Machinability of high silicon content aluminum alloys

Richárd Horváth ¹ – Sándor dr. Sipos ²

• – Institute Engineer, Óbuda University, Bánki Donát Faculty of Mech. Eng.

• – Associate Professor, Óbuda University, Bánki Donát Faculty of Mech. Eng.

Introduction

The quality of the products is increasing continuously: on the one hand, it can be led back to the development of the machine tools, on the other hand, to the presence of new geometries and tool materials. This progress is accompanied by ever-increasing productivity (reduced productive time, shorter non-productive times and production period) as well. It can be noticed especially in case of high technologies, characteristical for the modern automotive industry, defense industry, aircraft industry and aerospace industry.

The industries, mentioned earlier, use preferred aluminum cast alloys, especially the versions, alloyed with silicon, copper, magnesium. In these alloys excellent mechanical features (hardness, strength) and appropriate technological advances (excellent castability, machinability, corrosion resistance, weldability) are combined. Parts, made in the 80’ and made from aluminium with high silicium content, have spread in the automobile industry (for example, fixtures of engines, compressors, steering devices) and their unfavourable machinability causes several problems.

In our article we are going to introduce the difficulties, arising during the turning operations of the fixture of large-scale produced compressor.

1. Machinability of high silicon content aluminum alloys

The aluminum alloys, having a silicon content of higher than 11,8% are called hypereutectic, and almost all of them have good strength characteristics, higher fatigue limits and excellent wear resistance. One of the circumstances, making the machining difficult, is that aluminum is an easy-to-machine material, soft and ductile; but with increasing the Si-content, the abrasive effect of the alloys increases and the difficulties, arising during the machining, are on the increase. Because of the primary silicon crystalls, embedded in the aluminum matrix, the chips become easy-to-break, however, the presence of these hard particles leads to the quick wear of the insert, due to their strong adhesion and chemical reactions as well as low abrasive resistance with Al-Si alloys. In case if the primary Si-particles contact the tool edge in the cutting zone then they wear it intensive, furthermore, due to their hardness they hinder the formation of surface of good quality. Therefore, the precondition of the favourable surface roughness is the even dispersion, small grain size and favourable shape of the primary Si-particles, otherwise the particles, adhered to the edge on the „adhesion way”, can „plough” the surface completely, to be machined.

The situation can be even more complicated if the interdendritic region contains a high number of resistant intermetallic precipitates and inclusions. The cause-effect diagram (the so called Ishikawa diagram) of the unfavourable machinability of these cast alloys can be seen in Figure 1 [1, 2].

2. Description of the goals and circumstances of the tests

The main goal of the tests was to get clear picture how the tools of different materials and with different construction can meet the extremely rigorous roughness standards, applied in the automobile industry. Different surface roughnesses, made with hard metal from K group and polycristallic diamond, have been compared, where the dry turning operations have been performed with tools with different point angles and nose radiuses; the cutting speed has been varied in a very wide range (v_c=200…2000 m/min). The depth of cut – due to reason of saving materials – has been kept on a constant value (a=0,5 mm), other testing conditions can be seen in Table 1.

Table 1.

3. Test results

Due to space limitations it is not possible to introduce the results of our systematically performed tests; the research experiences of our far-reaching examinations will be summarised in the following parts. The below description contains the surface roughness values, measured along 3 different measuring lines, at setting every single of the test values.

3.1. Effects of the tool geometry

The most important features of the tool edge geometry are the point angle and nose radius of the insert and the tool edge quality.

In case of light metal alloys it is a general accepted principle to apply the point angle as small as possible: during the turning operation the surface roughness is considerably better if the chip has enough space to leave, with other words: the chip space is wide (is not limited) [3]. This value of angle is in case of CCMW insert 80°, while in case of DCMW it is only 55°. As it can be seen in Figure 2/a well: the value of Rz (the so called ten point height) is by 20 – 40% smaller in case of application of sharper point angles. What is surprising in this fact is the rate and this phenomenon can be noticed so significantly even in case of hypereutectic Al-Si alloys.

The volume of point angle has an evident effect: the increase of the insert radius favourably affects the surface roughness. In Figure 2/b the different (measured and calculated) surface roughness values can be seen: the Rth value means the surface roughness data, calculated with the Bauer-formula, the RBr values are calculated with Brammertz-equation. This last relationship can be convenient to provide a basis for the determination of a reasonable feed rate during the technological design process provided that we use the formula, applied by us as the method of continual approaches with computer, in the below form

RthBr=125⋅f2rε+hmin2⋅1+hmin⋅rεf2 μm 1

to define the value of the minimum undetachable chip thickness (h_min) [1]. Due to space limitations it is not possible to analyse the tool edge quality (surface roughness values on the rake face and flank land of the inserts, roughness of main cutting edge, edge sharpness, edge radius etc.)

