Theoretical and Practical Aspects of the Modification of Al-Si Alloys
Modification was first observed when Al-Si alloys were produced by electrolysis in Hall cells. These alloys contained dissolved sodium present from the partial decomposition of sodium cryolite. Pacz later stirred sodium fluoride into melts, and Alcoa researchers plunged Na metal into Al-Si melts. The dissolved sodium changes (modifies) the shape of silicon crystals formed during solidification. The change in microstructure is profound. (Figure 1.) Instead of large plates, the eutectic Si is present as fine filaments. Silicon crystals are hard and brittle. The fine Si crystals produced by modification improve elongation and tensile strength in castings. So, most Al-Si (3xx) alloy castings are modified.
The mode of solidification also changes. Figure 2 shows the internal structure of a cylindrical casting. The surfaces shown in this figure were in contact with a hot carbon plate. Halfway through the freezing process the carbon was rapidly quenched. What we see, therefore, is the state of the solid-liquid interface midway through freezing. In the absence of modification, small ‘islands’ of eutectic nucleate in the liquid ahead of the advancing solidification front. When sodium is added, the eutectic islands no longer form (nucleate), and the solid/liquid interface becomes planar. (Note also the top portion of Figure 1.) As will be shown later, this change in the mode affects how risers ‘feed’ the casting, and how and where shrinkage and porosity form.
In view of these difficulties, researchers sought other ways to produce the desirable modified structure. A number of other elements modify silicon. Strontium and antimony found extensive commercial application, and it will be useful to focus on these first. A detailed comparison between the two modifiers is instructive. Afterwards, we consider the use of Na and Ca.
Foundries started to use Sr in the 1970s, but they experienced problems with gas porosity. This was primarily because foundries at that time did not have an effective means to degas their metal. Also, the effect of Sr on porosity formation was poorly understood. However, there was also another problem that I encountered.
In 1980, I began working at a master alloy producer, called Kawecki Berylco Industries (later KB Alloys). One of their products was an aluminum master alloy containing strontium and silicon. The KBI master alloy also contained calcium, because it was made by alloying strontium silicide produced in calcium carbide furnaces. It was a terrible product and gave strontium a bad reputation. It took me years to convince KBI to stop producing the Al-Sr-Si product, and instead produce Al-Sr master alloys made from pure Al and Sr.
The situation at the time was outlined by Pechiney and Montupet engineers. They poured an end-chilled sand mold bar casting. The design of the casting is shown in Figure 3. Porosity was determined in this square bar by measuring the density (specific gravity) in sections cut along its length. Measurements were made in Na-, Sr-, Sb- and non-modified A356 alloys. In the Na- and Sr-modified alloys there was an increased amount of porosity near the riser. The results for the Sr-modified alloy are shown in Figure 3. (The results for Na-modified alloys were similar, but are not duplicated here.) The increase in porosity is most pronounced at higher addition levels of strontium and in the slowly cooled portions of the casting.
By contrast Sb-modification did not produce an increase in porosity. Moreover, there is no loss of Sb with holding or remelting. The modification was essentially permanent. This was the commercial basis for the marketing of the Calypso alloys. These alloys were used for many years to produce castings; especially wheels; primarily in France and Japan.
The porosity formation associated with Sr modification has been the subject of numerous studies over the years. There has been (and still is) a good deal of confusion about what is actually happening. A better understanding is useful for the purpose of this discussion, as well as having commercial relevance. Hence, a brief summary of the mechanisms involved is offered here. What follows is a personal view based on published studies and my experience in the foundry.
It is generally agreed that strontium additions do not result in any appreciable gas pickup. The evidence regarding the effect of Sr on gas pickup from the atmosphere is less clear. Experiments with small melts held under atmospheres with controlled humidity show that strontium additions do increase the gas, by changing the composition of the oxide on the melt surface. Other foundry experiments suggest that strontium has little effect. My own experience in the foundry is in agreement with the observations offered by Hurley and Atkinson. As long as excessive strontium contents and high melt temperatures (>1400F [760C]) are avoided, there is no significant increase in gas pickup from the atmosphere.
Strontium does affect porosity formation during solidification. Some of the best studies were made by Anson and co-workers because they were careful not to confuse gas porosity with shrinkage porosity. Figure 4 is reproduced from their 2000 AFS Transactions paper. As noted by many over the years, there is a ‘threshold’ gas content, below which very little porosity forms. Anson and co-workers found pores nucleate sooner when Sr is added to the melt. In some experiments, they employed a fast quench during solidification. In this way, they determined at what fraction solid porosity formed. Strontium additions were shown to cause pore formation earlier in the solidification process.
The cause of the results shown in Fig. 4 were debated for many years. Finally, Samuel and co-workers conducted a detailed study of pores in Sr-modified castings. They observed copious amounts of small Sr-containing oxides on the surfaces of the pores. Although Sr-containing oxide films were also observed, the very small oxides were presumably contained in the Al-Sr master alloy used to make Sr additions. Whatever their source, they are better at nucleating pores than the aluminum oxides found in non-modified alloys.
