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MBUYA, TO, Sinclair I, Soady KA, Reed PAS.  2012.  Three‐Dimensional Analysis of Microstructure in Cast Aluminium Piston Alloys.
MBUYA, TO, Crump J, Sinclair I, Soady KA, Thomson RC, Ree PAS.  2012.  Short Fatigue Crack Growth Micromechanisms in a Cast Aluminium Piston Alloy.


Mbuya, TO.  2011.  Analysis of microstructure and fatigue micromechanisms in cast aluminium piston alloys . Abstract

Light vehicle engine pistons have traditionally been cast from near eutectic Al-Si cast alloys due to several favourable functional and processing attributes. The increasingly demanding engine performance requirements have necessitated the need for the development of multicomponent alloys with high alloy content and highly complex microstructure. In this regard, recent trends in new piston alloy development have been to increase the level of various alloying elements such as Cu, Ni and even Si. However, low Si compositions of ∼7 wt% Si and ∼0.7 wt% Si have also been proposed largely due to observations that the large blocky primary Si particles found in the near-eutectic alloys are potent fatigue crack initiators. Nonetheless, previous research on these low Si piston alloys has demonstrated that their fatigue performance is significantly impaired by porosity which increases with decreasing Si content. With improved processing techniques, porosity can be reduced to levels that make it impotent in fatigue failure processes. The aim of this work was therefore to characterise the microstructure and fatigue micromechanisms of the low Si piston alloys after hot isostatic pressing (hipping) to reduce porosity. This was achieved using a combination of various imaging tools and fatigue testing to establish the role of microstructure on initiation and growth of fatigue cracks. It has been demonstrated using X-ray microtomography that hipping significantly reduces porosity, especially in the 0.7 wt% Si alloy, while the intermetallic structures remain largely unaffected. The eutectic Si particles in the 7 wt% Si alloy are however transformed from a fine fibrous interconnected structure to coarse, spheroidised and discrete particles. Hipping has also been observed to improve the fatigue performance of the 0.7 wt% Si alloy due to the significant reduction in porosity. Fatigue crack initiation has been observed to occur mainly at intermetallic particles in both alloys after hipping and, consistent with previous work, the most frequent crack initiating phase is found to be Al9 FeNi. Analysis of short fatigue crack growth profiles has shown that intermetallics and eutectic Si particles preferentially debond, thus providing a weak path for crack propagation along their interfaces with the α-Al matrix. However, grain boundaries as well as these hard particles have also been shown to frequently act as effective barriers to crack growth. On the other hand, long fatigue crack growth analysis has shown that fatigue cracks tend to avoid Si and/or intermetallic particles at low ΔK levels (up to ΔK∼7 MPa√m). At higher levels of ΔK, the cracks increasingly seek out these hard particles up to a ΔK of ∼9 MPa√m after which the crack preferentially propagates through them. It has also been observed that crack interaction with intermetallics causes significant crack deflection which may result in roughness related closure mechanisms to be activated.

Mbuya, TO, Sinclair I, Moffat AJ, Reed PAS.  2011.  Analysis of fatigue crack initiation and S–N response of model cast aluminium piston alloys. AbstractWebsite

Fatigue crack initiation and S–N fatigue behaviour of hipped model Al7Si–Sr and Al0.7Si piston alloys have been investigated after overaging at 260 °C for 100 h to provide a practical simulation of in-service conditions. The results show that hipping did not affect the S–N behaviour of Al7Si–Sr. This is attributed to the lack of significant change in porosity distribution in this alloy because of its low porosity levels even in the unhipped state. However, hipping profoundly improved the fatigue performance of alloy Al0.7Si due to the significant reduction in porosity. In this investigation, it was observed that porosity was rendered impotent as a fatigue crack initiator in both hipped alloys. Instead, fatigue cracks were observed to originate mainly from intermetallic particles (particularly the Al9FeNi phase) in both alloys and sometimes from oxide particles in Al0.7Si alloy. Fatigue cracking was also frequently observed at intermetallic clusters in hipped Al0.7Si. The observed scatter in fatigue life is discussed in terms of the size of fatigue crack initiating particles and the overall particle size distribution which follows a power law distribution function.


