How to improve casting quality of BMW K5X engine

How to improve casting quality of BMW K5X engine

Foreword :The engine front cover production project cooperated with our company in Germany. The front cover product was jointly developed by the German BMW Motorcycle Company and our company. It was produced by our company for die-casting production. The large-displacement engine company sprayed the powder and machined it for the German BMW K5X high-end motorcycle assembly. The product has a unique design and looks like a turtle(figureure1).
  The front cover is the crankcase cover of the engine, and the intrinsic quality of the front cover is related to the overall performance of the engine. The structure of the left crankcase cover wall of the motorcycle engine can prevent the leakage of the oil, reduce the runout of the crankshaft, reduce the noise of the engine, and improve the overall performance of the whole engine.
  This article describes the improvement process from the perspective of mold design - through the design of the casting system and equipment. Improve the casting process by improving the casting system, improve the fullness, solve the defects such as pores and shrinkage holes caused by the mold casting system, thereby improving the casting yield of the front cover.
1.Customer request
  The K5X front cover has very strict requirements on the quality: the main body thickness of the part is 2.56mm; the weight of the part is 1210±30g (finished product) (1305g die-casting); no blister and delamination are allowed after the outer surface is sprayed; The appearance of the parts must not be scratched, scratched, scratched, etc.; after the die casting, the flaw detection is carried out, and there are no defects such as pores and shrinkage holes exceeding the PK3 standard.
BMW engine front cover of left crankshaft
figureure1.BMW engine front cover of left crankshaft
2.3# mold improved design defective before die-casting products
Table1. Defect analysis table
Defect Classification Defect Description Picture Cause Analysis
Die casting defect Gas porosity in thicker wall See figureure 2.Position1,2,3more gas porosity. Thick wall thickness, affecting the feeding.
Individual location cold-shut See figureure 3.Position 4, 5 clod shut,Position 5 is especially serious. Die casting conditions are not good enough.
Affect the subsequent spray quality Individual location blister See figureure 4.Position6,7,8 material organization is not tight enough. Material organization is not tight enough.
K5X front cover #3 die casting defect
figureure2. Gas-porosity of the BMW engine front cover
/K5X front cover #3 die casting defect2
figureure3.Clod-shut of the BMW engine front cover
K5X front cover #3 die casting defect3
figureure4.Loosen of the BMW engine front cover
3.Summary of failure analysis of front cover
  1) Analyze relevant failure modes from mold, equipment, where the mold and equipment have the highest risk failure modes as shown in Table 2.
Table2.Front cover failure mode analysis
 
Item Failure mode Reason for failure
Die Casting Machine Die casting machine clamping force is too small The projected area of the die-cast product is not accurately calculated
Low fullness Punch gate set is too long
Die casting mold design Flashing problem is serious Insufficient sealing distance of the mold, insufficient rigidity, poor mold strength and parallelism
High aluminum filling speed Ingate size is thin
Aluminum melt produces turbulence during filling The position and direction of the gate interfere with each other
The overall casting system design is unreasonable Unsmooth size of sprue, ingate and gate, unbalanced ratio
  2)Front cover die casting condition analysis table 3
Table3 Front cover die casting condition analysis table
  Theoretical condition Actual status Equipment condition Disparity
Shadow area 111260mm²  111260mm²  / /
Die casting pressure 80MPa 77.8-83.3MPa / /
Bulging force 9234.6KN  8656KN-9268KN  
Clamping force 9234.6KN 8200KN  8200KN  Exceeding equipment capabilities
Pressure chamber fullness 30%-50% 22.30%   The actual fullness is less than the theoretical condition, increasing the possibility of gas filling
  According to the analysis of Tables 2 and 3 above: 1 The current injection force is 28-30MPa (the pressure gauge display value), the injection head is φ90mm, and the specific pressure is calculated to be 77.8-83.3MPa. 2 The actual expansion force (the safety factor has not been considered, usually the safety factor should be 1.1) has exceeded the clamping force of the equipment (800 tons press, 8200KN in January 2013), which increases the possibility of flashing. 3 The fullness of the pressure chamber is much smaller than the theoretical requirement, increasing the possibility of gas entrainment. Therefore, the design of the front cover die casting casting system must be improved to improve the pass rate of the front cover die casting.
4.Improvement
4.1.Die casting machine calculation choice

  In the development of the front cover die-casting mold in 2012, no potential failure mode analysis was performed, and the KTX front cover was produced using an 800T die-casting machine with empirical values. The result is that in the actual production, the flashing phenomenon of the mold is more prominent, and the quality and internal quality of the product are very unsatisfactory. Even the customer's supply is also seriously affected.