3.2. The effect of the tool material

The explicit tendency of the carbide tools to the strong adhesion and their chemical reaction with the high silicon content aluminum alloys enable cutting operations only at low cutting speed values and the surface roughness, achieved under such conditions, can not meet the increased requirements of the modern part production. The other material, coming into question, is the polycrystalline diamond: it has anti-adhesion characteristics, chemical inertness, better abrasive wear resistance and low coefficient of friction. The technological characteristics, mentioned earlier, make it possible, and, the expensive tool materials make it necessary (require) to use PCD inserts at increased cutting speed values.

The Figure 3/a introduces the case when the carbide insert – due to its better edge quality, but mainly due it favourable surface roughness on the rake face – takes the lead over the PCD tools. The measured Rz (ten point height) values inform us that the use of diamond inserts is not enough effective at low cutting speed values. The size and structure of the grains significantly affect the Rz value of the machined surface. As it can be seen well in Figure 3/b the KD1400 grade is fine-grained (~2 μm), the wear resistance of the KD1425 grade, having a grain size of 2 … 30 μm, is greater as it has a multi-modal structure.

This last one version can withstand moderate interruptions, appearing - in case of machining of aluminum alloys - in form of microporosity, caused by the hydrogen gas. At low cutting speed values and feed rates it is recommended to use the grade with higher wear resistance, in case of higher cutting speed values the recommendation „for high speed finishing”, given for the KD1400, can prevail. With the help of surface roughness (i.e. microtopographical) measurements we can get even more detailed picture about the textures of surfaces, machined with different tool materials. Figure 4 shows the waviness, filtered from the cylindricity, and the photosimulation visualisation of the surface roughness; furthermore, the most important parameters of the P-topography. As it can be seen well in this figure, in case of surfaces with nearly the same roughness parameters (arithmetical mean deviation, sPa and total height of profile, sPt) the profile peak heights, machined with carbide tools, are significant higher (sPp), the profile valley depths are much lower (sPv) than in case of surfaces, turned with diamond inserts. It means that diamond tool produces much more even surfaces and it machines the surface in its real meaning; while carbide insert produces wavier surface and detaches chip, causing great plastic deformation. It is confirmed by the kurtosis value of profile (sP Ku > 3), characterising the topography height distribution of the 3D surface roughness values as well.

The surfaces, machined with PCD tools, show us that whole „series” of primary Si-crystalls appear in the feed grooves, while in case of surfaces, machined with hard metal tools, one small piece of chip can be seen as great protrusion on the workpiece. The micro-mechanisms of cutting operations have been confirmed by the photos, taken with electron microscope type JEOL JSM-4510.

3.3. Tests of PCD tools under HSC conditions

HSC of high silica aluminum alloys means the cutting operations with speed values, exceeding 1000 m/min. The tests, carried out with the inserts of famous tool manufacturers, have confirmed that the average surface roughness values (Ra) can be significantly affected not only by the feed, but by the point angle of the insert as well; furthermore, the Ra value of < 0,4 μm can be achieved without any difficulties under reasonable defined machining conditions, even in case of inserts with small nose radius. It can be seen in Figure 5.

Figure 6 shows us an example for the determining role of nose radius of PCD insert. The tests, carried out in a wide range of cutting speed values, have confirmed that if the cutting speed values have been changed, then very diverse average surface roughness values have been measured by us in case of inserts with small nose radius; while in case of inserts with bigger nose radius only a slight increase in surface roughness has been noticed.

4. Summary, further tasks

During our short term tests, carried out to examine the machinability of the grade, mentioned earlier, we can get picture about the roughness parameters of surfaces, machined with tools, having different materials, shapes, constructions and produced by different manufacturers. We have carried out 2D and 3D measurements; moreover, our tests have been extended by electron-microscopic analyse as well. Not having the approval from the manufacturers, we do not wish to publish the results of our research in details.

We wish to extend our examinations to the long-lasting tests and (wear and tool life) evaluation of the insert, considered to have the best performance. Furthermore, we would like to examine one special version of diamond inserts (wiper edge geometry) under similar testing conditions.

Acknowledgments

Authors wish to thank Mr. Tornyi and Mr. Nagy, working at Delphi Thermal Hungary Kft. in Balassagyarmat, for their valuable comments and for the company as well, having provided us with the casting components.

Literature

[1] www.forgacsolaskutatas.hu

[2] Haizhi Ye: An Overview of the Developement of Al-Si-Alloy Based Material for Engine Applications JMEPEG (2003) 12:288-297

[3] Rábel, Gy. – Sipos, S. dr. – Csiszár, G.: Alumíniumötvözetek forgácsolásának tapasztalatai Coromant szerszámokkal végzett esztergáláskor

Gépgyártástechnológia, 1992/11-12. p. 533-537.