Gerry Gang Wang,Jeremy Bos

Meridian Lightweight Technologies,Strathroy,Ontario N7G 4H6,Canada N7G 4H6

Abstract With tremendous weight saving potential,magnesium alloy high pressure die casting components have been widely used for automotive applications.Magnesium fastening technology is thus becoming increasingly important to design engineers.Joining as-cast holes of magnesium high pressure die casting components with thread forming fasteners provides significan advantages for the assembly process,overall cost benefi and joint integrity.This type of joint is thus preferred for structural applications.Designing the thread forming fasteners with as-cast holes follows the general rules for designing for the machined holes,including carefully designing the variables such as the assembly torque range,the length of thread engagement,the hole diameter and required failure mode.In addition,special attention needs to be paid to the draft angles of the magnesium cast components that are required for the die casting process.In this paper,the effects of above key factors,individually and combined,on the joint performance of thread forming fasteners with as-cast blind holes of AM60B magnesium components are studied.A joint design philosophy was proposed to optimize both joint performance(the prevailing torque,the failure torque and the failure mode)and manufacturing easiness(the hole diameter and corresponding draft angle).The detailed design considerations for as-cast holes of magnesium HPDC are discussed and explained through a hypothetical example.

Keywords:Magnesium;Die casting;Fastening;Joining;Thread forming.

1.Introduction

Magnesium alloy high pressure die casting(HPDC)components have been widely used for structural applications due to their low density,high castability,high strength and high ductility[1–5].Magnesium alloys have some unique characteristics that make them especially suitable for joining with thread forming fasteners[6–7].Thread forming fasteners form mating threads in the as-cast or drilled holes of magnesium alloy components.These holes are designed with boss structures as shown in Fig.1.The advantages of this type of joint include:(1)the use of steel nuts are eliminated since the magnesium alloy components act as the nut members with internal threads;(2)no tapping is required since the fasteners form threads during assembly;and(3)the boss structures can be designed with various reinforcement structures(ribbing patterns),resulting in strong and stiff joints(Fig.2).Most importantly,modern die casting technologies and improved engineering designs have enabled the hole and boss structures of the magnesium HPDC components to be manufactured with near-net shape and tight tolerance.This makes it possible to use thread forming fasteners in as-cast holes of the magnesium components such that no drilling is needed[8].

Fig.1.Magnesium HPDC boss structures with holes to receive thread forming fasteners.

The as-cast holes of magnesium HPDC components have draft angles which are needed for the die cast process.These draft angles are the tapers given to the core to permit easy removal of the component in the die casting process[9].Usually the draft angles are larger than 1°/side,depending on the geometry(diameter and height)of the boss structure and the design requirements.Because of the draft angles,the as-cast holes have bigger diameter on the top than on the bottom as illustrated in Fig.3.This means the radial thread engagement is not consistent along the length of the fastener.This brings some challenges to properly designing the joints with threading forming fasteners.The performance of this type of joint is dependent on the length of thread engagement(LOE,define as the fastener length with fully engaged threads),the hole diameter and the corresponding draft angles,the assembly torque and the required joint failure modes.Effects of these factors,individually and combined,on the joint performance are systematically studied in this work.The design philosophy for as-cast holes of the magnesium HPDC is discussed and explained.

2.Experimental procedure

2.1.Magnesium alloy HPDC samples

AM60B magnesium alloy HPDC boss samples(Fig.1)for M6 fasteners were employed in this study.The holes are 5.30mm in diameter and 20mm deep(all hole diameter values presented in this paper are the diameters measured on the top of the holes).These samples were cast on a 1200 ton cold chamber HPDC machine.The holes were then machined to achieve specifi hole diameters and draft angles designed for the study.

Fig.3.A typical as-cast hole of the magnesium HPDC component(A=hole diameter,α=draft angle).

To study the effect that machining the hole surface had on the joint performance,a comparative study was conducted on the machined holes(with draft angles)and the as-cast holes(with the same draft angles).The as-cast drafted holes were from AM60B cross-car beam castings and are 5.66mm in diameter with 0.75°/side draft angle.The cast boss samples were machined to have the same hole diameter and draft angle.This hole diameter and draft angle were chosen to ensure the failure mode was the magnesium hole strip.Therefore the effect of the machining could be evaluated by comparing the strength of the joints with the machined hole surface vs.the as-cast hole surface.

The chemistry of the magnesium alloy is shown in Table 1.

2.2.Fastener tests

Fig.2.Reinforced magnesium HPDC boss structures.

Table 1 Chemical composition of as-cast AM60B(mass fraction,%).

MR6×1.0 Taptite 2000?and MG6-2.0 Mag-Form?thread forming fasteners were used in this work.These fasteners are 20mm long and coated with a Zn plated coating.The different LOEs were achieved by adding steel washers with different thicknesses.Fastening tests were conducted using an Atlas Copco ETV S7-70-13 CTADS Electric Right Angle Nutrunner.The torque test rotation speed was 350 RPM.Ten to sixteen samples from each test condition were tested.