One consequence is that pores in modified melts tend to be larger. This is undesirable, especially for applications where fatigue life is important. Fortunately, good grain refinement tends to counteract this growth in pore size.
This is not the whole picture, however. Strontium modification changes the shape of the solid/liquid interface during solidification of the eutectic. One result is that the liquid ‘channels’ in the freezing (semi-solid) metal are larger. This allows for feeding of liquid from risers to occur longer (until a greater fraction solid) during solidification. This apparently occurred in Anson’s experiments. When melts were prepared having low gas contents—below threshold values—the results in Figure. 5 were obtained. The Sr-modified melt is seen to be lower in porosity than the non-modified alloy. The larger amounts of porosity formed at the end of solidification in Anson’s non-modified melts is presumably related to shrinkage formation caused by poor feeding.
This provides only a glimpse into a complex subject. It is, however, germane to the present discussion. In the 1980s, rotary impeller degassers were introduced into foundries. The resulting ability to easily reach low gas contents has largely removed the stigma of ‘gas problems’ associated with Sr modification. Strontium is also easy to add, and its loss or ‘fade’ with time in the furnace is relatively slow. In North America only a handful of foundries still use sodium to modify their metal. The other foundries now use strontium.
Sodium is still used today as a modifier by some foundries, especially in Europe, but the addition is usually made now in the form of Na-containing salts. This procedure is easier to control; recovery of Na is more consistent; and the salts help to remove oxide inclusions suspended in the melt.
Antimony is no longer used. The reasons include (1) cost of addition––five to 10 times more that strontium; (2) recycling––essentially permanent (Figure 6); (3) heat treatment––the saving of reducing solution time from eight to four hours is still 10 times the cost of the Sr addition; (4) quality––Figure 8 plots the quality index from a study of mechanical properties in low-pressure die castings.
In summary, improved properties and shorter solution times are possible with strontium. The widespread use of rotary impeller degassers now makes it easy to obtain low hydrogen contents in our melts. Antimony is a heavy metal and many are concerned about possible health risks associated with its use in the foundry. It is also more costly to add. For all of these reasons, antimony is now rarely used commercially in Al-Si casting alloys.
BENEFITS AND BEST PRACTICES
In laboratory studies, nearly a dozen elements produce modification in Al-Si alloys. However, only four have been used commercially in foundries: Na, Sr, Sb and Ca. The use of antimony was discussed in detail above, and will not be considered further. The other three are considered individually below.
Strontium is most commonly used today, so this is where we begin.
Modification with Sr offers the following advantages:
· Castings have improved tensile properties, especially elongation
· Solution heat treat times can usually be shortened. This provides an important cost saving and allows for greater productivity through the furnaces.
· Feeding from risers is usually improved, although this depends on mold design. I have seen many examples where Sr additions have ‘cured’ a shrinkage problem. However, this is frequently not a ‘miracle cure.’ (When shrinkage is replaced by microporosity that appears on machined surfaces.)
And there are disadvantages:
· Porosity tends to increase in areas of the casting that cool more slowly.
· Changes in the mode of solidification may mean a mold design must be changed. That is, a mold producing sound castings in non-modified (or Na-modified) metal may have unacceptable shrinkage when Sr-modified.
The following are recommended best practices when using strontium as a modifier:
Sr addition options: Adding strontium is easy. There are several master alloys available for this purpose. Rod products are generally fast acting, and give high recoveries. Waffle products are slower to dissolve, and recovery may be more erratic. Higher metal temperatures promote more rapid dissolution. The exception is the 90Sr-10Al product, which is exothermic and goes into solution best when the melt temperature is low. Strontium recoveries are generally high, 90 percent or more, with the possible exception of waffle products
Addition levels: The best modification in A356 alloy occurs at a level of about 0.012% Sr, or 120 ppm.26 Alloys containing higher silicon or copper may require more Sr, than the 120 ppm recommended for A356 alloy However, good modification can be obtained at lower concentrations, depending on alloy purity. In primary metal, the P level is usually below 10 ppm and other ‘bad’ impurities (Sb and Bi) are absent, so full modification can be obtained with strontium contents as low as 60 to 70 ppm. Good modification can also be obtained at higher levels of Sr, although this tends to produce more porosity. Strontium levels more than approximately 0.02% result in coarser silicon particles and a slight reduction in elongation.
Larger Sr additions may result in the formation of an undesirable Al2Sr2Si phase. A recent study of the Al-Si-Sr system27 has shown that the ternary eutectic:
Liquid → Al + Si + Al2Sr2Si
occurs at 0.2 wt % (2,000 ppm) Sr. Because of strontium segregation during solidification, this phase can form at levels less than 2,000 ppm. For example, it is commonly found in die castings made with Mercalloy, which contains 0.05 to 0.07 % Sr for improved resistance to die sticking. However, Al2Sr2Si does not form at concentrations less than about 0.03%.
