Abstract: Die-cast aluminum alloy is one of the more common metal parts materials nowadays. However, when processing this type of material, various problems often occur, causing considerable economic losses to production enterprises. This article focuses on analysis and research from perspectives of cutting fluid, cutting tools, processing methods, etc., and puts forward solution suggestions, hoping to serve as a reference.

Die-cast aluminum alloys are increasingly used in engines, machine tools, aerospace and other industrial fields in today's society. It is particularly necessary for related industries to analyze problems that arise during processing and solve them.

Take K15 oil pump body mentioned in this article as an example. Pump body is made of YL113 GB/T15115-2009 die-cast aluminum, which is characterized by high wear resistance, low thermal expansion coefficient, and excellent thermal crack resistance. However, it performs poorly in terms of corrosion resistance and polishability, and oxidation protective layer is a significant flaw.

(1) Mold appears during processing. Aluminum naturally oxidizes in air to form mildew, which is essentially loose aluminum oxide. Mildew will worsen over time. Die-cast aluminum molds faster than aluminum alloy due to its relatively loose material.
(2) Processing holes of product are exposed. Outer surface of die-cast aluminum alloy product has a fairly dense layer of structure, and some small holes will appear inside workpiece due to shrinkage. If processing amount exceeds thickness of dense layer, holes will increase significantly.
(3) Tool wear or chipping. When casting is mixed with impurities or hard spots are formed due to segregation, tool may be damaged. If coating layer of coated tools also contains aluminum, chipping will often occur.
(4) Poor surface finish and dimensional accuracy. Die-cast aluminum alloy pump shell parts are generally made of YL113 material in China. Due to its low hardness and good plasticity, it is easy to produce built-up edge during cutting, which seriously affects roughness and dimensional accuracy of machined surface. Elastic recovery after cutting also brings difficulties to improving dimensional accuracy of machined surface.
Below, we will take a problem encountered in process of processing K15 oil pump body as an example to illustrate: When processing the two holes K1 and K2 in Figure 1, there are certain difficulties whether it is diameter of a single hole or position between the two holes. For this reason, a proper solution must be found.

 

Purpose of using cutting fluid is to cool, clean, lubricate and reduce early mildew during processing. For processing of die-cast aluminum alloy products, its main functions are the latter two.
Lubrication: If degree of lubrication is not enough during machining process, it is easy to produce built-up burrs, which will affect dimensional accuracy. Reduce early mildew during processing: Aluminum products are susceptible to corrosion in acid and alkali environments, but as long as corrosion inhibitor is properly selected, occurrence of mildew can be reduced.

(1) Dilution ratio of cutting fluid is generally 1:10~1:20.
(2) Requirements for dilution water: If temperature of dilution water is too low, it is easy to form colloidal substances and difficult to dissolve, which is not conducive to preparation of emulsified cutting fluid.
(3) PH value of emulsified cutting fluid: PH value when processing die-cast aluminum cannot be too high, generally around 8.5.

Dense layer on the surface of die-cast aluminum alloy products is only about 1mm thick, so machining allowance is smaller than this value.

Selection of tools is divided into selection of tool type and selection of tool geometric parameters.

Taking processing of YL113 die-cast aluminum alloy as an example, one of following three types of tools can be used:
(1) Uncoated ultra-fine grain carbide cutting tools. Main choice for processing aluminum alloys is K type (WC+Co) carbide, K10 type carbide tools have sharp cutting edges, good peeling resistance, and are not prone to bonding. They are preferred material for cutting silicon-aluminum alloys in the first, second, third, and fourth processing.
(2) Carbide cutting tools using physical coating (PVD) method. Coatings reduce diffusion and chemical reactions between tool and workpiece, thereby reducing pocket wear and built-up edge formation. Coated tools can extend tool life by more than 3 to 5 times than uncoated tools, increase cutting speed by 20% to 70%, improve machining accuracy by 0.5 to 1 level, and reduce tool consumption costs by 20% to 50%. It should be noted that composition of coating cannot contain aluminum.
(3) Medium-grained diamond (PCD) tools. Polycrystalline diamond tools, which are widely used, can keep cutting edge sharp for a long time, and tool life is 10 to 500 times that of carbide (WC matrix) tools. Generally, medium-grained diamond tools are selected when W (Si) ≤ 13%, and coarse-grained tools are selected for aluminum alloys with W (Si) > 13%.

(1) Under premise of fully considering factors such as tool tip strength and workpiece surface finish, select the smallest possible tool tip arc radius.
(2) Using a positive rake angle and relief angle of about 25%, cutting is light and chip removal is easy.
(3) Contrary to milling, choose a larger main deflection angle to avoid generating large radial component forces that cause bending deformation of shaft workpiece.
(4) Secondary declination angle generally takes a smaller value, which is slightly larger during roughing and slightly smaller for finishing.
(5) Generally, blade inclination angle is +10°, rake surface should be ground smooth, and a chip groove should be cut out.
For processing of die-cast aluminum alloy surfaces, aluminum alloy processing milling cutter (YG8) and ordinary 45-gauge steel processing milling cutter (YT30) were selected for precision milling comparison. Parameters are shown in Table 1 and Table 2.
Tool materials and tool parameters

Processing workpiece data comparison

Results show that method one can make cast aluminum alloy parts have a roughness of Ra1.6 and an accuracy of IT8 level; method two can make cast aluminum alloy parts have a roughness of Ra3.2 and an accuracy of IT9 level. Method one can better complete processing task.

When processing, feed amount should not be too large. This can not only extend service life of tool, but also ensure that dense layer on product surface will not be easily destroyed, and it can also reduce elastic recovery caused by excessive cutting force.

To sum up, in order to overcome difficulties in processing K1 and K2 holes in Figure 1, following measures have been taken:
(1) Replace cutting fluid with a special cutting fluid for die-cast aluminum alloy products.
(2) Considering that silicon content of K15 oil pump body material is about 11%, fine-tuning boring tool model TPGT080204L-WT1200A is selected to avoid tool damage easily and to ensure position of the two holes.
(3) In order to retain dense layer on the surface as much as possible, feed tool multiple times during processing, and set feed amount to F25.
(4) Elastic recovery also depends on selection of support points and pressing positions, as well as size of pressing force.

 

During processing, select surface B as support surface, use surface A to resist vertical plate of compression tooling, then use K3 and K4 holes in Figure 3 as positioning holes to achieve final positioning of pump body. When pressing, position circled by oval in Figure 4 is used as pressing position. Firstly, it is convenient to arrange pressing plate, and secondly, it is considered that there is no suspension or other holes between these two positions and vertical plate. It is also particularly important to control size of pressing force. If force is too small, pump body may loosen and shift during processing, causing processing dimensions to completely deviate, even cause serious consequences such as pump body rupture and tool damage; if force is too large, pump body may loosen and shift during processing. When measuring processing results, it will be found that there is obvious elastic recovery, and dimensional accuracy does not meet processing requirements at all. After calculation, it is more appropriate to control pressing force at 105~115kg.
(5) According to usage requirements, avoid interference of other equipment on processing machine tool, because such interference will affect accuracy of machine tool.

Factors that determine processing quality of die-cast aluminum alloys mainly include cutting fluid, cutting tools, processing methods, pressing positions and pressing forces. Accurately grasping above points can ensure dimensional accuracy of workpiece, which is conducive to large-scale production and thus expands economic benefits of production enterprise.