the Renaissance, it was standard practice of ruling figures to hire “men whose genius lay in the fact that they knew how to do everything.” These so-called “Renaissance men” were in essence universal engineers, and Fillipo Brunelleschi, Leonardo da Vinci and Galileo Galilei represent great minds that were either members or products of this system. As we will show, the emergence of these figures led to technical progress in the era.

I am Leonardo da Vinci, born in 1452 but most of my work was done in the late 15th and early 16th century. Although my illegitimate birth left me with a very low social standing, it was increased through my apprenticeship to the prominent painter and sculptor Andrea del Verrocchio. As I eventually worked my way up from apprentice to master my social status was further increased, peaking with my employment as the court artist and engineer under the Duke of Milan, Ludorico Sforza. In my time, I was considered to be an engineer, which was defined as an inventor and builder of ingegni, or complex and simpler machines. This was reinforced by my creation of military contraptions, such as artillery and fortresses, as well as a system of locks for canals in Milan. In addition to these technical inventions, I also was responsible for a number of paintings and sculptures in my lifetime, true to my training. Last but not least, while I resided as the official engineer, architect and artist to the King of France later in my lifetime, I focused on examining cadavers to create anatomical drawings of the human body, as well as drawing up architectural plans and doing set designs for court entertainment. Although these accomplishments in my lifetime gained me social prestige and respect during my time period as an engineer, by modern definitions I am viewed more as an inventor. This is due to my more theoretical and observational methods to designing machines. Now the term “engineer” implies a more specialized and technical area of focus than what I am remembered for.

The Duomo in Milan, Italy is a gothic cathedral completed in 1418, after 32 years of construction. The immense building is constructed of marble, with an intricate system of repeated arches and flying buttresses to support the massive stone structure. Additionally, as described in Gordon's writings, marble statuary is added to the pillars flanking the pointed arches, thereby providing increased weight to stabilize the building. The enormity of the arches, buttresses and cathedral overall were awe-inspiring to witness, and caused me to consider how they could have possibily constructed such large structures. It doesn't surprise me that it took 32 years to construct the building when I contemplate the massive quantities of marble that it was required to hoist up to such heights, and furthermore the skill it must have taken to piece together the marble blocks such that the finished cathedral maintains it's stability even now, over half of a millenium after the initial construction. In addition to the impressive engineering skill needed for this building, I also was amazed at the sculptural abilities needed to carve the impressive statuary that covers every inch of the building's fascade for both aestetic and structural purposes. Each flying buttress is intricately and almost identically carved, while all of the gothic arches is given stability through the addition of sculpted figures both on the top of the supporting pillars as well as in rows around them. I suppose my fascination with the Milan Duomo can best be summed up by this amazement at the vast quantity of art used with an engineering motive and not just for aesthetic purposes.