3.Test results and discussion

3.1.Torque curves

Due to the difference in fastener threads and pitch designs,Magform?fasteners and Taptite?fasteners have different torque curves,as seen in Fig.4.Generally speaking,the relationship between the assembly torque and the fastener rotation angle can be characterized by two stages:The drivingin stage and the clamping-up stage.During the driving-in stage,the assembly torque increases slowly when the fastener threads into the hole.The highest torque in this stage is called the ‘prevailing torque’or ‘drive torque’,which is the torque needed to form threads in the casting holes of the magnesium HPDC components.During the clamping-up stage,the assembly torque increases sharply to form the required clamping force to join the members together.The highest torque at which the joint fails is called the ‘failure torque’or ‘ultimate torque’.The joint failure modes can be categorized into 3 types:(1)the magnesium hole strip(the internal thread strip);(2)the fastener break and(3)a combination of the above.The failure mode of steel fastener strip(external thread strip)does not exist in steel/magnesium joint combinations.

The difference between the prevailing torque and the failure torque is the ‘a(chǎn)ssembly torque window’.For a properly designed joint,the lower bound of the assembly torque should be higher than the prevailing torque to ensure the joint is seated properly.Additionally,the upper bound of the assembly torque should be lower than the failure torque to avoid any pre-mature joint failure during assembly,as will be discussed later.

3.2.The effects of machining of the hole surface on the joint performance

The effects of machining the hole surface on the joint performance are illustrated by comparing the joint performance between the machined drafted holes and the as-cast drafted holes(Fig.5).The prevailing torques and the failure torque values are presented as the average values with 3 standard deviations.Even though the machined holes show a higher scatter(a larger standard deviation),the performance of the two types of holes are essentially the same.

The effects of machining on the material properties are two-fold.The as-cast magnesium HPDC components have a layer of skin surface with very fin microstructure that is stronger and harder than the inner portion of the components[10,11].Machining the as-cast hole will remove or partially remove this skin layer and expose the relatively softer inner structure of the castings.This may lower the prevailing torque and failure torque.On the other hand,machining will induce plastic deformation on the machined surface of the magnesium components that will strengthen the surface through strain hardening.This may increase the prevailing torque and the failure torque.Fig.5 shows that neither the prevailing torque nor the failure torque is influence by these effects.Machining is found to have no effects on the joint performance under the current sample configuration and testing conditions.Therefore,the results obtained from machined drafted holes are also applicable to the corresponding as-cast drafted holes.

Fig.4.Typical torque curves of thread forming fasteners(PT=prevailing torque,FT=failure torque).

Fig.5.Effect of the hole surface machining on the prevailing torque and the failure torque of M6 thread forming fasteners(error bars represent 3 standard deviations from the averages,LOE=1.8X D).

Fig.6.Torque curves and joint failure modes of M6 thread forming fasteners in AM60B HPDC with different LOEs(Failure Mode:Ms=Mg strip,Fb fastener break,Mix=Mg strip/fastener break mix)(D is the fastener diameter;D=6mm for a M6 fastener)(a).5.2mm hole diameter.(b).5.6mm hole diameter.

3.3.The effects of the length of thread engagement(LOE)

Fig.6 illustrates the effects of the LOE on the prevailing torque,the failure torque and the joint failure mode with 2 hole diameters.At a hole diameter of 5.2mm(Fig.6a),both prevailing torque and failure torque increase with the LOE.The failure mode changes from the magnesium hole strip when the LOE is below 2.0X D,to the mix of the hole strip and the fastener break when the LOE is 2.0X D,and to the fastener break when the LOE reaches 2.5X D.Similarly,at a larger hole diameter of 5.6mm(Fig.6b),both prevailing torque and failure torque increase with the LOE.However,the failure mode remains as the magnesium hole strip even at 2.5X D due to the lack of radial thread engagement.If the fastener break is the desired joint failure mode,a LOE of around 2.5 X D is recommended with the appropriate hole diameter range as discussed later.

3.4.The effects of the hole design

Fig.7.Effect of the hole diameter on the prevailing torque,the failure torque and the joint failure mode of M6 thread forming fasteners in AM60B HPDC(error bars represent 3 standard deviations from the averages)(Failure Mode:Ms=Mg strip,Fb=fastener break).

Fig.7 shows the effect of the hole diameter on the joint performance of thread forming fasteners.The prevailing torques and the failure torque values are presented as the average values with 3 standard deviations.As expected,the prevailing torque decreases with the increase in thehole diameter.This is because the larger the hole diameter,the smaller the radial thread engagement.The failure torque changes with the hole diameter depending on the failure mode.In the tested hole diameter range,the failure torque does not change significantl with the hole diameter when the failure mode is the fastener break.When the hole diameter is above 5.50mm,the joint failure mode changes from the fastener break to the magnesium hole strip.In this case,the failure torque will decrease with the increase in the hole diameter.

