Acoustic resonance analysis in manufacturing - processes and practical applications -

Abstract

Acoustic resonance analysis for non-destructive testing of materials in mass-production, as offered by RTE Akustik + Prüftechnik GmbH is a proven technology, having been successfully implemented in various industries. Acoustic testing technology for non-destructive testing of materials is very old: for instance, merchants excite glass or porcelain goods by tapping, thus causing them to vibrate; the object's quality is assessed by the sound produced. Cracked glass "rattles" instead of ringing.What is new is the industrial application of these methods to everyday manufacturing of various products in many industries. Modern, high-power computer systems can "audit" human hearing. Reliable integration into a production cycle of a few seconds, under mass-production conditions, is possible without any difficulties.

1. Introduction

Nowadays, rationalisation and reliable, automated mass-production testing technology are increasingly important in order to deal economically with German manufacturing issues and with the increasing cost pressures. 
The requirements of DIN ISO 9000 for demonstrated and documented product quality cannot easily be satisfied by subjective testing or by destructive testing of sampled specimens. The result of cumbersome and costly testing technologies is clients who respond too late to changes in process parameters, the "rebooting" of the production process remains a critical problem, in-line measurement and product evaluation is not implemented. With RTE's testing technology, production and process reliability can be increased significantly. By using acoustic methods, the detection of cracks and structural defects can be carried out during the manufacturing cycle itself. 
Conventional procedures applied nowadays to non-destructive materials testing are in part too expensive and complex, and in part require a considerable amount of previous knowledge to be available. Non-destructive materials testing using resonance analysis, has already proven itself in everyday industrial practice as the new way forward.

2. Acoustics

Acoustic deals with sound fields, i.e. with the description and explanation of the phenomena of sound generation, sound emission, sound propagation and sound absorption. Sound (aerial sound = tone, noise) in the narrow sense is understood as elastodynamic vibrations and waves in the medium of air, in the frequency range audible to humans i.e. approximately 20 Hz to 16 kHz.
Mechanical vibrations in a structure (solid-borne sound) and radiated vibrations in the surrounding air (audible sound) carry information. The resonances depend uniquely on the object's material, geometry and condition. 
Acoustic measurement technology is very sensitive; even minor changes in the oscillatory behaviour of mechanical structures can be detected. Resonance analysis is a qualitative process that compares the actual oscillatory situation with the target one derived from a learning base. This learning base is established by using defined standard parts.

3. The acoustic resonance analysis process

Acoustic resonance analysis is a new, non-destructive testing process that allows rapid and cost-effective 100% testing of a wide spectrum of test objects. It exploits the well-known physical effect in which a body, having been suitably excited (e.g. by tapping), vibrates in certain characteristic forms and frequencies (its natural resonances). The vibrations are, so to speak, the test object's 'language', its 'fingerprint' that can be captured with a sensor and analysed digitally.Material-specific acoustic parameters can be calculated from the resonant frequencies, and assigned to quality characteristics such as:

• Pass,
• Cracked,
• Material and structure
• hardened / tempered yes/no.
The characteristics depend on dimensions, on the material and on the (internal) structure. If one of these properties changes, then individual (or several) resonant frequencies that represent this property change as well.
Key properties of acoustic resonance analysis (AR)

• AR evaluates the entire test object, regardless of its size. It does not sample or examine any local regions for faults.
• AR is simple to automate. Computer-aided testing is rapid, cost-effective and accurately reproducible (no subjective evaluation).
• AR is a qualitative procedure. Statements about the dimensions of defects are possible, and are based on comparable reference objects.
• AR is a dry procedure without chemistry or environmental problems.
• AR is proven and reliable ('ringing analysis' of glass and porcelain is an ancient method).

4. Practical applications

RTE's acoustic resonance analysis is tried and tested, having been successfully applied to mass-production in various industries. Here are a few examples:
• Testing of spherical-cast brake callipers for structural changes, cracks and cavities
• Testing of cast pipes (6 m length) for cracks and cavities
• Testing of camshafts for white irradiation
• Testing of graphite electrodes for cracks
• Testing of sintered metal rings and cog belt wheels for cracks
• Testing of forged synchronous rings for cracks
• Testing of roof tiles for cracks
• Testing of glass for cracks and tensile conditions
• Testing of Al and Mg die castings

