Automotive Door Inner Panel Manufacturing Engineering Essay

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This research paper is aim at demonstrating why using stamping to manufacture wrought magnesium alloy sheet for door inner panel in automobiles is suitable. The inner panel is an important part of car and critical to the safety, which need high performance. Wrought magnesium alloy is still on developing which can reduce the car weight replace the conventional steel, while supply similar strength and rigidity. Applying stamping process to manufacture the inner panel will result in greater alloy performance than using casting process.

With the rising of the gas price, more attention should be pay to the energy efficiency of vehicle, each 100 pounds increase of weight will lead to the reduce of gas mileage by as much as 4 percent[5]. So the age that using steel to make vehicles is nearly end, using advanced materials has become the fashion to boosting the fuel economy while maintaining driving performance and safety. At present, aluminum alloys and some polymers have already being used to decrease the car weight, but as the lightest materials, magnesium alloys can supply substantial additional reductions. Using magnesium alloys to fabricate car door, clutch case, steering wheel, transmission housing, chair frame and some other parts has become the trend, and weight can be reduced by 50% called lightweight car. Following parts will mainly focus on using magnesium to manufacturing door inner panel.

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Problem Statement:

As the manufacturing engineer, you have been asked to explore the use of magnesium sheets in the manufacture of automotive door inner panel.

Objectives:

Explain the consider factors of Mg alloys that affect product.

Select suitable Mg sheet alloy to manufacture the inner panel.

Identify a suitable manufacturing process for forming the panels.

Explore environmental impact.

Body

Automotive door inner panel:

Doors are often used activity components in vehicle components systems, relatively independent, and it is critical to the safety of the car. They are highly structure almost contain everything that a car has except the powertrain parts such as window regulator, control circuit for windows, automatic lock system. At the same time, doors should bear and disperse the impact from head-on and side collision.

For conventional doors which are produced by stamping, are mainly composed by three parts, inner panel, outer panel and window frame.

Figure 1 The door frame [2]

Consider of function of door inner panel, to manufacture the inner panel, such structures are needed,

1. Flanging process is used to form a panel owning the same thickness with door.

2. Panel should be stamped out different shapes of pallet, pothole, mounting holes, to set up the accessories.

3. Stiffeners are necessary to improve the rigidity of inner panel to reduce vibration and noise.

4. Enough strength and rigidity to satisfy the position requirement to install accessories, especially the window regulator

From the actual structure of inner panel, if using magnesium to make inner panel, following factors need to be taken into account,

Figure 2 The door inner panel [From Google]

Price

Since magnesium is 75% lighter than steel and 33% lighter than aluminum but it also costs three to four times as much as aluminum, but on the other hand, with the develop of widely using magnesium alloys to fabricate vehicle, it will encourage magnesium sheet metal producers to make more, which will bring down the costs[7].

corrosion resistance

Since leakage of rain may sometimes occur, fall into the gap between door frame and windows. The water will cause the rust when contact with alloys, so as a result the strength and beautiful degree of vehicle door will be reduced or destroyed. So select Mg sheet alloy should has some corrosion resistance. Furthermore, many anticorrosion methods can be used to the door made by steel or aluminum alloy, such as galvanization, painting and waxing. But to the magnesium alloy, since the magnesium oxide easily generated and hard to clear, it is hard to galvanize on magnesium alloy surface. Using other methods will enhance the cost, so this part should be solve to using Mg alloy.

volume of production

Compare to conventional steel sheet stamping process which can be manufactured at room temperature, before forming magnesium sheets into auto parts, they should be heat to 842 degrees Fahrenheit, even so, this process is very slow [7]. By contrast, aluminum has gotten much better through innovations, but with the vehicle industry gradually distract attention to magnesium, more researches have being doing on magnesium sheet forming, finally Mg sheet alloy can forming into parts at room temperature.

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strength and rigidity

Since the position requirement to install accessories and to bear and disperse the impact from collision, strength and rigidity is the key point to decide whether we can use Mg sheet alloy.

Table 1 Comparison properties of Mg alloys, Al alloys and steel [6]

Property

Magnesium and its alloys

Aluminum and its alloys

Steel

Crystal Structure

Hcp

Fcc

Bcc

Density at 20C (g/cc)

1.74

2.70

7.86

Yield point

130~305

35~550

205~1725

UTS

240-380

90-600

415-1750

Coefficient of thermal expansion 20-100 (x106/C)

25.2

23.6

11.7

Elastic modulus (106 psi)

6.4

10

30

Melting point (C)

650

660

1536

The yield point and elastic modulus in table above, shows that specific strength of magnesium is the highest compare to aluminum and iron, and its specific rigidity is in close proximity to aluminum alloys and iron. So Mg alloy has enough strength and rigidity to fabricate the door inner panel. Meanwhile, compare to magnesium alloy use to fabricate the engine block which need high heat resistance, high creep resistance, magnesium alloy use to make inner panel require low heat resistance and creep resistance, hence, cost is much lower.