Timing of the addition: Several have found that Sr is slow acting, showing an ‘incubation time’ of one or two hours. That is, modification was observed to improve with time after the addition, even though part of the Sr was lost to oxidation. Recent research has established that most of this delay is caused by the time needed for intermetallic particles in waffle products to dissolve. Strontium-containing rod products usually have small particles of SrAl4 and are fast dissolving and fast acting. With rod additions, no more than about 10 or 15 minutes is needed to obtain a good modification. The melt loss of strontium is relatively slow, so it will often be convenient to have Sr added by your metal supplier. If you add Sr in the foundry, make sure the addition is made before degassing.
Degassing: Degas with nitrogen or argon. Do not use chlorine to degas, or use chloride- or fluoride- containing fluxes during degassing. These will react with and ‘burn off’ Sr dissolved in the melt.
Holding: Avoid excessive strontium additions and high melt temperatures (> 1400F [760 C]).
Quality Control: Under normal conditions a spectrographic analysis for Sr is sufficient to control modification when using primary metal. However, some tramp elements can 'poison' the effect of modifiers. These may become a concern when secondary metals or scrap are used. In this case, it would be wise to check modification using thermal analysis.
The advantages and disadvantages of Na-modification are similar to those previously outlined for strontium. However, there are differences as follows:
· At higher concentrations Na produces a thick oxide film on the liquid melt. This oxide was frequently called ‘elephant skin’ in older German publications. This oxide can cause fluidity problems in some castings.
· As mentioned, alloying with metallic sodium makes it difficult to control modification. One of the results is a sporadic problem called ‘over-modification’. Two examples are shown in Figure 9.
The structures in Figure 9 are caused by the relatively low solubility of Na in liquid metal, and by the planar solidification front (shown top of Fig. 1). The latter requires a higher velocity of the liquid/solid interface compared to non-modified alloys. This makes it difficult for Na to diffuse away from the advancing solid. The Na concentration builds up until the intermetallic compound NaAlSi forms. According to Ransley and Neufield,7 the compound is NaAlSin, where n = 1.25-1.33. This depletes the Na and some of the Si at the liquid/solid interface, and a ’halo’ of aluminum is deposited. This grows until Si (and Na) increase and the eutectic once again appears.
These two disadvantages can be ameliorated by adding Na via a salt flux. Foseco in Europe has developed a suitable Na-containing flux. Sodium is added using a combined flux feeding/degassing system called the metal treatment system (MTS). A degassing rotor is first placed in the crucible or ladle. It generates a vortex by spinning at 600−750 rpm. The flux is fed into the vortex. This takes 20−30 seconds. The rotor speed is then decreased and a baffle plate is lowered into the melt. This eliminates the vortex and the melt is then degassed. The recovery of sodium is relatively high, and more importantly, more consistent. Also, the flux treatment cleans the metal by removing oxides.
The published information on optimum Na content is fragmentary, but the desired spectrographic analysis of Na appears to be in the range of 60-90 ppm for primary A356 alloy. (Note: lab technicians will need to handle coupons carefully. Fingerprints on the surface will result in false high values for sodium.) Eutectic or secondary alloys may require an addition as high as 90 to 120 ppm.
Calcium lies above strontium in the periodic table, and the two elements are chemically very similar. Laboratory studies show that Ca is similar to Sr as a modifier.
However, there are important differences to consider:
· Calcium is not as effective as a modifier, in castings (or sections of castings) where the cooling rate is slow.
· Calcium tends to increase porosity, and makes the alloy more susceptible to gas pickup from moisture in the atmosphere. When I worked for Alcoa some customer complaints of excessive porosity were traced to high Ca levels in the ingot. I recommended a maximum permissible level of 35 ppm Ca.
· There is an undesirable reaction between Ca and Sr, which reduces the effectiveness of modification, when both elements are present.
· To a limited extent, Ca reduces the size of iron-bearing intermetallics. This improves ductility and impact strength.
Calcium was demonstrated to be useful commercially as a modifier by the Chrysler casting plant in Etobicoke, Ontario. Previously, they had difficulty maintaining control of sodium content, and in the 1950s switched to calcium. Metal was supplied in liquid form from a secondary smelter, located 30 miles away. The Ca was present in delivered metal, and any ‘touch up’ additions were made by adding Ca metal. Two products were produced using Ca-modified metal: pistons cast in 332 alloy and master cylinders cast in 322. Both were small permanent mold castings, so they should be relatively tolerant to gas dissolved in the metal. A paper describing Chrysler’s 30-year experience with calcium modification was presented at a 1986 AFS Metalcasting Conference. The gas content of metal delivered to the mold was reported to be 0.2 cc/100 grams. This is a moderate gas level.
Calcium appears to be a viable modifier for many smaller permanent mold castings. However, Ca and Sr are approximately the same price, so there is no real incentive not to use Sr—the preferred choice.
We have learned a lot in the 100 years we have been modifying the structure of Al-Si alloy castings. But there surely still is a great deal yet to be revealed to us.