Select die casting machine: clamping force calculation
The vertical parting surface projection area of the front cover casting is measured by three-dimensional graphics:
F piece = 72470mm
For the pouring and draining system, the projected area on the vertical parting surface is calculated as:
∑ pouring F piece x (50%~55%)=72470mm²x53%=38520mm²           (1)
The total area: 
∑ F = F + ∑ Pouring =72470+38520+111260mm²=1112.6cm²             (2)
P clamping force ≥ Q expansion force = ∑ F XP ratio ÷ K = 1054T     (3)
  The result calculated above: the production of a die casting machine of 1000-1100T is required.
Finally, according to the equipment already in the workshop, it is decided to use the DC1250T model die casting machine to produce the front cover of the K5X, in order to solve the problem of running water and insufficient clamping force. The parameters of the die casting machine are shown in Table 4.
4.2.Design improvement of front cover die casting and pouring system
  
The pouring system consists of the remaining cake, the ingate, the overflow trough, cross gate, and the exhaust passage. Its size, shape size, and position directly affect the filling amount of the casting, filling time and speed. However, these factors have a significant impact on the quality of die casting products and therefore need to be fully considered in the design of the gating system.
  According to the parameters of die casting, the thickness of the residual cake is determined, d = 25 ~ 36 mm; the role of the sprue is mainly to adjust the heat balance to ensure the final product solidification. Through the gates, over-gates, and ingates, the final pressure is transmitted to the castings to ensure that the castings crystallize under pressure, which is beneficial to the feeding of the hot joints and gas compression, effectively reducing and reducing the shrinkage cavities, and adjusting the melting. The size of the cup increases the fullness and prevents slag and gas from being injected.
The model of the front die casting machine was not improved: DC-800T, and the diameter of the punch was selected as d=φ90mm.
   Fullness I=V metal liquid ÷V melting cup = (3.3÷2.4)/(πRH)=1315.7÷(πX0.462X6.25)= 36.11%                     (4)
After modification, the parameters of the die casting machine are: DC-1250T; when φ90mm is selected;
   Fullness II=V metal liquid ÷V melting cup =(3.3÷2.4)/(πRH)=1315.7÷(πX0.462X7.45)=30.62%                      (5)
   Fullness I> Fullness II, indicating that DC-1250T uses a cup with a size of φ90mm, and the material utilization rate is not high;
  The reason why the DC-1250T does not use the φ80mm punch to increase the fullness is due to the injection force of the DC-1250T die casting machine 105 T, and the 1150mm punch length, which poses a great risk to the handpiece;
 In order to improve the fullness of the DC-1250T model, we have adopted a high gate technology, which not only improves the fullness, but also adjusts the heat balance of the mold. The height of the splitter cone is designed to be 125mm, and the fillet size is shortened to φ80mm.
   Fullness III = V metal liquid ÷ V melting cup = (3.3 ÷ 2.4) / (πRH) = 1315.7 ÷ (πX0.462X6.35) = 34.87%         (6)
Table 5 Improved fullness comparison table
  Die casting machine model Punch  dimensional Fullness 
Before improvement DC-800T φ90mm 36.11%
After the first improvement DC-1250T φ90mm 30.62%
After the second improvement DC-1250T φ80mm 34.87%
  From the data in Table 5, the improved fullness increased from 30.62% to 34.87%, an increase of 4 percentage points. The die-casting product adopts high-gate casting technology to improve the fullness, and at the same time, it can adjust the heat balance inside the mold, which is beneficial to the feeding of the hot joint and gas compression, and can directly reduce or reduce the shrinkage hole.
The improved sprue size is shown in figureure 5.