Example 1: 100 % inspection of mass-produced cast components

Product: Spherical-cast safety parts for car brakes 
The task: Structural and casting faults, cracks and cavities should be reliably detected as part of the components' total quality assurance tests. Fault localisation is only given to a limited degree. There are many different types; costly setting-up times are not tolerated. During the process inspection task on "starting up" the process, the important point is to define objective measured parameters right from the start, from which process status and thus product quality can be evaluated. The cast parts should be approved without tedious destructive testing. The ambient conditions are typical of a foundry (among others, high background noise level, graphite dust) 
The solution concept: the design is 100% production line testing on a revolving transfer table, installed specially for the final tests. The measured parameter is sound, with the specimen excited to oscillate in a defined way. Recording and analysis occur in a few hundred milliseconds on the SR20 testing system. As a result of implementing a heuristic process ("reference run", "inspection run"), changing between types takes place by push-button. Monitoring by the test system of all the parameters relevant to the acoustic test leads to high system availability and test result reliability. Parameters can be altered by authorised personnel during operation via the facility's SPS, by using the "online parameter setting" function. Inputs are checked by the system for physical plausibility, so as not to endanger the testing process. Time-domain and frequency-domain parameters are calculated from the acoustic signal in real time, and compared against preset limits. 
Result: cost-savings through rapid process optimisationReduction in customer complaints through assured quality standards (reliability aspect!). High process reliability (detection of structural defects and accurate evaluation of cracks: so far not possible with other methods).


Spherical-cast safety parts for car brakes

Example 2: Detection of the structural defect 'White irradiation' in camshafts

Diagram 2: Temporal signal: the acoustic signal over time, 100 milliseconds are sufficient

Diagram 3: Frequency spectrum of the acoustic signals; each peak corresponds to one resonant frequency and can be utilised as a test feature

Product: Cast camshafts for cars 
White irradiation defect: Camshafts suffering from white irradiation lead to high machine tool wear during subsequent processing, and are brittle. 
Task: During total quality testing of the parts, the white irradiation defect must be detected reliably. There are many different types; costly setting-up times are not tolerated. The ambient conditions are typical of a foundry (among others, high background noise level, graphite dust) 
TThe solution concept: solution and implementation of the test task are through the compact, industrial SR20 test system. The measured parameter is sound, with the specimen excited to oscillate in a defined way. Recording and analysis occur in a few hundred milliseconds. Time-domain and frequency-domain parameters are calculated from the acoustic signal in real time, and checked against preset limits (comparison process). As a result of implementing a heuristic process ("reference run", "inspection run"), changing between types takes place by push-button. Monitoring by the test system of all the parameters relevant to the acoustic test leads to high system availability and test result reliability. 
Result: Reduction in customer complaints through assured quality standards, high process reliability (detection of white irradiation and the achieved evaluation accuracy not possible so far with other methods!). Presentation of production-line results: 80 production-line camshaft specimens of one type and defined condition were analysed by acoustic testing. After acoustic measurement, specimen quality was verified (in part through destructive processes). The temporal signal is shown below in Diagram 2, the frequency spectrum of a defined measurement window in Diagram 3, an overview of the distribution of resonant frequencies (1 frequency line given as example) across the specimens in diagram 4. Table 1 summarises the statistics. 
Conclusions: the detection of specimens 60 and 61 with resonant frequencies 8263 Hz (specimen 60) and 8258 Hz (specimen 61) from among the 80 specimens of verified quality, is statistically significant.

Minimum	Maximum	Mean	STD	MW + 3 x STD
8053 Hz	8263 Hz	8117 Hz	28,9 Hz	8204 Hz
Table 1: Statistics of the examined 80 production-line specimens of verified quality

Diagram 4: Resonant frequency (as an example, frequency characteristics at ca. 8 kHz) shown for 80 production-line specimens (x-axis: specimen number, y-axis: resonant frequency in Hz)

Example 3: Aluminium die casting - comparison against dye-penetration test

Product: Steering housing for cars 
Defects: Cracks, cavities 
Task: Production line testing, directly after casting
• All the specimens that proved unusual during crack testing with the dye penetration method, also show significant differences against the mean non-defective pattern during acoustic testing. Several specimens, originally classified as non-defective as per dye penetration test, proved acoustically unusual and were re-tested for cracks by the penetration method. The result of the acoustic resonance analysis was confirmed.
• Of 20 specimens judged defective by the dye penetration test, 18 were also identified as defective through their acoustic properties. The two non-identified specimens (i.e., passed by the resonance analysis), were found to be non-defective through destructive sectioned pattern analysis.
• Of 50 'sale quality' specimens (classified as Passed by x-ray and dye penetration tests), 2 specimens were found to have unusual acoustic characteristics (Failed by resonance testing). These were thoroughly re-tested with x-rays. A clear defect (long cavity) was identified. Hence the parts were reclassified as defective and not sent out.
• Reproducibility - process comparison Resonance analysis: 100 % agreement of acoustic parameters Dye penetration test: repeat testing by different testers established a level of agreement of only a few percent between the Pass/Fail test results!

