Determination of the mechanical properties of foundry moulding sand bonded with grade 3 Nigerian acacia species exudates.
Nigeria produces four grades of acacia species in commercial quantities. These are grade 1 (acacia Senegal); grade 2 (acacia Seyal); grade 3 (combretum) and grade 4, (neutral) . The annual total average production of the material rose to 20,000 tonnes in 2005 making the country the second largest producer in the world after Sudan . Acacia species exudate is a natural resin that contains arabin; a semi solidified sticky fluid oozing from incision made on bark of acacia trees . Gary and Ryan  reported that the trees grow more in Borno, Yobe, Sokoto, and Bauchi than the other states in northern Nigeria. According to Hirst et al  major acacia species is used for pharmaceutical, confectionary, food, textile and beverage production but not used much in the metal manufacturing processes. Ayeni  reported it as most abundant natural exudates in Nigeria. Okatahi  studied the tree characteristics and reported that the average minimum and maximum temperatures that make them thrive in Nigeria are 14[degrees]C and 40[degrees]C respectively.
Ominijei  investigated the use of it as an admixture in cement concrete mortar. Based on past uses it is believed that its mechanical properties may be compatible with those desired of sand binders that it could be used as foundry binders especially in Nigeria where foundries use imported materials as good ones are not locally available . Fennema  described acacia species as a complex variable compound mixture of arabino galactan oligosaccharide, polysaccharide and glycoprotein less consistent than the other hydrocolloids. As a mixture it varies significantly with source and exact molecular structure is uncertain. An IR analysis of Nigerian acacia exudates by Ominijei  revealed that it is composed of polysaccharide chains that have flexible and compact conformation like glycol protein dissolving in water to low viscosity Newtonian fluid at high concentration. The material has low chemical reactivity, excellent emulsifying, foam stabilizing and adhesive properties .
The main objective of this research is to use grade 3 Nigerian acacia species exudates (combretum) to bond moulding sand specimens, test them for above mentioned properties and compare result to the standard shown in table 1.1  to ascertain the potency and type of alloy for which it is suitable.
Materials and Methods
According to Dietert  green/dry compressive strengths; green permeability and harness; shatter index and moisture content test are the most tested of all mechanical properties of moulding sand. The tests give basic information of the other properties and desirability of sand. These tests were then carried out on prepared moulding sand specimens bonded with grade 3 Nigerian acacia species exudates using foundry equipment in laboratory and foundry workshop of Ajaokuta Steel Company Limited, Nigeria. Silica sand of known source with 0.03% clay and BS standard sieve size 40-72 was used to produce specimens. The sand was oven dried at 110[degrees]c to remove free water . A quantity of washed and dried sand was sieved with BS mesh to obtain required grain sizes . The grade 3 Nigerian acacia species sample was milled to the smallest possible grain size to enable even particle distribution within mixes.
The selected sand grains were thoroughly mixed in a roller for 10 minutes and then moulded into test specimens with different percentages of grade 3 acacia exudates as the binder. 2inches diameter by 2inches height test specimens of average weight of 130g were rammed in three blows with 6.5Kg weight from a height of 2inches . Two schedules of specimen preparation were used. The first involved specimens bonded with solutionized grade 3 acacia species in the ratio of 30/70% exudates/water. Binder content was varied from 2% and 4%, through 6% and 8% to 10% and 12%. The second schedule involved specimen bonded with powdered acacia species in percentages ranging from 3.0% and 4.5% through 6.0%, 8.0% and 9.0% to 11.5% and 13% by weight.
Speedy moisture teller was used and instantaneous readings of percentage of moisture were made from the gauge of instrument . Standard air pressure of 9.8 x [10.sup.2]N\[m.sup.2] was passed through the specimen tube containing green sand specimen placed in parameter of the permeability meter and time the taken for 2000 [cm.sup.3] of air to pass through the sand specimen was read to determine the permeability . The green and dry compressive strength was measured with universal strength testing machine. A steadily increasing compressive force was applied on the specimen until failure just occurred and the compressive strength in KN\[m.sup.2] read instantaneously. The dry compressive specimens were dried at 110[degrees]C for an hour and cooled to room temperature before the tests. A shatter apparatus was used to measure the shatter index of specimens .