MBUYA, TO, ODERA BO, Ng'ang'a SP, Oduori FM.  2010.  Effective Recycling of Cast Aluminium Alloys for Small Foundries. AbstractWebsite

A method of effective recycling of aluminium castings suitable for small foundries was investigated. Automotive cast aluminium scrap obtained from various scrap vendors was sorted into groups of similar components, namely; pistons, cylinder heads and housings (gearbox and rear-axle housings). This sorting method was adopted with the hypothesis that the resulting alloys could be closely equivalent to the commercial alloys that were originally used to make the components. The remainder of the scrap was grouped as miscellaneous scrap and contained various parts such as alternator covers, exhaust manifolds, oil sumps and other assorted scrap. As hypothesised, the chemistry of the resulting alloys were found to be consistently equivalent to the commercial alloys commonly used to cast the various components that were melted. For example, the alloy chemistry of secondary alloys from piston scrap was consistently equivalent to commercial piston alloys such as AC8B and LM26. Furthermore, alloys from cylinder head scrap were equivalent to commercial alloys such as 319, LM27 and AC2B. On the other hand, the alloy chemistry from unsorted scrap was not found to be consistent nor equivalent to a specific group of commercial alloys except for the 319 and 380.0 workhorse alloys. These results are discussed against the possibility of reusing the alloys in casting components similar to those that they were recycled from, in addition to other possible applications.


Mose, BR, Maranga SM, MBUYA TO.  2009.  Effect of Minor Elements on the Fluidity of Secondary LM25 and LM27-Type Cast Alloys. AbstractWebsite

In this study, the effect of individual and combined additions of Fe, Mn, Cr, Sr and Al-5Ti-1B grain refiner on the fluidity of secondary LM25 and LM27-type cast alloys was investigated. No change in fluidity was observed when the Fe level was increased from 0.14 to 0.4% in LM25, but it increased by 21% when Fe was raised to the critical content of 0.48%. Furthermore, a decrease of 32% resulted when the Fe level was increased to 0.6%. A combination of 0.3%Mn or 0.6%Cr with 0.6%Fe in LM25, resulted in a fluidity increase of 13 and 8%, respectively compared to the base alloy, but a combination of 0.6%Fe, 0.3%Mn and 0.2%Cr decreased the fluidity by 9%. A 34% increase in fluidity was observed when the Fe content in LM27 was raised from 0.41% to the critical level of 0.6%Fe with further increase when Mn was raised to 0.3%Mn. Increasing the Fe content to 1% in LM27 led to a drop in fluidity of 9%. Addition of 0.015%Sr and 0.02%Sr increased the fluidity of LM25 and LM27 by 9% and 21% respectively. Furthermore, a 0.28% Al-5Ti-1B grain refiner addition decreased the fluidity of LM 25 and LM27 by 2% and 19% respectively while a combined addition of 0.02%Sr and 0.28%Al-5Ti-1B decreased the fluidity of LM27 by 8%. Possible reasons for these observations are discussed.


MBUYA, TO, ODUORI MF, Rading GO, WEKESA MS.  2006.  Effect of runner design on mechanical properties of permanent mould aluminium castings. AbstractWebsite

A comparison of the influence of different runner designs on the mechanical properties of aluminium castings has shown that filtered rectangular runners (FRRs) yield aluminium castings with higher and more reliable mechanical properties than the conventional unfiltered rectangular runners. Unfiltered vortex flow runners have also been shown to improve the reliability of the modulus of rupture of cast aluminium alloys over unfiltered rectangular runners. In the present paper, experimental results of a comparative study on the effect of the unfiltered vortex flow and FRR designs on the tensile strength of permanent mould aluminium castings are reported. The results show that an FRR yields castings with higher and more reliable tensile strengths than the unfiltered vortex flow runner. Castings poured into a mould with an FRR had strengths between 269 and 291 MPa and a Weibull modulus of 50·2 while castings poured into the vortex flow runner had strengths between 255 and 280 MPa and a Weibull modulus of 40·3.

OCHUKU, MRMBUYATHOMAS.  2006.  T.O. Mbuya, G. O. Rading, M. F. Oduori, and M. S. Wekesa . KLB. : WFL Publisher
OCHUKU, MRMBUYATHOMAS.  2006.  T.O. Mbuya, B.O. Odera, S.P. Ng. KLB. : WFL Publisher
OCHUKU, MRMBUYATHOMAS.  2006.  M.F. Oduori and T.O. Mbuya, . KLB. : WFL Publisher
OCHUKU, MRMBUYATHOMAS.  2006.  T.O. Mbuya, B.O. Odera, S.P. Ng. KLB. : WFL Publisher
OCHUKU, MRMBUYATHOMAS.  2006.  M.F. Oduori, D.M. Munyasi, and T.O. Mbuya, . KLB. : WFL Publisher