Diagram 5: Examples of the tested property 'Decay behaviour of a resonant frequency' for 2 passed specimens (upper curves) and 2 failed ones (lower curves). The failed specimens decay significantly faster.

Example 4: Magnesium die casting

Product: Steering wheel framework 
Defect: Cracks 
Task: Production line testing 
Results of production-line experiments:
• There is good correlation between acoustic parameters and specimen quality.
• Analysis of parameter effect
  • Specimen temperature and relaxation time (internal tensile conditions!): high
  • Nest dependence: minor
• All cracked specimens were detected; this was reproducible.
• All non-defective specimens were identified as such; this was reproducible.
• 1 specimen originally classified as non-defective was acoustically unusual. Destructive verification discovered a structural fault (material separation).

Diagram 6: Demonstration of the resonant effect's frequency distribution

Tooling Design for Die Casting

Introduction
Die casting or mold is a closed vessel, is injected into the molten metal under high pressure and temperature, then rapidly cooled to solidification ed part is sufficiently rigid to allow the expulsion of the mold.To in this environment, diecasting mold must meet are constructed from tool steel of high quality for the required hardness and structure of heat treatment, machining with the dimensions of the punch and cavity demanding cation. The two mold halves in a die casting machine operation, which is operated at the temperatures and pressures necessary to produce a quality part of net shape or near net shap customers specifications.The products customer product design requires direct impact on the size , type, equipment, and the cost of the tools. The elements of the decision tool plate, covering the number of cavities, the number of core or slide, the weight of the matrix, machining, finish requirements, ie painting, polishing, plating, to name a few. A useful checklist for casting considerations for use in discussions with their own molding, end this section.Explanation of key terms related to the design are to die down.

The various alloys available for die casting,from aluminum to zinc, require unique and special features in the die that produces them. Because of these differences, the descriptions and parameters described in this text are generic. Where possible, options are listed but should be used only as a general guide, with the final decisions discussed between the customer and the die caster.

1 Types of Die Casting

Dies There are various types of die casting dies and each serves a critical need for the customer. The choice of which type of die casting die the customer requires is usually determined by the following:
• Size of the part to be cast
• Volume of parts required
• Requirements for “family” sets of parts
• Desirability of core slides
• Requirements for cast-in inserts.

   

 Prototype Dies Prototype dies are usually requested by the customer to produce a small number of castings under production conditions. They enable thorough product testing and market exposure before committing to full production dies. For eventual high-volume programs more than one prototype die may be produced.Only production from an actual die casting die can yield a part with precise die cast characteristics. However, there is a range of prototyping strategies that can be employed to approximate a die cast part for eventual production die casting. Among them: gravity casting, including the plaster mold process; machining from previously die cast parts or from wrought and sheet stock; and new rapid prototyping techniques such as stereolithography.

Production Dies These are the most common types of tools produced. They range from a single-cavity die, with no slides, to a mulitple-cavity die with any number of slides. The cavities are made from high-quality tool steel, retained in a quality holder block.Production dies are built to critical dimensions, coring the maximum amount of stockfrom the casting, and allowing the agreed upon amount of machining. A unit die is aspecial type of production die.

 Unit Dies A unit die is a lower cost production tool that has a standardized main die frame and replaceable cavity units. These replaceable units are designed to be removed from the main die frame without removing the standard frame from the die casting machine. The most common commercial types of unit dies are single and double unit holders. These types of dies are generally used for smaller parts, or a family of parts, with no slides or a minimum number of slides. Unit dies limit the use of core slides because of the configuration needed for interchangeable unit inserts and the limited space available.

Trim Dies The trim die is a tool that trims the runner, overfl ows, and fl ash from the casting. The trim dies are single or multiple cavity tools, made in the same confi guration as the die casting die.Depending on the shape of the casting,the trim die may be a simple open-and-closetrim die or it may include as many slides asthe die casting tool. In some cases multiple station trim dies will be used for successive trimming operations.Trim dies require as much attention to detail in design as the die casting tools andthe use of quality materials should be specified to extend their productive life.