Presentation of Results
The results for tests on the specimens moulded with the solutionized grade 3 Nigerian acacia species exudates and those bonded with powdered exudates are as presented in tables 3.1 and 3.2 respectively. Key mechanical properties investigated are the green and dry compressive strength; green permeability and hardness; shatter index and moisture content of moulded sand specimens. Moisture test determines the dampness of the sand casting mould; green and dry compressive strength called the bonding strength, measures the ability of the sand mould to withstand the pressure of molten metal during casting in the green or dry state; while green hardness measures the resistance of the mould against abrasion for dimensional accuracy. Permeability measures the ease at which gasses escape from the sand mould to forestall defects such as porosity and gas inclusions; while shatter index measures the collapsibility of sand for easy shake out and cleaning of casting. The results are transformed into graphs as shown in the appendix.
Discussion of Results
The result in table 3.1 and appendix A1a-A1c are for specimens bonded with the grade 3 Nigerian acacia species exudates in solution form. The solutionized exudate was added as the binder to the sample moulding sand in percentage weights ranging from 2.0% to 12.0%. The moisture content of moulded sand specimens increased from 2.0% to 4.0% with increase in binder content. This is because as the binder is in solution of ratio 3:7 (exudate: water) meant that more binder brought higher moisture into the mixture. The green compressive strength increased from 28KN\[m.sup.2] at 2% binder to the maximum of 43KN\m2 at 8% and then decreased to 39KN\[m.sup.2] at 12% binder. The moisture increased from 3.0% at 8.0% to 4% at 12% binder. Weakening of strength was caused by the increment in moisture showing that optimum moisture range and binder content were 2-3% and 8% respectively for green sand moulding. The result when compared with table 1.1 shows that the material in solutionized form is suitable as a binder for dry sand casting of aluminium, brass, bronze, light and malleable iron. Green compressive strength showed that in solutionized form it is unsuitable for both ferrous and non-ferrous alloy castings.
Though the green permeability decreased from 187No at 2% binder to 124No at 12% binder, by standard data in table 1.1, the value at each composition investigated is very suitable for above mentioned range of castings. The green hardness of specimens increased with increased binder from 44 to 60No with the values being suitable within the range of 8-12& binder content. Shatter index decreased with increased binder from 60No at 2% to 20No at 12% binder. The values are suitable in the range of 2%-8% solutionized binder. Optimum composition therefore is 8% solutionized grade 3 acacia exudates.
Table 3.2 and appendix A2a-c present the result of experiment with specimens moulded with powdered grade 3 Nigerian acacia species. The composition was made up of grade 3 powdered acacia binder content that varied from 3% to 13%, 3% water and silica sand grain for each specimen mix. The moisture decreased from 3.0% at 3% binder, through 2.5% at 8.0% binder to 2.0% at 13% binder. The moisture decreased with increasing binder content because the powdered acacia exudates reacted with more water for partial dissolution and to wet the sand grain to enable bonding action between sand and binder.
The green compressive strength of specimen increased sharply from 8.0KN\[m.sup.2] at 3% binder to 43KN\[m.sup.2] at 4.5% binder. Thereafter it increased steadily to 80KN\[m.sup.2] at 13% binder content. The dry compressive strength followed the same trend to maximum of 380KN\[m.sup.2] at 13% binder content. A comparison of the green and dry strength result to the standard in table 1.1 shows that the mix with 4.5-9.0% binder is suitable for green and dry sand mould casting of aluminum, brass, bronze, magnesium, light and malleable grey iron alloys. The specimen bonded with 11.5%13% dry powdered grade 3 Nigerian acacia is suitable for green sand moulding for light steel casting.
The green permeability decreased with increased binder from 201.1No at 3% to 143.5No at 13% binder. The result when compared with standard in table 1.1 showed the permeability values are suitable for the categories of castings determined by the compressive strength results. Specimen hardness increased from 40No at 3% binder to 68No at 13% binder. Shatter index decreased with increased binder from 102 at 3% to 70No at 13% binder. The result of green hardness and shatter index are satisfactory for range of castings determined above in respect of green and dry compressive strength of the test specimens. Though compressive strength continued to increase with binder the addition of more exudates could decrease the permeability and shatter index numbers to below the standard values for the applications specified above.