Mbuya, TO.  2003.  studies on the microstructure and mechanical properties of recycled cast aluminium scrap . Abstract

The aluminium casting industry in Kenya is strongly dependent on aluminium scrap recycling. There is, however, little organised information on appropriate recycling procedures and the expected chemical composition of the resulting secondary alloys. Furthermore. little is also , known on the expected mechanical properties of castings produced from the secondary alloys . under different processing conditions. Consequently, the quality of locally produced castings is poor compared to their imported counterparts as most Kenyan foundries rely on chance to make good castings. The main objectives of this preliminary study were: (a) to determine the chemical composition of the secondary alloys obtained from recycling various cast aluminium scrap components available in Kenya; and (b) to determine the microstructure, tensile strength. percent elongation and hardness obtainable from these alloys using both green sand and permanent mould casting processes and under different process conditions. The process parameters for both green sand and permanent mould casting processes, whose influence on the said properties was investigated . are: the type of mould filling system, melt handling and pouring temperature. The influence of the thickness and initial temperature of permanent moulds on these properties was also investigated. It was found in this study that the common types of scrap components available in Kenya are automotive engine parts. Most of these components were found to be pistons, cylinder heads, gearbox housings and to a lesser extent, rear axle housings. Pistons and cylinder heads were classified as individual groups while gearbox and rear axle housings were grouped together. The rest of the scrap items, most of which could not be identified, were lumped together to form another separate group. Samples were randomly picked from these scrap groups and • individually remelted to obtain secondary alloys. Other secondary alloys were also prepared by • blending the above scrap groups in predetermined proportions. • The resulting secondary alloys from these groups of scrap •c9mponents were all hypoeutectic aluminium-silicon-copper (AI-Si-Cu) based alloys. The samples in similar scrap groups were v \. < fairly equivalent in chemical composition although some minor variations were observed. Some samples from different groups were also found to be fairly equivalent In their chemical composition. Furthermore, all the samples were also fairly equivalent. In their chemical composition, to several common commercial hypoeutectic AI-Si-Cu based alloys. Permanent mould casting resulted in castings with higher tensile strength, percent elongation and hardness compared to green sand casting in all scrap samples tested. Both quiescent melt handling and quiescent bottom filled mould systems yielded castings with higher aforementioned properties than turbulent melt handling and turbulent top filled mould systems . . respectively. In addition, these mechanical properties were found to decrease with increasing pouring temperature and mould preheat. Increasing the mould thickness was, however, found to increase these mechanical properties. Heat treatment of some of the alloys to the T6 condition increased their tensile strength and hardness, but decreased their percent elongation. The microstructure of all the recycled alloys was typically similar and contained cc-aluminium matrix, eutectic silicon particles, iron-bearing and copper-bearing intermetallics. The tensile strength and percent elongation of the recycled alloys did not correlate well with those of their respective commercial counterparts. This was particularly so with top filled green sand castings. The mechanical properties of bottom filled samples were. however. closer to those of their commercial counterparts albeit falling on the lower end in the range of reported . mechanical properties of their respective equivalent commercial alloys. The hardness values of the samples, however, correlated well with those of their commercial counterparts. It is inferred from the results of this study that using the type of scrap component as the sorting criterion is of limited usefulness because some samples cast from different scrap components were fairly equivalent in their chemical composition. Furthermore. some scrap items are either too few to recycle separately and/or difficult to identify. However, in the absence of appropriate facilities to determine the alloy chemistry on-line and make appropriate melt corrections. this sorting criterion together with the chemical compositions of the samples studied in this study can be a useful guide. Also inferred is that blending scrap samples is not useful because no . definite trend in the chemical composition was observed with blending. Finally the processing conditions strongly influenced the mechanical properties of the samples . . It is therefore important to choose the right casting process and to closely control the mould design, molten metal processing and other process parameters like pouring temperature and initial mould temperatures in permanent mould casting. In particular. permanent mould casting should be preferred to green sand casting if maximising mechanical properties is the major factor for process selection. In addition, mould filling systems should be skilfully designed to quiescently introduce molten metal into the mould cavity preferably via bottom filling. The pouring temperature should also be as low as possible.

OCHUKU, MRMBUYATHOMAS.  2003.  T.O. Mbuya, B.O. Odera and S.P. Ng. KLB. : WFL Publisher
OCHUKU, MRMBUYATHOMAS.  2003.  T. O. Mbuya . KLB. : WFL Publisher


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