How To Weld Aluminum Die Casting

The chemical properties of precious metal die casting must be taken into account for welding. Some of the key include the shedding heat range, color change, nonmagnetic features, electrical qualities and heat qualities. One of the main issues with welding precious metal die launching is that internal gas in the die launching creates both the weld and the precious metal next to the weld very permeable. This makes a sluggish weld then when becoming a member of other types of precious metal. Die-casting contains birdwatcher which creates welding precious metal more difficult because the precious metal is more subject to heat breaking.

1. Select an appropriate environment for precious metal die toss welding. You want to weld inside your home in a dry place free of writes. Any wetness or wetness in the air can cause problems with the weld. Try to prevent welding when the air wetness is higher if welding outside. However, it is best to prevent any wetness completely in a devoted room designed specifically for this form of welding.

2. Remove the welding combined with acetone-soaked towel to eliminate any fat or oil. Do not use water to clear the combined. Make sure you clear the combined and the location. Increase this washing to 25 mm on either side of the combined.

3. Get rid of slim coating of oxide found on the precious metal die launching with a precious metal clean that has only been used on precious metal. Do not use a farming hard drive as this will put in parts of dust and contaminants on the welding surface. It is best to start welding directly after washing though you can wait as long as three hours after washing.

4. Check that the welding materials are correct, as precious metal die launching uses an assortment of argon and helium that is 99.99 percent genuine. Clean the welding cable and product supports that will be used if they are filthy. Clean the product supports with precious metal fleece coat right before using.

5. Pre-heat the die casting before welding. The necessary preheating heat range will depend on the size and form of die launching. Aluminum die casting that is 8 mm wide will need a starting heat range of 100 levels Celsius. Larger die launching and those that are very complicated need preheating conditions of 350-400 levels Celsius.

6. Select the welding product content based on the form of welding you are doing. Restoring welds, repairing breaks, cosmetic fixes and becoming a member of of several castings need different stuffing content. For precious metal welding any form of welding can be used though MIG, precious metal inert gas, and TIG, tungsten inert gas, welding are suggested for resilient and high-quality welds. TIG is suggested for light evaluate work and MIG for wider welds.

Molding For Success

Die casting is a process where molten metal is put into a mould cavity using considerable pressure. The die casting method is a low cost and high product generating method. It is mainly used for producing small or medium sized parts.
Die casting is used in a variety of industries including the industrial, consumer and commercial industries, which require using different kinds of metals for different products. Die castings are usually made of metal alloys like zinc, copper, aluminium, tin and lead. Companies that specialize in die casted parts use various methods to manufacture the components.

The major ones are gravity die casting, high pressure aluminium casting and low pressure aluminium casting. Alloy wheels are made using the low pressure aluminium casting process. Aluminium die casting is one of the more popular methods.

Aluminium is a popular metal alloy used. The reason behind this is that aluminium is a light weight metal, but also has an ability to remain stable when moulding it into complex shapes. It retains its strength at high temperatures. It has a very good resistance to corrosion. Aluminium is known its good mechanical properties and its high electrical conductivity.

use parts that have been manufactured using the aluminium die casting process. Many automotive companies have replaced steel with aluminium for the cars frame and automotive components. The reason for this is that aluminium is lighter than steel, making the car leaner and faster. Lighter cars mean less consumption of fuel. It is easier to mould it into complex parts. Also, the labour cost for aluminium is less.

With a high demand for aluminium parts, many aluminium companies have come up that cater to the industries, manufacturing aluminium die casted parts. The die casting process also allows the manufacturers to produce the product according to the clients specifications. The products can be light weight, ranging from a few 100 gms to a few kgs, depending on the requirement.
The work of aluminium die casting manufacturers is majorly influenced by the research and development that goes into creating better systems to improve the production process. R&D usually comprises a highly qualified team that handles a large amount of imported and indigenous equipment. Automotive components manufacturers use the aluminium die casting process so that the component will have a long life. They can be designed to add to the visual appeal of the vehicle. That is why many automotive designers ask for parts that can be produced using the aluminium die casting method.

One of the newer processes that have been introduced in die casting is the pressure injection process. A number of methods are being used to automate the die casting process. This helps to continuously provide quality control. Automated systems are used to ladle metal into chambers, lubricate dies. Microprocessors adjust pressure and velocity to create the product according to the specification provided. Thus, it is simple to produce a component of according to ones needs.