Improved sprue dimension drawing
figureure5.Improved sprue dimension drawing
4.3.Ingate improved
Ingate total cross-sectional area
∑Fin=G⁄ρxVxtxK1xK2xK3xK4=1305/2.4x15x0.06x1.25x 0.7x0.89x1.25=618.3mm²      (7)
∑Fin:Ingate total cross-sectional area  (mm²)
G:Die casting part weight                     (g);
ρ:Aluminum liquid density                   (2.3g/cm²)
V:Filling speed                            (15m/s)
t:Filling time                                      (0.05s)
K1:K1=1.25 when the wall thickness speed correction coefficient δ=2.7mm
K2:Specific pressure rate correction coefficient Pb=900 when K2=0.6
K3:Alloy physical property correction factor. Aluminum alloy: K3=0.9
K4:Wall thickness characteristic correction factor. Uniform wall thickness: K4=1.25
 Ingate thickness: in order to ensure that the casting can not be solidified; aluminum liquid can not be formed into aluminum, spray, strain, inclusions, scouring, gas, etc.;
 Insulation thickness:
 d=δxKnxK1xK2=2.6x0.48x1.25x1.6=2.49mm                                                               (8)
 δ:Die casting thickness (mm)
 Kn:Ingate thickness and casting wall thickness ratio
    Kn=d/δ=(Tj-Tn)/(Tj-Tx)=0.47                                                                                       (9)
Tj: Molten metal temperature Tj=ΔT1+Tjj=32+582+614°C
Tn: Temperature at the ingate: Tn=Txc + (tj-txc) xKt =576℃
Tx: Die cavity temperature: Tx=Tn-32=544°C
Txc:Die cavity initial temperature: Txc=260°C
Tjj:Metal crystallization temperature ADC12, Tjj=582°C
ΔT1:Crystallization process minimum temperature difference aluminum alloy  ΔT1=32°C
Kt:Metal liquid temperature coefficient of variation in the cavity when flowing, V=15m/s
Filling time t=0.05s,Kt=0.89;
K1:K1=1.25 when the wall thickness speed correction coefficient δ=2.7mm  (δ becomes smaller, K1 increases)
4.4.Cross gate improved
The total cross-sectional area of the gate is 3 to 4 times that of the gate; the thickness of the gate is 3 to 5 times the thickness of the gate; the total cross-sectional area of the gate:
∑ F horizontal = 3.5x ∑ inner =2361.3mm²                                                                                (10)
The specific distribution of the gate and the 2.3.4 ingate
Calculation results for the gate and gate:
∑ inside = 613.8 mm²
d = 2.97mm
∑cross=2361.3mm²
  Since the front cover casting product is easy to produce air holes at both ends and the largest number of shrinkage holes, the scrapped area is large, and the two ends of the gate are respectively increased by 6x3mm two gates, so ∑F = 582+2X18=618mm
The position of the gate is distributed as shown in Table 6 below:
Table6.Specific layout of the inner gate
Position 1 2 3 4 5 6 7 8
Area (mm²)LengthxWidth 3X6 3X24 3X45 3X35 3X24 3X26 3X24 3X6
  Because in some areas, the largest number of shrinkage cavities and pores are most likely to occur, the product is scrapped, and also to ensure that the aluminum water is sequentially filled upward from the lower gate and does not generate turbulence. Therefore, the thickness of the cross gate  is increased by 1.5mm, and the total cross-sectional area of the six gates is 2387mm².
The specific layout of the gate is shown in Table 7.
Position 1 2 3 4 5 6
Area (mm)LengthxWidth 19x16 46x17 35x12 24x12 23x18 18x16
The improved shape of gate is shown in figureure. 6
After the mold gate is improved
figureure 6.The improved shape of gate
(1) By changing the direction and position of the No. 1 gate, the order of filling the M9 to the overflow tank and the slag collection bag is solved to avoid turbulence, and the filling end serves as an overflow.
(2) The direction of the No. 2 gate has not changed. The width has been widened by 2 mm in the direction of the No. 3 gate, and the width of the No. 1 gate has been increased by 4 mm to 46 mm, which solves the bubble formed at the N 4 position and the surrounding area. Holes, poor molding, etc., while the amount of filling is also increased.
(3) After the sprue direction of No. 3 was adjusted, the sharp angle of the No. 2 gate was cut off and increased by 2 mm to 35 mm, which solved the shrinkage, poor molding and air bubbles in the I 7 area.
(4) The direction of No. 4 gate has not changed.
(5) No. 5 sprue direction has not changed. Widening the transition section contour of the gate to increase the filling amount.
4.5.overflow system improved  
  The overflow system consists of a venting groove, an overflow sump and a slag collection bag. The overflow sump and the slag collection bag are increased from 5 groups to 6 groups; an independent venting groove is added to the outer edge of the M9 area; The planar dimensions of the package are correspondingly reduced and the depth is increased. 1# The slag bag and the overflow port are moved upward by 4mm, and are located in the middle of the two cores outside the N4 area, which plays the role of slag discharge and filling guide. A set of slag bag and overflow port are added between the 2# and 3# slag bags before the improvement, which plays a good filling guiding role for the K2 area, avoiding the occurrence of turbulence and slag discharge. The exhaust effect is improved.
4.6. 3# mold defects and 4# mold improvement measures
  The previous section 4.3-4.5 has been designed to improve the design of the 4# mold pouring and draining system. The results of the dimensional adjustment of the casting system have been improved, and the defects of the casting products produced by the 3# mold have been solved. The corresponding solutions are shown in Table 8 below.