Magnesium Alloy and Manufacture Process selection:

Classifying Mg alloy by forming process, they can be divided into casting Mg alloy and wrought Mg alloy. Casting Mg alloys use casting method to forming while wrought Mg alloys are forming with stamping, extrusion, rolling and forging. But in general, two methods are common used to fabricate car door inner panel, die casting and stamping.

Using die casting to manufacture the inner panel has its own advantages. Relatively low melting temperature of Mg made die casting process more easily, die casting surface finish has a good roughness that reduces the further machining, much lower scrap rate than other manufacture processes. These advantages made die casting become a traditional processing method for Mg alloys while wrought Mg alloys technique is not so developed [10].

But with more researches focus on wrought Mg alloys, the defect of using die casting to machine the Mg alloy is gradually expanding. Compare to automatic forming production line, production efficiency of die casting is not so acceptable while the demand of Mg alloy inner panel is increasing, which can be achieved by using stamping. And since the structure of inner panel is complex, some pallets and potholes cannot cast directly that need secondary machining, lower the production efficiency and gain the cost. What’s more, casting Mg alloys have defects such as segregation of composition, minimum thickness too large, lower mechanical properties, cannot give full play to the advantages of Mg alloy. Meanwhile, using stamping, can tremendously improve the strength and ductility of Mg alloy by change the microstructure through plastic deformation [11]. So to fabricate inner panel with better performance, we should use wrought Mg alloys and stamping process.

Wrought Mg alloys can be classified into following types, Mg-Al-Zn-Mn, Mg-Li, Mg-Mn, Mg-Zn-Zr, Mg-Mn-Re, Mn-Th. Consider the strength and corrosion resistance requirement of the inner panel, choose Mg-Al-Zn-Mn group. Compare the common used wrought magnesium alloys in table 2 with the cast alloy AZ91D, it is evident cast alloy AZ91D has lower ultimate tensile strength and elongation, which is very important in stamping process. Since strength and plasticity requirement of inner panel, contrast the data in the table, AZ61A maybe the best choice due to it has the largest UTS and superior elongation.

Table 2 Comparison of Different Mg alloys (from Google)

Alloy

UTS

Yield

Elongation

AZ40M

260

160

14

AZ41M

270

170

17

AZ61A

310

230

16

AZ80A

380

275

7

AZ91D(cast)

240

160

3

As the table 3 shown, AZ61A has approximately 7 % aluminum and 1% zinc, an obvious higher aluminum content compare to alloys, indicates a substantial precipitation of β phase in AZ61A which improve the strength of alloy coincide with the data in table above [9]. On the other hand, since the share of Zn is smaller than 2.5%, alloy AZ61A has a good corrosion resistance [9], which will reduce the cost in the following anti-corrosion treatments.

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Table 3 Comparison of Different Mg alloys (ASTM B39)

Brand

Al

Zn

Mn

Si

Fe

Cu

Ni

AZ40M

3.0-4.0

0.20-0.80

0.15-0.50

0.10

0.05

0.05

0.005

AZ41M

3.7-4.7

0.80-1.4

0.30-0.60

0.10

0.05

0.05

0.005

AZ61A

5.8-7.2

0.40-1.50

0.15-0.50

0.10

0.005

0.05

0.005

AZ80A

7.8-9.2

0.20-0.80

0.12-0.50

0.10

0.005

0.05

0.005

AZ91D(cast)

8.5-9.5

0.45-0.90

0.17-0.40

0.08

0.004

0.025

0.001

After select the alloy and stamping process, to forming a complex, specific steps should be consider. Generally, forming steps are,

Uncoiling shearing blanking TailoredBlandLaserWelding

Bevel punching punching trimming profiling drawing

Figure 3 The forming steps of inner panel [10]

Environmental Impact:

The main environment issue resulting from using Mg alloy to fabricate car door is,

Heat during the process. At the present stage, Mg alloy sheet should be heat to 842 degrees Fahrenheit before machining [7].

In magnesium alloy production and stamping process, to protect the molten Mg from react with air, a kind of greenhouse gases, sulphur hexafluoride (SF6) is widely used [12]. Along with the greenhouse effect is more and more serious, it will limit the developing of Mg industry.

Scrap rate of stamping process is very high, easy to cause the waste of resources. It is necessary to enhance the recycling of scrap.

After solve these environment problems, the application of Mg alloys on the vehicle will become much more widely, save more energy on weight.

Conclusion:

In conclusion this paper explored the basics of door inner panel, the characteristic of different Mg alloys and selection of forming process. It was determined that the wrought Mg alloy is a perfect fit for manufacturing the automotive inner panels due to their complex, the increasing requirement of light-weight car and the better performance of wrought alloys than cast alloys. Although the technique of Mg alloys stamping process is still not so developed, it is somewhat slow, causing environment issues and generates more wasted than casting, the performance of fabricated inner panel is economical, especially the strength and weight.