Die Casting Aluminum Advantages To Various Company Industrial Sectors

A ton of production firms these days acknowledge the reliability and strength of light weight aluminum. It is not apparent that aluminum is as safe and powerful as those most typical metals utilized in large firms however the truth is indeed, this kind of part is undoubtedly dependable and excellent to use. In large business these days, die casting aluminum is a lot more preferred than utilizing high end metals and components. Main reason is the product's affordable nature and versatility on utilization depending on people's demands.


Actually, the buzz of die casting aluminum being utilized in large processing firms can easily be experienced more in the aircraft sector. This type of business needs accurate and top-notch quality products in all their airplane needs; and they found that using this kind of element or metal is a bright concept because of its solidity and outright resistance against bad weather conditions. Aside from utilizing it in aviation, casted light weight aluminum is now also being used in business business. It is merely since this sort of material blends well with additional sorts of metals and definitely provide wonderful flexibility.


Die Casting Aluminum to Aviation Business


Apart from its fantastic perk for the aircraft company, die casting aluminum pieces are more effective when compared with those plastic molded parts. The following qualities of aluminum metal will certainly further explain why it is far better than plastic when used in large companies:

1. When we talk about durability and quality, aluminium gloats itself for its normal defense in removing any radio regularities.


2. Best molded light weight aluminum metal parts are extremely resistant to heat which of course can prevent immediate fire develop when emergencies arise.


3. Aluminum metal parts have the biggest opportunity of sustainability and threshold. They can easily offer you the best casted output when compared with other products utilized in die casting.


4. Casted aluminium parts are better when mentioning coherence and design or size accuracy.


5. Highly advisable and appropriate to the aerospace sector because of its light weight attribute and sturdiness. Many aluminium casted parts are used in the seats and additional traveler elements like the baggage areas and cabins. These specific attributes make aluminium more effective in die casting.


Die Casting Aluminum to Other Industries

Actually apart from its owning perks in the aerospace field, die casting aluminum additionally produces remarkable uses to other things like developing electronic devices such as MP3 users, mobile phones, flat display TVs and screens, digital video cameras and a great deal more. Casted light weight aluminum parts also are of excellent usage to the construction sectors because they can be created into preciseness products or parts at reduced prices even in wonderful volume. And since of light weight aluminum's smooth finish or area, there is no demand for post completing of the products produced. Casted aluminiums are commonly recognized in worldwide because they pass the requirements for superior candidness. To top it all off, aside from the aviation location, these types of casted metals are widely utilized in telecom, electronic, vehicle and even plastic companies.

Diecasting Techniques And Refines

Have it ever before entered your thoughts of how many of your daily favorite things, especially metal ones, were produced? Essential aluminum or metal items from the huge ones like autos or trucks to little ones like door handles and ladles are made through the procedure of diecasting. This process is shaping a metal, alloy or aluminum to a wanted form especially designed based on its application.


Actually, some individuals point this procedure as casting metals where, as discussed above, produce metal structures by molding them correctly. This procedure obviously utilizes high-powered machines that apply excessive tension on the product as a way to make the wanted output. Many components used right here are zinc, aluminum and alloy which are known to emit strong item outputs.


Given that this strategy or treatment calls for using massive, technological machines to finish the molding procedure, this is clearly an expensive solution. Thus, productions in excellent amounts with sizes varying from little to medium are more preferred. The measurements of these output castings differ according to design. But in each design, you can immediately see the uniformity in sizes which will inform you right away that the item is a die cast.

There are actually four steps included in die casting. These procedures are as follows:

a. groundwork of the die

b. stuffing

c. ejection

d. shakeout

The complete Method

Diecasting procedure starts from your die groundwork where it is sprayed with wax or oil to make the removal of casting a great deal easier. It additionally assists in maintaining the temp within the product. After which, these freshly developed dies are sealed and injected with molten metal. This process then needs maximum force and should be preserved until the item hardens. The produced mold will then be injected with casting making use of pins. The final process, which is the shakeout, takes place when waste are stripped away from the output. And then the product will then be sent in for final assessment to evaluate if the manufacturing is all fine and if it passes the standards for offering or making use of. This whole procedure simply should be duplicated all over and over and if finished with hot chamber tools or devices, the manufacturing price is extremely quick.

There are actually 2 known appliances made use of in diecasting - the hot chamber and cold chamber. The standard and the most used tool is the very first one due to the fact that it is really typical in developing high fluidity materials. The cold chamber is used on materials or items that do not call for the utilization of hot chamber. The majority of products that are produced with cold chamber are those with high melting points. The only drawback is its longer processing and manufacturing time compared to making use of hot chamber machineries.