Elsevier

Wear

Volumes 442–443, 15 February 2020, 203153
Wear

Wear of form taps in threading of Al-Si alloy parts: Mechanisms and measurements

https://doi.org/10.1016/j.wear.2019.203153Get rights and content

Highlights

  • Wear mechanisms assessment of form taps in Threading of Al-Si Alloy.

  • Tapping machining process using uncoated HSS form taps.

  • New measurement metrics to quantify wear during threading.

  • Volumetric wear progression on lobes at different stages.

  • Critical wear region of a form tap identified.

Abstract

Wear of form taps in machining of Al-12Si die-cast alloys has been a critical problem for automotive engine manufacturing. Poor quality of formed threads results in high rework cost, and scrap. This research is focused on investigating the wear mechanisms and quantify wear by developing new measurement metrics. In addition, a performance criterion has been proposed to limit machining and evaluate wear on form taps. Abrasion and Adhesion wear mechanisms have caused severe deterioration of lobes on the chamfered length of tap tools during form tapping. A detailed evaluation of the complexity of wear propagation has been presented for critical regions of a lobe in a form tap.

Introduction

The Aluminum-Silicon (Al-Si) alloy is a eutectic alloy system which is widely used for its wear resistance applications. Since the 1950s, the die-cast Al-Si alloys were commercially used to produce engine blocks due to their high strength to weight ratio, good thermal conductivity, higher wear resistance and excellent castability. However, the machining of Al-Si alloys containing silicon greater than 10% is difficult due to rapid tool wear caused by the abrasive, hard silicon particles in the form of primary-phase silicon and eutectic silicon [1,2].

Tapping is a common machining process for generating threads in both ferrous and non-ferrous metals and their alloys. The cost of rework or scrapping due to the poor quality of internal threads must be avoided in large-scale production such as in the case of the automotive industry [3]. Form tapping on Al-Si alloy engine blocks has almost replaced cut tapping due to superior quality of formed threads. This could be attributed to the continuous material flow along the tap profile [4,5] and cold work hardening at the flanks and roots of the formed thread. Higher geometrical precision of formed threads is possible due to minimal error of flank straightness and thread pitch [6]. Both cut and formed threads have three distinct features-root, crest and flanks as shown in Fig. 1. The characteristic feature of a formed thread is the split crest which weakens the overall thread strength due to susceptibility towards chipping in assembly [7]. The split crest is directly dependent on the drilled hole diameter [8]. Elosegui et al. [9] studied the effect of surface pre- and post-treatments, and surface coatings of cut taps for tapping of austempered ductile iron (ADI-900), a similar material like Al-Si alloy, that causes severe abrasive and adhesive wear of taps. Although significant improvement in wear resistance was observed with AlTiSiN-G coating but delamination of coating was seen after tapping 216 threads [9]. Piska et al. [10] concluded that PVD TiN+DLC coating on form taps showed higher tool life of 1000 threads than that of cut taps which lasted for 600 threads in tapping 42CrMo4V steel alloy. As form taps do not have cutting edges since it's a chipless process, such surface treatments and coatings could significantly extend tool life by improving the wear resistance. Recently, Urbikain et al. [5] proposed a novel method that combined friction drilling and form tapping to produce internal threads on dissimilar metal alloys such as Al 5754-AISI 1045 and Al 5754-304 stainless steel. This new method is very promising for sheet metal applications as it is economical and lowers process time.

The sharpest corner of each crest on the tap's threaded section is called the lobe. Internal threads on drilled holes are formed by the penetration of a series of lobes at the tapered entry region of the tap. Fig. 1(a) shows these lobes on the chamfered length of tap. The guiding lobes move the tap forward into the hole. A back taper on the guiding lobes reduces the contact area between the surfaces of tap's lobe and the formed thread. Although there is no cutting edge, the crest can be divided into two sections-rake edge and relief edge (see Fig. 1(b)) [11]. The rake edge is in contact with the workpiece material, whereas the relief edge provides clearance between the tap's lobe and the formed thread.

The lobes on chamfered length experience forces primarily due to the plastic deformation, and subsequently contact pressure due to restricting elastic recovery. The guiding lobes experience forces due to friction with the elastically recovered workpiece. The surface of a threaded section on tap can be divided into 9 faces (see Fig. 1(c)) [12]. F1, F2, and F3 are the leading faces that perform plastic deformation. F4, F5, and F6 are the central faces that restrict elastic recovery occurring immediately after deformation. F7, F8, and F9 are the trailing faces that provide relief between formed thread and tap's surface.