Table 8 K5X front cover 3# die casting defect and 4# mold corresponding measures
Defect Solution Note
Shrink1 see picture2 See measures 5-7 Improve the pressure chamber filling degree, reduce the appearance of air entrainment
Shrink2 see picture2 See measures 1-4 Increase the casting amount in this direction and make the casting direction as parallel as possible to improve the feeding capacity
Shrink3 see picture2 See measures 1-4 Increase the casting amount in this direction and make the casting direction as parallel as possible to improve the feeding capacity
Cold shut 4  see picture3 See measures 5-9 Increase exhaust, deepen slag package, reduce cold material residue
Cold shut 5  see picture3 See measures 5-9 Increase exhaust, deepen slag package, reduce cold material residue
Defect 6 see picture4 See measures 1-4 Reducing punch diameter, increasing specific pressure and improving casting compactness
Defect 7 see picture4 See measures 1-4,6 Reducing punch diameter, increasing specific pressure and improving casting compactness
Defect 8 see picture4 See measures 5-7 Reducing punch diameter, increasing specific pressure and improving casting compactness
 Solution1:Position2 thickness is increased from the original 15mm to 18mm to improve the pressure chamber filling degree, reduce the appearance of air entrainment.
#3 mold improvement measures 1
figureure7.4# mold improvement measures 1
  Solution2:The thickness of position 2 is increased from 14 mm to 17 mm as shown in figure. 8. The flow of aluminum liquid shown in figure. 9 is increased to improve the filling effect and provide the filling degree.
MW engine casting quality  mold improvement measures 2-1
Figure8.4# mold improvement measures 2
4#mold improvement measures
Figure9.
Improvement measure 2 brings changes - increase the flow of aluminum fluid at arrow positions
   Solution3. Adjust the red point R, adjust the direction of the flow channel here, to ensure that the flow direction of the aluminum liquid here is parallel to several other gates, increase the flow of aluminum liquid, improve the feeding capacity, reduce product gas-porosity, shrink and other defects, such as Figure 10.
4# mold improvement measures 3f
Figure10.4# mold improvement measures 3
  Solution4.Adjust the glue at the 4 positions to the core position, increase the flow of aluminum liquid, and make the flow direction of aluminum liquid as parallel as possible as shown in figure. 11
4# mold improvement measures 4
Figure11.4# mold improvement measures 4
  Solution5.The splitter cone length is increased from now 70mm to 120mm, increasing the packing density of the press chamber, as shown in Figure 12.
4# mold improvement measures 5
Figure12.4# mold improvement measures 5
  Solution6.The diameter of the injection head is changed from the current φ90mm to φ80mm. increasing the packing density of the press chamber, as shown in Figure 13.
4# mold improvement measures 6
Figure13.4# mold improvement measures 6
  Solution7.The shape of the splitter cone is changed from a circular shape to a rectangular shape, which increases the wide coverage of the sealing surfaces at both a and b. The sealing surface of the sealing surface in other places is at least 80 mm. As shown in Figure 14.
4# mold improvement measures 7
Figure14.4# mold improvement measures 7
Solution8.The projected area of the slag package is reduced, and the thickness is increased from 14mm to 20mm, as shown in Figure 15.
4# mold improvement measures 8
Figure15.4# mold improvement measures 8
  Solution9.Increase the exhaust groove in the position a shown, see Figure 16.
4# mold improvement measures 9
Figure16.4# mold improvement measures 9
4.7. 4# mode improvement and 3# mode comparison chart
4# Improved mold VS 3# mold1
Figure17.4# mode improvement after comparison with 3# mode
  In Figure 17, the runner is deepened and widened by 2-3mm, mainly to increase the flow rate and solve the castings.The problem of poor molding.
4# Improved mold VS 3#mold2
Figure18.4# mode improvement after comparison with 3# mode
  In Figure 18, the two green parts in the 4# mold are additional exhaust slots with a depth of 0.1mm, which are used to solve the casting bubbles, poor molding and other defects in this part.
4# Improved mold VS 3#mold3
Figure19.4# mode improvement after comparison with 3# mode
  In Figure 19,the shunt cone is changed from circular to square, mainly to increase the sealing surface and prevent the mold from running water during the die casting process. The splitting cone is changed from φ90(left view red part) to φ80 (right view green part)mainly to increase the filling pressure and ensure the compactness of the product. Shrink.
4# Improved mold VS 3#mold4
Figure20.4# mode improvement after comparison with 3# mode
  In Figure 20, the splitter cone of the 4# mold is higher than the 3# mold, mainly to reduce the length of the punch, increase the fullness of the aluminum alloy solution in the punch, and also reduce the thermal energy loss of the aluminum alloy solution. 
4# Improved mold VS 3#mold5
Figure21.4# mode improvement after comparison with 3# mode
  In Figure 21, the 4# mold has more inserts than the 3# mold sprue, mainly to increase the sealing surface of the near pouring(right view bule part) and prevent flashing during the die casting process.
5.conclusion
  Through the analysis and improvement in section 4, the product yield rate of 4# die casting has been greatly increased to ensure timely delivery and win the praise of customers, which provides valuable experience for the design and development of die casting die in the future.

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