A comparison of the result in table 3.1 for specimens bonded with solutionized exudates with that in table 3.2 for sand moulded with powdered acacia showed that it gave better properties when added in powdered form than otherwise. Green compressive strength averagely increased by about 30%, dry strength 20%, permeability 25%, hardness 15% and shatter index 30%
The research result showed that grade 3 Nigerian acacia species exudate is suitable for binding moulding sand. It gave better bonding properties when added in its powdered form than in its solutionized form. In the solutionized form it is suitable for dry sand casting of non-ferrous, light and malleable grey iron at optimum mix of 8% binder composition. In powdered form it is suitable for above applications and light steel green sand mould casting. The efficacy of grade 4 Nigerian acacia species exudates would be studied next in the series.
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Nuhu A. Ademoh and A.T. Abdullahi
Dept. of Mechanical Engg., Bayero University, Kano, Nigeria E-mail: firstname.lastname@example.org
Table 1.1: Satisfactory mould property ranges for sand castings . Metal Green Compressive Permeability Dry Strength Strength No [KN|[m.sup.2]] [KN|[m.sup.2]] Heavy Steel 70-85 130-300 1000-2000 Light Steel 70-85 125-200 400-1000 Heavy Grey iron 70-105 70-120 350-800 Aluminum 50-70 10-30 200-550 Brass & Bronze 55-85 15-40 200-860 Light Grey iron 50-85 20-50 200-550 Malleable Iron 45-55 20-60 210-550 Medium Grey Iron 70-105 40-80 350-800 Table 3.1: Measured Foundry Property of Sand bonded with varying percentages Solutionized Grade 3 Nigerian acacia species exudates (30% exudates: 70% water). SAMPLE: Gum Arabic A B C D Grade 3 (Solution) Binder Content (%) 2.0 4.0 6.0 8.0 Moisture Content (%) 2.0 2.0 3.0 3.0 Green Strength (KN\[m.sup.2]) 28.0 33.0 36.0 43.0 Dry Strength (KN\[m.sup.2]) 253.0 260.0 268.0 274.0 Green Permeability (No) 187.1 120.3 107.4 126.9 Green Hardness (No) 44.0 45.0 49.0 50.0 Shatter Index (No) 60.0 57.0 55.0 45.0 SAMPLE: Gum Arabic E F Grade 3 (Solution) Binder Content (%) 10.0 12.0 Moisture Content (%) 3.5 4.0 Green Strength (KN\[m.sup.2]) 40.0 39.0 Dry Strength (KN\[m.sup.2]) 292.0 280.0 Green Permeability (No) 138.8 124.0 Green Hardness (No) 55.0 60.0 Shatter Index (No) 25.0 20.0 Table 3.2: Measured Foundry Property of Sand bonded with varying Percentages Powdered Grade 3 Nigerian acacia species and 3% water. SAMPLE: Gum Arabic A B C D E Grade 3 (Powdered) Binder Content (%) 3.0 4.5 6.0 8.0 9.0 Moisture Content (%) 3.0 3.0 2.8 2.5 2.2 Green Strength (KN\[m.sup.2]) 8.0 43.0 62.0 68.0 69.0 Dry Strength (KN\[m.sup.2]) 136.0 312.0 320.0 337.0 346.0 Green Permeability (No) 201.0 197.9 176.9 167.1 187.9 Green Hardness (No) 40.0 47.0 56.0 60.0 62.0 Shatter Index (No) 102.0 99.0 98.0 96.0 95.0 SAMPLE: Gum Arabic F G Grade 3 (Powdered) Binder Content (%) 11.5 13.0 Moisture Content (%) 2.0 2.0 Green Strength (KN\[m.sup.2]) 76.0 80.0 Dry Strength (KN\[m.sup.2]) 354.0 380.0 Green Permeability (No) 158.3 143.5 Green Hardness (No) 64.0 68.0 Shatter Index (No) 75.0 70.0