Literature review suggests that torque is an important metric for assessing the performance of tap. Tapping torque depends on the workpiece material [3,11,13], tap geometry [8], drilled hole diameter [8,13], percentage of thread height [8], spindle speed [8,13], and metalworking fluids (MWF) [3,11,14]. Bierla et al. [14] performed tap tests to conclude that addition of Sulphur additives in MWF could lower torque by 15% due to reduced friction between tap and work material. Generally, drilled hole diameter is equal to the thread minor diameter in cut tapping, but in the case of form tapping, the drill hole diameter is bigger than the thread minor diameter, and selection of correct drill is critical for maintaining proper thread height percentage [8]. The decrease in the initial hole diameter leads to an increase in thread height percentage but also leads to higher mean torque and greater chances of tool breakage [6,13]. Agapiou [8] experimentally showed that higher tapping speeds did not influence the mean torque, but the peak torque increased with speed. Generally, the tapping speed is limited due to the use of a floating tap holder. This tap holder is required to eliminate pitch error but causes chatter issues at high speeds [8].

There has been limited research reported on the wear of form taps. In 2004, Fromentin [15] did an extensive study thread formation and tool wear analysis in form tapping. The author used a profile projector to measure the radial change at the lobe when the crest was worn. The length of the worn region at each lobe was measured using SEM upto 500 holes. In 2015, Landeta et al. [11] conducted tool wear study of coated HSSE form taps of three types. These taps were measured after every 1000 threads and up to 5000 threads. A linear measurement system was developed to record the wear on crest of every lobe of the tap. The wear was measured on both the rake and relief edges of each lobe's crest from the point of thread's maximum diameter. The total wear was the sum of wear on both rake and relief edges. In general, the author found higher wear of lobes on the chamfered length than the rest. Landeta et al. [11] monitored torque upto 5000 threads using DDU4 Artis system and found that 3-pitch taps with oil grooves have considerably lower torque under lubricated condition than other two taps without oil grooves. Sufficient lubrication with efficient oil grooves could lower friction and torque, as well as prevent adhesion wear. Bustillo et al. [16] performed similar wear measurement method as Landeta et al. and investigated several data-mining models for tap wear prediction. Rotation forest was concluded as the most accurate model with unpruned REPTree as its base regressor. Tallai et al. [17] performed tool wear analysis of five different coatings on HSSE form taps in machining a commercial grade tool steel. Wear was measured on the crest and maximum wear for the failure of the tool was restricted to 1.5 mm. Maximum torque was monitored and an upper limit to the torque was set beyond which the quality of the formed threads was considered inferior. Although the torque and wear had a positive correlation, the wear behavior was not explained and the basis for setting failure criterion was not explained.

In the above-mentioned studies, the linear wear measurement of the crest is very exhaustive and time-consuming. Real-time detection of tool wear has been possible lately by online monitoring of cutting forces using dynamometers and cutting power from the spindle servo motors [18]. Online tool condition monitoring for taps in high volume production is a significant gap to be addressed. Also, there is a high possibility that positioning the maximum diameter point might not be consistent and possibly cause positional errors. An effective tool fixture design must be proposed to ensure the same location of the reference point in every measurement during the experimentation. Previous researchers have mentioned the need for such a fixture to wear measurement under an optical microscope [11,15].

Literature review indicates the demand to reduce long inspection process of the wear of the lobes on chamfered length. Hence, a feasible methodology for wear measurement of a form tap correlating with performance metrics is presented. In this study, the wear mechanism of uncoated HSS roll form tap is investigated during threading Al-12Si alloy. In addition, a performance criterion for comparative tool wear analysis is proposed to facilitate tool life study.

Section snippets

Experimental methodology

Fig. 2 shows the experimental setup used to perform the form tapping process on a horizontal 5 axis - CNC milling machine. This setup ensures swift evacuation of chips during drilling under flooded lubrication. The drilling and tapping tests were performed with the same metal working fluid-HYSOL MB 500 having a concentration between 9-10% under flooded condition. Table 1 states the process parameters for drilling and tapping test which were kept constant for the entire test. The geometrical

Results and discussion

The performance of the roll form tap in terms of torque and wear was analyzed after tapping of 1020, 2160, 3240 and 4320 holes. The wear mechanism has been discussed in detail for different machining stages of the tap. An attempt has been made to identify the critical region for wear initiation, wear modes and wear propagation pattern.

Wear assessment of taps at end of service life

Based on industrial data, tool life of the same form tap with ZrN coating is estimated to be 20,000 holes. Geometrically correct formed threads on the workpiece is ensured at this limit. Investigations performed by SEM and EDS of the lobes on the chamfered length showed similar wear pattern on the crest of second lobe (see Fig. 18). The condition of this thread is similar to the second lobe of uncoated tap after 2160 holes (see Fig. 11(a)). This suggest that the coating was effective in

Conclusions

In the current work, the wear mechanism of form taps in threading Al-Si alloy with high Si contents (about 12%) has been studied. Different methods used to measure the wear of the tap's lobes. The following conclusions can be derived from this work:

  • 1.

    The wear mechanism of the uncoated tap was established as a combination of both adhesion wear and abrasion wear. Severe pluck-out occurred on the lobe's crest due to the removal of aluminum built-up during tapping. Hard silicon precipitates in α-Al

Acknowledgment

This research was supported by the Natural Sciences and Engineering Research Council of Canada (NStRC) under the CANRIMT Strategic Research Network Grant NETGP 479639- 15.

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