Fig.8 shows the effects of the hole draft angle on the joint performance of thread forming fasteners.Again,the prevailing torques and the failure torque values are presented as the average values with 3 standard deviations.For the same tophole diameter,the prevailing torque increases with the draft angle.This is because when the top-hole diameter is f xed,increasing the draft angle means decreasing the bottom hole diameter,thus increasing the radial thread engagement with the fastener going deeper into the hole.For the same reason,the joint failure mode changes from hole strip to fastener break,which results in the increase in the failure torque.However,it should be noted that when the draft angle is too large(over 1.2°/side in this study),the failure torque will decrease with the increase of draft angle.This is because when the bottom diameter is too small,the shearing force on the fastener during assembly will increase,which will result in the decrease of the total failure torque.

As explained earlier,a draft angle is required for the ease of ejecting the casting from the cavity in the die casting process.The larger the draft angle,the easier the ejection of the parts during die casting.Therefore,manufacturing engineers usually prefer larger draft angles.However,as shown in Fig.8,when the draft angle is too large,the prevailing torque will increase and the failure torque could decrease.Therefore,the assembly torque window(between the prevailing torque and the failure torque)will lessen which might result in a pre-mature joint failure during assembly or degradation in the joint performance.

When designing a joint with thread forming fasteners in as-cast holes of the magnesium HPDC components,all above factors need to be considered.The assembly torque range is determined by the requirements of the joint functionality and the assembly process variation.The LOEis determined by the fastener length and the mating component thickness.When designing the hole diameter and the draft angle of the magnesium HPDC components,both the assembly torque range and the manufacturing(casting)requirements need to be considered.As a general rule,the holes of magnesium HPDC have to be designed such that the prevailing torque is lower than the lower bound of the assembly torque,and the failure torque higher than the upper bound of the assembly torque.In addition,variations from the manufacturing process(such as hole diameter variation)need to be taken into consideration.Fig.9 shows a hypothetical example of a M6 fastener joint design with a LOE of 2.0X diameter,and the assembly torque range of 12 to 15Nm.In this design,the 3 standard deviations in the torque values are taken into consideration,i.e.the average failure torque is lowered by 3 standard deviations and the average prevailing torque is increased by 3 standard deviations.According to Fig.9,when the hole size is below 5.60mm,the prevailing torques(withvariations)will be close to the lower bound of assembly torque range(12Nm).In other words,if the hole size is smaller than 5.60mm,there is a chance thatthe joint won’t seat properly during the assembly process.On the other hand,when the hole size is above 5.80mm,the failure torques(with variations)willbe close to the upper bound of the assembly torque range(15Nm).This meansthat if the hole size is bigger than 5.80mm,there is a chance that the joint willfail(magnesium stripping)during the assembly process.This hole size range between5.60mm to 5.80mm can satisfy the joint design rule discussed above.Thus thehole size range should be designed for 5.60 to 5.80mm or 5.65±0.15 mm in this case.

Fig.8.Effect of the magnesium boss draft angle on the prevailing torque,the failure torque and the joint failure mode of M6 thread forming fasteners in AM60B HPDC(the error bars represent 3 standard deviations from the averages,the LOE=2.2X D)(Failure Mode:Ms=Mg strip,Fb=fastener break).

Fig.9.A hole design for AM60B HPDC with M6 thread forming fasteners(error bars represent 3 standard deviations from the averages)(Failure Mode:Ms=Mg strip,Fb=fastener break,Mix=Mg strip/fastener break mix).

4.Conclusion

The performance of a magnesium joint with thread forming fasteners is dependent on the length of engagement,the hole diameter and the hole draft angle.The prevailing torque will increase with the increase in the length of engagement,the decrease in the hole diameter and the increase in the draft angle.The failure torque will be determined by the above factors as well as the joint failure mode.Under the magne-sium hole strip failure mode,the failure torque will increase with an increase in the length of engagement,a decrease in the hole diameter and an increase in the draft angle.

When using thread forming fasteners with as-cast holes of magnesium HPDC components,the hole diameter and corresponding draft angle should be designed such that the joint should be seated at the lower bound of the assembly torque range and should not fail at the upper bound of the assembly torque range.

Acknowledgments

The authors would like to thank D.C.Goss and J.Salisbury for the discussions and critical review of the manuscript.K.Thacker is acknowledged for his contribution to fastener testing.

?Mag-Form?is a registered trademark of Acument Intellectual Properties,LLC

?Taptite?is a registered trademark of Research Engineering&Manufacturing,Inc.